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1.  A DNA Adenine Methyltransferase of Escherichia coli That Is Cell Cycle Regulated and Essential for Viability 
Journal of Bacteriology  2004;186(7):2061-2067.
DNA sequence analysis revealed that the putative yhdJ DNA methyltransferase gene of Escherichia coli is 55% identical to the Nostoc sp. strain PCC7120 gene encoding DNA methyltransferase AvaIII, which methylates adenine in the recognition sequence, ATGCAT. The yhdJ gene was cloned, and the enzyme was overexpressed and purified. Methylation and restriction analysis showed that the DNA methyltransferase methylates the first adenine in the sequence ATGCAT. This DNA methylation was found to be regulated during the cell cycle, and the DNA adenine methyltransferase was designated M.EcoKCcrM (for “cell cycle-regulated methyltransferase”). The CcrM DNA adenine methyltransferase is required for viability in E. coli, as a strain lacking a functional genomic copy of ccrM can be isolated only in the presence of an additional copy of ccrM supplied in trans. The cells of such a knockout strain stopped growing when expression of the inducible plasmid ccrM gene was shut off. Overexpression of M.EcoKCcrM slowed bacterial growth, and the ATGCAT sites became fully methylated throughout the cell cycle; a high proportion of cells with an anomalous size distribution and DNA content was found in this population. Thus, the temporal control of this methyltransferase may contribute to accurate cell cycle control of cell division and cellular morphology. Homologs of M.EcoKCcrM are present in other bacteria belonging to the gamma subdivision of the class Proteobacteria, suggesting that methylation at ATGCAT sites may have similar functions in other members of this group.
PMCID: PMC374390  PMID: 15028690
2.  The CcrM DNA methyltransferase is widespread in the alpha subdivision of proteobacteria, and its essential functions are conserved in Rhizobium meliloti and Caulobacter crescentus. 
Journal of Bacteriology  1997;179(18):5869-5877.
The Caulobacter crescentus DNA methyltransferase CcrM (M.CcrMI) methylates the adenine residue in the sequence GANTC. The CcrM DNA methyltransferase is essential for viability, but it does not appear to be part of a DNA restriction-modification system. CcrM homologs are widespread in the alpha subdivision of gram-negative bacteria. We have amplified and sequenced a 258-bp region of the cerM gene from several of these bacteria, including Rhizobium meliloti, Brucella abortus, Agrobacterium tumefaciens, and Rhodobacter capsulatus. Alignment of the deduced amino acid sequences revealed that these proteins constitute a highly conserved DNA methyltransferase family. Isolation of the full-length ccrM genes from the aquatic bacterium C. crescentus, the soil bacterium R. meliloti, and the intracellular pathogen B. abortus showed that this sequence conservation extends over the entire protein. In at least two alpha subdivision bacteria, R. meliloti and C. crescentus, CcrM-mediated methylation has important cellular functions. In both organisms, CcrM is essential for viability. Overexpression of CcrM in either bacterium results in defects in cell division and cell morphology and in the initiation of DNA replication. Finally, the C. crescentus and R. meliloti ccrM genes are functionally interchangeable, as the complemented strains are viable and the chromosomes are methylated. Thus, in both R. meliloti and C. crescentus, CcrM methylation is an integral component of the cell cycle. We speculate that CcrM-mediated DNA methylation is likely to have similar roles among alpha subdivision bacteria.
PMCID: PMC179479  PMID: 9294447
3.  The Brucella abortus CcrM DNA Methyltransferase Is Essential for Viability, and Its Overexpression Attenuates Intracellular Replication in Murine Macrophages 
Journal of Bacteriology  2000;182(12):3482-3489.
The CcrM DNA methyltransferase of the α-proteobacteria catalyzes the methylation of the adenine in the sequence GAnTC. Like Dam in the enterobacteria, CcrM plays a regulatory role in Caulobacter crescentus and Rhizobium meliloti. CcrM is essential for viability in both of these organisms, and we show here that it is also essential in Brucella abortus. Further, increased copy number of the ccrM gene results in striking changes in B. abortus morphology, DNA replication, and growth in murine macrophages. We generated strains that carry ccrM either on a low-copy-number plasmid (strain GR131) or on a moderate-copy-number plasmid (strain GR132). Strain GR131 has wild-type morphology and chromosome number, as assessed by flow cytometry. In contrast, strain GR132 has abnormal branched morphology, suggesting aberrant cell division, and increased chromosome number. Although these strains exhibit different morphologies and DNA content, the replication of both strains in macrophages is attenuated. These data imply that the reduction in survival in host cells is not due solely to a cell division defect but is due to additional functions of CcrM. Because CcrM is essential in B. abortus and increased ccrM copy number attenuates survival in host cells, we propose that CcrM is an appropriate target for new antibiotics.
PMCID: PMC101938  PMID: 10852881
4.  The CcrM DNA Methyltransferase of Agrobacterium tumefaciens Is Essential, and Its Activity Is Cell Cycle Regulated 
Journal of Bacteriology  2001;183(10):3065-3075.
DNA methylation is now recognized as a regulator of multiple bacterial cellular processes. CcrM is a DNA adenine methyltransferase found in the alpha subdivision of the proteobacteria. Like the Dam enzyme, which is found primarily in Escherichia coli and other gamma proteobacteria, it does not appear to be part of a DNA restriction-modification system. The CcrM homolog of Agrobacterium tumefaciens was found to be essential for viability. Overexpression of CcrM is associated with significant abnormalities of cell morphology and DNA ploidy. Mapping of the transcriptional start site revealed a conserved binding motif for the global response regulator CtrA at the −35 position; this motif was footprinted by purified Caulobacter crescentus CtrA protein in its phosphorylated state. We have succeeded in isolating synchronized populations of Agrobacterium cells and analyzing their progression through the cell cycle. We demonstrate that DNA replication and cell division can be followed in an orderly manner and that flagellin expression is cyclic, consistent with our observation that motility varies during the cell cycle. Using these synchronized populations, we show that CcrM methylation of the chromosome is restricted to the late S phase of the cell cycle. Thus, within the alpha subdivision, there is a conserved cell cycle dependence and regulatory mechanism controlling ccrM expression.
PMCID: PMC95206  PMID: 11325934
5.  Computational and Genetic Reduction of a Cell Cycle to Its Simplest, Primordial Components 
PLoS Biology  2013;11(12):e1001749.
Mathematical modelling and genetics in the bacterium Caulobacter crescentus identified redundancy in asymmetric cell cycle regulation through the dispensability of key transcription factor GcrA and methyltransferase CcrM, which together form a genetic module.
What are the minimal requirements to sustain an asymmetric cell cycle? Here we use mathematical modelling and forward genetics to reduce an asymmetric cell cycle to its simplest, primordial components. In the Alphaproteobacterium Caulobacter crescentus, cell cycle progression is believed to be controlled by a cyclical genetic circuit comprising four essential master regulators. Unexpectedly, our in silico modelling predicted that one of these regulators, GcrA, is in fact dispensable. We confirmed this experimentally, finding that ΔgcrA cells are viable, but slow-growing and elongated, with the latter mostly due to an insufficiency of a key cell division protein. Furthermore, suppressor analysis showed that another cell cycle regulator, the methyltransferase CcrM, is similarly dispensable with simultaneous gcrA/ccrM disruption ameliorating the cytokinetic and growth defect of ΔgcrA cells. Within the Alphaproteobacteria, gcrA and ccrM are consistently present or absent together, rather than either gene being present alone, suggesting that gcrA/ccrM constitutes an independent, dispensable genetic module. Together our approaches unveil the essential elements of a primordial asymmetric cell cycle that should help illuminate more complex cell cycles.
Author Summary
Cell cycle regulation is remarkably complex and the fundamental principles difficult to understand, even in simple cells. The bacterium Caulobacter crescentus is a popular model organism to study cell cycle regulation due to the two different daughter cells resulting from cell division: a mobile “swarmer” cell and a “stalked” cell that adheres to surfaces. Here, we use mathematical modelling and genetic experiments to identify the core components of the asymmetric cell cycle of these bacteria. Using our mathematical model we predicted and confirmed experimentally that the transcription factor and cell cycle regulator, GcrA, hitherto thought to be essential, is in fact dispensable. We also identified another master regulator, the methyltransferase, CcrM as dispensable. Furthermore, simultaneous deletion of both GcrA and CcrM removes the severe cell division defects observed on either single deletion, returning cells to near wild-type morphology. We found that GcrA and CcrM constitute an independent, dispensable, genetic module that regulates transcription of cytokinetic proteins during the cell cycle. Phylogenetically, the module is conserved in Alphaproteobacteria, the class of Caulobacter, but is not present in the tree root of the class, suggesting that we have identified the primordial core of the asymmetric cell cycle regulatory circuit in the Alphaproteobacteria.
PMCID: PMC3885167  PMID: 24415923
6.  The functions of DNA methylation by CcrM in Caulobacter crescentus: a global approach 
Nucleic Acids Research  2014;42(6):3720-3735.
DNA methylation is involved in a diversity of processes in bacteria, including maintenance of genome integrity and regulation of gene expression. Here, using Caulobacter crescentus as a model, we exploit genome-wide experimental methods to uncover the functions of CcrM, a DNA methyltransferase conserved in most Alphaproteobacteria. Using single molecule sequencing, we provide evidence that most CcrM target motifs (GANTC) switch from a fully methylated to a hemi-methylated state when they are replicated, and back to a fully methylated state at the onset of cell division. We show that DNA methylation by CcrM is not required for the control of the initiation of chromosome replication or for DNA mismatch repair. By contrast, our transcriptome analysis shows that >10% of the genes are misexpressed in cells lacking or constitutively over-expressing CcrM. Strikingly, GANTC methylation is needed for the efficient transcription of dozens of genes that are essential for cell cycle progression, in particular for DNA metabolism and cell division. Many of them are controlled by promoters methylated by CcrM and co-regulated by other global cell cycle regulators, demonstrating an extensive cross talk between DNA methylation and the complex regulatory network that controls the cell cycle of C. crescentus and, presumably, of many other Alphaproteobacteria.
PMCID: PMC3973325  PMID: 24398711
7.  DNA methylation by CcrM activates the transcription of two genes required for the division of Caulobacter crescentus 
Molecular Microbiology  2013;88(1):203-218.
DNA methylation regulates many processes, including gene expression, by superimposing secondary information on DNA sequences. The conserved CcrM enzyme, which methylates adenines in GANTC sequences, is essential to the viability of several Alphaproteobacteria. In this study, we find that Caulobacter crescentus cells lacking the CcrM enzyme accumulate low levels of the two conserved FtsZ and MipZ proteins, leading to a severe defect in cell division. This defect can be compensated by the expression of the ftsZ gene from an inducible promoter or by spontaneous suppressor mutations that promote FtsZ accumulation. We show that CcrM promotes the transcription of the ftsZ and mipZ genes and that the ftsZ and mipZ promoter regions contain a conserved CGACTC motif that is critical to their activities and to their regulation by CcrM. In addition, our results suggest that the ftsZ promoter has the lowest activity when the CGACTC motif is non-methylated, an intermediate activity when it is hemi-methylated and the highest activity when it is fully methylated. The regulation of ftsZ expression by DNA methylation may explain why CcrM is essential in a subset of Alphaproteobacteria.
PMCID: PMC3708114  PMID: 23480529
8.  DNA Binding of the Cell Cycle Transcriptional Regulator GcrA Depends on N6-Adenosine Methylation in Caulobacter crescentus and Other Alphaproteobacteria 
PLoS Genetics  2013;9(5):e1003541.
Several regulators are involved in the control of cell cycle progression in the bacterial model system Caulobacter crescentus, which divides asymmetrically into a vegetative G1-phase (swarmer) cell and a replicative S-phase (stalked) cell. Here we report a novel functional interaction between the enigmatic cell cycle regulator GcrA and the N6-adenosine methyltransferase CcrM, both highly conserved proteins among Alphaproteobacteria, that are activated early and at the end of S-phase, respectively. As no direct biochemical and regulatory relationship between GcrA and CcrM were known, we used a combination of ChIP (chromatin-immunoprecipitation), biochemical and biophysical experimentation, and genetics to show that GcrA is a dimeric DNA–binding protein that preferentially targets promoters harbouring CcrM methylation sites. After tracing CcrM-dependent N6-methyl-adenosine promoter marks at a genome-wide scale, we show that these marks recruit GcrA in vitro and in vivo. Moreover, we found that, in the presence of a methylated target, GcrA recruits the RNA polymerase to the promoter, consistent with its role in transcriptional activation. Since methylation-dependent DNA binding is also observed with GcrA orthologs from other Alphaproteobacteria, we conclude that GcrA is the founding member of a new and conserved class of transcriptional regulators that function as molecular effectors of a methylation-dependent (non-heritable) epigenetic switch that regulates gene expression during the cell cycle.
Author Summary
Methylation of genomic DNA at a specific regulatory site can impact a myriad of processes in eukaryotic cells. In bacteria, methylation at the N6 position of adenosine (m6A) is known to mediate a non-adaptive immunity response to protect cells from foreign DNA. While m6A marks are not known to govern expression of cell cycle genes in Gammaproteobacteria, cell cycle transcription in the model alphaproteobacterium Caulobacter crescentus requires the m6A methyltransferase CcrM that introduces m6A marks at GAnTC sequences and the enigmatic factor GcrA. Investigating if a functional and biochemical relationship exists between CcrM and GcrA, we found that CcrM-dependent m6A marks recruit GcrA to the promoters of cell cycle genes in vitro and in vivo and is required for efficient transcription. GcrA interacts with RNA polymerase, explaining how cell cycle transcription is affected. Importantly, m6A-dependent binding is also seen in GcrA orthologs, indicating that this transcriptional regulatory mechanism by CcrM and GcrA is conserved in Alphaproteobacteria.
PMCID: PMC3667746  PMID: 23737758
9.  Coordinate cell cycle control of a Caulobacter DNA methyltransferase and the flagellar genetic hierarchy. 
Journal of Bacteriology  1995;177(7):1662-1669.
The expression of the Caulobacter ccrM gene and the activity of its product, the M.Ccr II DNA methyltransferase, are limited to a discrete portion of the cell cycle (G. Zweiger, G. Marczynski, and L. Shapiro, J. Mol. Biol. 235:472-485, 1994). Temporal control of DNA methylation has been shown to be critical for normal development in the dimorphic Caulobacter life cycle. To understand the mechanism by which ccrM expression is regulated during the cell cycle, we have identified and characterized the ccrM promoter region. We have found that it belongs to an unusual promoter family used by several Caulobacter class II flagellar genes. The expression of these class II genes initiates assembly of the flagellum just prior to activation of the ccrM promoter in the predivisional cell. Mutational analysis of two M.Ccr II methylation sites located 3' to the ccrM promoter suggests that methylation might influence the temporally controlled inactivation of ccrM transcription. An additional parallel between the ccrM and class II flagellar promoters is that their transcription responds to a cell cycle DNA replication checkpoint. We propose that a common regulatory system coordinates the expression of functionally diverse genes during the Caulobacter cell cycle.
PMCID: PMC176791  PMID: 7896686
10.  The CtrA Response Regulator Mediates Temporal Control of Gene Expression during the Caulobacter Cell Cycle 
Journal of Bacteriology  1999;181(8):2430-2439.
In its role as a global response regulator, CtrA controls the transcription of a diverse group of genes at different times in the Caulobacter crescentus cell cycle. To understand the differential regulation of CtrA-controlled genes, we compared the expression of two of these genes, the fliQ flagellar gene and the ccrM DNA methyltransferase gene. Despite their similar promoter architecture, these genes are transcribed at different times in the cell cycle. PfliQ is activated earlier than PccrM. Phosphorylated CtrA (CtrA∼P) bound to the CtrA recognition sequence in both promoters but had a 10- to 20-fold greater affinity for PfliQ. This difference in affinity correlates with temporal changes in the cellular levels of CtrA. Disrupting a unique inverted repeat element in PccrM significantly reduced promoter activity but not the timing of transcription initiation, suggesting that the inverted repeat does not play a major role in the temporal control of ccrM expression. Our data indicate that differences in the affinity of CtrA∼P for PfliQ and PccrM regulate, in part, the temporal expression of these genes. However, the timing of fliQ transcription but not of ccrM transcription was altered in cells expressing a stable CtrA derivative, indicating that changes in CtrA∼P levels alone cannot govern the cell cycle transcription of these genes. We propose that changes in the cellular concentration of CtrA∼P and its interaction with accessory proteins influence the temporal expression of fliQ, ccrM, and other key cell cycle genes and ultimately the regulation of the cell cycle.
PMCID: PMC93667  PMID: 10198005
11.  N6-methyl-adenine: an epigenetic signal for DNA-protein interactions 
Nature Reviews. Microbiology  2006;4(3):183-192.
N6-methyl-adenine is found in the genomes of bacteria, archaea, protists, and fungi. Most bacterial DNA adenine methyltransferases are part of restriction-modification systems. In addition, certain groups of Proteobacteria harbor solitary DNA adenine methyltransferases that provide signals for DNA-protein interactions. In γ-Proteobacteria, Dam methylation regulates chromosome replication, nucleoid segregation, DNA repair, transposition of insertion elements, and transcription of specific genes. In Salmonella, Haemophilus, Yersinia, Vibrio, and pathogenic E. coli, Dam methylation is required for virulence. In α-Proteobacteria, CcrM methylation regulates the cell cycle in Caulobacter, Rhizobium, and Agrobacterium, and plays a role in Brucella abortus infection.
PMCID: PMC2755769  PMID: 16489347
Adenine; analogs & derivatives; metabolism; physiology; Bacteria; genetics; metabolism; pathogenicity; Bacterial Proteins; metabolism; Cell Cycle; Chromosomes, Bacterial; metabolism; DNA Methylation; DNA Repair; DNA, Bacterial; genetics; metabolism; Epigenesis, Genetic; Genes, Bacterial; genetics; Mutagenesis, Insertional; Proteobacteria; genetics; physiology; Site-Specific DNA-Methyltransferase (Adenine-Specific); genetics; metabolism; Transcription, Genetic
12.  The diversity and evolution of cell cycle regulation in alpha-proteobacteria: a comparative genomic analysis 
BMC Systems Biology  2010;4:52.
In the bacterium Caulobacter crescentus, CtrA coordinates DNA replication, cell division, and polar morphogenesis and is considered the cell cycle master regulator. CtrA activity varies during cell cycle progression and is modulated by phosphorylation, proteolysis and transcriptional control. In a phosphorylated state, CtrA binds specific DNA sequences, regulates the expression of genes involved in cell cycle progression and silences the origin of replication. Although the circuitry regulating CtrA is known in molecular detail in Caulobacter, its conservation and functionality in the other alpha-proteobacteria are still poorly understood.
Orthologs of Caulobacter factors involved in the regulation of CtrA were systematically scanned in genomes of alpha-proteobacteria. In particular, orthologous genes of the divL-cckA-chpT-ctrA phosphorelay, the divJ-pleC-divK two-component system, the cpdR-rcdA-clpPX proteolysis system, the methyltransferase ccrM and transcriptional regulators dnaA and gcrA were identified in representative genomes of alpha-proteobacteria. CtrA, DnaA and GcrA binding sites and CcrM putative methylation sites were predicted in promoter regions of all these factors and functions controlled by CtrA in all alphas were predicted.
The regulatory cell cycle architecture was identified in all representative alpha-proteobacteria, revealing a high diversification of circuits but also a conservation of logical features. An evolutionary model was proposed where ancient alphas already possessed all modules found in Caulobacter arranged in a variety of connections. Two schemes appeared to evolve: a complex circuit in Caulobacterales and Rhizobiales and a simpler one found in Rhodobacterales.
PMCID: PMC2877005  PMID: 20426835
13.  Roles of DNA adenine methylation in host-pathogen interactions: mismatch repair, transcriptional regulation, and more 
FEMS microbiology reviews  2009;33(3):488-503.
The Dam methylase of gamma-proteobacteria and the CcrM methylase of alpha-proteobacteria catalyze an identical reaction (methylation of adenosine moieties using S-adenosyl-methionine as methyl donor) at similar DNA targets (GATC and GANTC, respectively). Dam and CcrM are of independent evolutionary origin. Each may have evolved from an ancestral restriction-modification system that lost its restriction component, leaving an “orphan” methylase devoted solely to epigenetic genome modification. Formation of 6-methyladenine lowers the thermodynamic stability of DNA and changes DNA curvature. As a consequence, the methylation state of specific adenosine moieties can affect DNA-protein interactions. Well known examples include binding of the replication initiation complex to the methylated oriC, recognition of hemimethylated GATCs in newly replicated DNA by the MutHLS mismatch repair complex, and discrimination of methylation states in promoters and regulatory DNA motifs by RNA polymerase and transcription factors. In recent years, Dam and CcrM have been shown to play roles in host-pathogen interactions. These roles are diverse and only partially understood. Especially intriguing is the evidence that Dam methylation regulates virulence genes in E. coli, Salmonella, and Yersinia at the postranscriptional level.
PMCID: PMC2941194  PMID: 19175412
Dam; CcrM; Pathogenic bacteria; Transcription; GATC regulation
14.  The Ruegeria pomeroyi acuI Gene Has a Role in DMSP Catabolism and Resembles yhdH of E. coli and Other Bacteria in Conferring Resistance to Acrylate 
PLoS ONE  2012;7(4):e35947.
The Escherichia coli YhdH polypeptide is in the MDR012 sub-group of medium chain reductase/dehydrogenases, but its biological function was unknown and no phenotypes of YhdH− mutants had been described. We found that an E. coli strain with an insertional mutation in yhdH was hyper-sensitive to inhibitory effects of acrylate, and, to a lesser extent, to those of 3-hydroxypropionate. Close homologues of YhdH occur in many Bacterial taxa and at least two animals. The acrylate sensitivity of YhdH− mutants was corrected by the corresponding, cloned homologues from several bacteria. One such homologue is acuI, which has a role in acrylate degradation in marine bacteria that catabolise dimethylsulfoniopropionate (DMSP) an abundant anti-stress compound made by marine phytoplankton. The acuI genes of such bacteria are often linked to ddd genes that encode enzymes that cleave DMSP into acrylate plus dimethyl sulfide (DMS), even though these are in different polypeptide families, in unrelated bacteria. Furthermore, most strains of Roseobacters, a clade of abundant marine bacteria, cleave DMSP into acrylate plus DMS, and can also demethylate it, using DMSP demethylase. In most Roseobacters, the corresponding gene, dmdA, lies immediately upstream of acuI and in the model Roseobacter strain Ruegeria pomeroyi DSS-3, dmdA-acuI were co-regulated in response to the co-inducer, acrylate. These observations, together with findings by others that AcuI has acryloyl-CoA reductase activity, lead us to suggest that YdhH/AcuI enzymes protect cells against damaging effects of intracellular acryloyl-CoA, formed endogenously, and/or via catabolising exogenous acrylate. To provide “added protection” for bacteria that form acrylate from DMSP, acuI was recruited into clusters of genes involved in this conversion and, in the case of acuI and dmdA in the Roseobacters, their co-expression may underpin an interaction between the two routes of DMSP catabolism, whereby the acrylate product of DMSP lyases is a co-inducer for the demethylation pathway.
PMCID: PMC3338564  PMID: 22563425
15.  Epigenetic Gene Regulation in the Bacterial World 
Like many eukaryotes, bacteria make widespread use of postreplicative DNA methylation for the epigenetic control of DNA-protein interactions. Unlike eukaryotes, however, bacteria use DNA adenine methylation (rather than DNA cytosine methylation) as an epigenetic signal. DNA adenine methylation plays roles in the virulence of diverse pathogens of humans and livestock animals, including pathogenic Escherichia coli, Salmonella, Vibrio, Yersinia, Haemophilus, and Brucella. In Alphaproteobacteria, methylation of adenine at GANTC sites by the CcrM methylase regulates the cell cycle and couples gene transcription to DNA replication. In Gammaproteobacteria, adenine methylation at GATC sites by the Dam methylase provides signals for DNA replication, chromosome segregation, mismatch repair, packaging of bacteriophage genomes, transposase activity, and regulation of gene expression. Transcriptional repression by Dam methylation appears to be more common than transcriptional activation. Certain promoters are active only during the hemimethylation interval that follows DNA replication; repression is restored when the newly synthesized DNA strand is methylated. In the E. coli genome, however, methylation of specific GATC sites can be blocked by cognate DNA binding proteins. Blockage of GATC methylation beyond cell division permits transmission of DNA methylation patterns to daughter cells and can give rise to distinct epigenetic states, each propagated by a positive feedback loop. Switching between alternative DNA methylation patterns can split clonal bacterial populations into epigenetic lineages in a manner reminiscent of eukaryotic cell differentiation. Inheritance of self-propagating DNA methylation patterns governs phase variation in the E. coli pap operon, the agn43 gene, and other loci encoding virulence-related cell surface functions.
PMCID: PMC1594586  PMID: 16959970
16.  Insights into the Cellular Function of YhdE, a Nucleotide Pyrophosphatase from Escherichia coli 
PLoS ONE  2015;10(2):e0117823.
YhdE, a Maf-like protein in Escherichia coli, exhibits nucleotide pyrophosphatase (PPase) activity, yet its cellular function remains unknown. Here, we characterized the PPase activity of YhdE on dTTP, UTP and TTP and determined two crystal structures of YhdE, revealing ‘closed’ and ‘open’ conformations of an adaptive active site. Our functional studies demonstrated that YhdE retards cell growth by prolonging the lag and log phases, particularly under stress conditions. Morphology studies showed that yhdE-knockout cells transformed the normal rod shape of wild-type cells to a more spherical form, and the cell wall appeared to become more flexible. In contrast, YhdE overexpression resulted in filamentous cells. This study reveals the previously unknown involvement of YhdE in cell growth inhibition under stress conditions, cell-division arrest and cell-shape maintenance, highlighting YhdE’s important role in E. coli cell-cycle checkpoints.
PMCID: PMC4319933  PMID: 25658941
17.  Genetic and Transcriptional Analyses of the Vibrio cholerae Mannose-Sensitive Hemagglutinin Type 4 Pilus Gene Locus 
Journal of Bacteriology  1999;181(4):1110-1117.
The mannose-sensitive hemagglutinin (MSHA) of the Vibrio cholerae O1 El Tor biotype is a member of the family of type 4 pili. Type 4 pili are found on the surface of a variety of gram-negative bacteria and have demonstrated importance as host colonization factors, bacteriophage receptors, and mediators of DNA transfer. The gene locus required for the assembly and secretion of the MSHA pilus has been localized to a 16.7-kb region of the V. cholerae chromosome. Sixteen genes required for hemagglutination, including five that encode prepilin or prepilin-like proteins, have been identified. Examination of MSHA-specific cDNAs has localized two promoters that drive expression of these genes. This evidence indicates that the MSHA gene locus is transcriptionally organized into two operons, one encoding the secretory components and the other encoding the structural subunits, an arrangement unique among previously characterized type 4 pilus loci. The genes flanking the MSHA locus encode proteins that show homology to YhdA and MreB of Escherichia coli. In E. coli, the yhdA and mreB genes are adjacent to each other on the chromosome. The finding that the MSHA locus lies between these two E. coli homologs and that it is flanked by a 7-bp direct repeat suggests that the MSHA locus may have been acquired as a mobile genetic element.
PMCID: PMC93486  PMID: 9973335
18.  Acrylyl-Coenzyme A Reductase, an Enzyme Involved in the Assimilation of 3-Hydroxypropionate by Rhodobacter sphaeroides 
Journal of Bacteriology  2013;195(20):4716-4725.
The anoxygenic phototroph Rhodobacter sphaeroides uses 3-hydroxypropionate as a sole carbon source for growth. Previously, we showed that the gene (RSP_1434) known as acuI, which encodes a protein of the medium-chain dehydrogenase/reductase (MDR) superfamily, was involved in 3-hydroxypropionate assimilation via the reductive conversion to propionyl-coenzyme A (CoA). Based on these results, we speculated that acuI encoded acrylyl-CoA reductase. In this work, we characterize the in vitro enzyme activity of purified, recombinant AcuI using a coupled spectrophotometric assay. AcuI from R. sphaeroides catalyzes the NADPH-dependent acrylyl-CoA reduction to produce propionyl-CoA. Two other members of the MDR012 family within the MDR superfamily, the products of SPO_1914 from Ruegeria pomeroyi and yhdH from Escherichia coli, were shown to also be part of this new class of NADPH-dependent acrylyl-CoA reductases. The activities of the three enzymes were characterized by an extremely low Km for acrylyl-CoA (<3 μM) and turnover numbers of 45 to 80 s−1. These homodimeric enzymes were highly specific for NADPH (Km = 18 to 33 μM), with catalytic efficiencies of more than 10-fold higher for NADPH than for NADH. The introduction of codon-optimized SPO_1914 or yhdH into a ΔacuI::kan mutant of R. sphaeroides on a plasmid complemented 3-hydroxypropionate-dependent growth. However, in their native hosts, SPO_1914 and yhdH are believed to function in the metabolism of substrates other than 3-hydroxypropionate, where acrylyl-CoA is an intermediate. Complementation of the ΔacuI::kan mutant phenotype by crotonyl-CoA carboxylase/reductase from R. sphaeroides was attributed to the fact that the enzyme also uses acrylyl-CoA as a substrate.
PMCID: PMC3807440  PMID: 23955006
19.  A Quantitative Study of the Division Cycle of Caulobacter crescentus Stalked Cells 
Progression of a cell through the division cycle is tightly controlled at different steps to ensure the integrity of genome replication and partitioning to daughter cells. From published experimental evidence, we propose a molecular mechanism for control of the cell division cycle in Caulobacter crescentus. The mechanism, which is based on the synthesis and degradation of three “master regulator” proteins (CtrA, GcrA, and DnaA), is converted into a quantitative model, in order to study the temporal dynamics of these and other cell cycle proteins. The model accounts for important details of the physiology, biochemistry, and genetics of cell cycle control in stalked C. crescentus cell. It reproduces protein time courses in wild-type cells, mimics correctly the phenotypes of many mutant strains, and predicts the phenotypes of currently uncharacterized mutants. Since many of the proteins involved in regulating the cell cycle of C. crescentus are conserved among many genera of α-proteobacteria, the proposed mechanism may be applicable to other species of importance in agriculture and medicine.
Author Summary
The cell cycle is the sequence of events by which a growing cell replicates all its components and divides them more or less evenly between two daughter cells. The timing and spatial organization of these events are controlled by gene–protein interaction networks of great complexity. A challenge for computational biology is to build realistic, accurate, predictive mathematical models of these control systems in a variety of organisms, both eukaryotes and prokaryotes. To this end, we present a model of a portion of the molecular network controlling DNA synthesis, cell cycle–related gene expression, DNA methylation, and cell division in stalked cells of the α-proteobacterium Caulobacter crescentus. The model is formulated in terms of nonlinear ordinary differential equations for the major cell cycle regulatory proteins in Caulobacter: CtrA, GcrA, DnaA, CcrM, and DivK. Kinetic rate constants are estimated, and the model is tested against available experimental observations on wild-type and mutant cells. The model is viewed as a starting point for more comprehensive models of the future that will account, in addition, for the spatial asymmetry of Caulobacter reproduction (swarmer cells as well as stalked cells), the correlation of cell growth and division, and cell cycle checkpoints.
PMCID: PMC2217572  PMID: 18225942
20.  tmRNA regulates synthesis of the ArfA ribosome rescue factor 
Molecular microbiology  2011;80(5):1204-1219.
Translation of mRNA lacking an in-frame stop codon leads to ribosome arrest at the 3´-end of the transcript. In bacteria, the tmRNA quality control system recycles these stalled ribosomes and tags the incomplete nascent chains for degradation. Although ubiquitous in eubacteria, the ssrA gene encoding tmRNA is not essential for the viability of Escherichia coli and other model bacterial species. ArfA (YhdL) is a mediator of tmRNA-independent ribosome rescue that is essential for the viability of E. coli ΔssrA mutants. Here, we demonstrate that ArfA is synthesized from truncated mRNA and therefore regulated by tmRNA tagging activity. RNase III cleaves a hairpin structure within the arfA coding sequence to produce transcripts that lack stop codons. In the absence of tmRNA tagging, truncated ArfA chains are released from the ribosome. The truncated ArfAΔ18 protein (which lacks 18 C-terminal residues) is functional in ribosome rescue and supports ΔssrA cell viability when expressed from the arfA locus. Other proteobacterial arfA genes also encode hairpins, and transcripts from Dickeya dadantii and Salmonella typhimurium are cleaved by RNase III when expressed in E. coli. Thus, synthesis of ArfA from truncated mRNA appears to be a general mechanism to regulate alternative ribosome rescue activity.
PMCID: PMC3103599  PMID: 21435036
21.  Peptidoglycan Hydrolase LytF Plays a Role in Cell Separation with CwlF during Vegetative Growth of Bacillus subtilis 
Journal of Bacteriology  1999;181(10):3178-3184.
Peptidoglycan hydrolase, LytF (CwlE), was determined to be identical to YhdD (deduced cell wall binding protein) by zymography after insertional inactivation of the yhdD gene. YhdD exhibits high sequence similarity with CwlF (PapQ, LytE) and p60 of Listeria monocytogenes. The N-terminal region of YhdD has a signal sequence followed by five tandem repeated regions containing polyserine residues. The C-terminal region corresponds to the catalytic domain, because a truncated protein without the N-terminal region retained cell wall hydrolase activity. The histidine-tagged LytF protein produced in Escherichia coli cells hydrolyzed the linkage of d-γ-glutamyl-meso-diaminopimelic acid in murein peptides, indicating that it is a d,l-endopeptidase. Northern hybridization and primer extension analyses indicated that the lytF gene was transcribed by EςD RNA polymerase. Disruption of lytF led to slightly filamentous cells, and a lytF cwlF double mutant exhibited extraordinary microfiber formation, which is similar to the cell morphology of the cwlF sigD mutant.
PMCID: PMC93774  PMID: 10322020
22.  Identification of RpoS (ςS)-Regulated Genes in Salmonella enterica Serovar Typhimurium 
Journal of Bacteriology  2000;182(20):5749-5756.
The rpoS gene encodes the alternative sigma factor ςS (RpoS) and is required for survival of bacteria under starvation and stress conditions. It is also essential for Salmonella virulence in mice. Most work on the RpoS regulon has been in the closely related enterobacterial species Escherichia coli. To characterize the RpoS regulon in Salmonella, we isolated 38 unique RpoS-activated lacZ gene fusions from a bank of Salmonella enterica serovar Typhimurium mutants harboring random Tn5B21 mutations. Dependence on RpoS varied from 3-fold to over 95-fold, and all gene fusions isolated were regulated by growth phase. The identities of 21 RpoS-dependent fusions were determined by DNA sequence analysis. Seven of the fusions mapped to DNA regions in Salmonella serovar Typhimurium that do not match any known E. coli sequence, suggesting that the composition of the RpoS regulon differs markedly in the two species. The other 14 fusions mapped to 13 DNA regions very similar to E. coli sequences. None of the insertion mutations in DNA regions common to both species appeared to affect Salmonella virulence in BALB/c mice. Of these, only three (otsA, katE, and poxB) are located in known members of the RpoS regulon. Ten insertions mapped in nine open reading frames of unknown function (yciF, yehY, yhjY, yncC, yjgB, yahO, ygaU, ycgB, and yeaG) appear to be novel members of the RpoS regulon. One insertion, that in mutant C52::H87, was in the noncoding region upstream from ogt, encoding a O6-methylguanine DNA methyltransferase involved in repairing alkylation damage in DNA. The ogt coding sequence is very similar to the E. coli homolog, but the ogt 5′ flanking regions were found to be markedly different in the two species, suggesting genetic rearrangements. Using primer extension assays, a specific ogt mRNA start site was detected in RNAs of the Salmonella serovar Typhimurium wild-type strains C52 and SL1344 but not in RNAs of the mutant strains C52K (rpoS), SL1344K (rpoS), and C52::H87. In mutant C52::H87, Tn5B21 is inserted at the ogt mRNA start site, with lacZ presumably transcribed from the identified RpoS-regulated promoter. These results indicate that ogt gene expression in Salmonella is regulated by RpoS in stationary phase of growth in rich medium, a finding that suggests a novel role for RpoS in DNA repair functions.
PMCID: PMC94696  PMID: 11004173
23.  In-Depth Profiling of the LiaR Response of Bacillus subtilis▿ †  
Journal of Bacteriology  2010;192(18):4680-4693.
The Lia system, a cell envelope stress response module of Bacillus subtilis, is comprised of the LiaRS two-component system and a membrane-anchored inhibitor protein, LiaF. It is highly conserved in the Firmicutes bacteria, and all orthologs investigated so far are activated by cell wall antibiotics. In response to envelope stress, the systems in Firmicutes cocci induce the expression of a number of genes that are involved in conferring resistance against its inducers. In contrast, a complete picture of the LiaR regulon of B. subtilis is still missing and no phenotypes could be associated with mutants lacking LiaRS. Here, we performed genome-wide transcriptomic, proteomic, and in-depth phenotypic profiling of constitutive “Lia ON” and “Lia OFF” mutants to obtain a comprehensive picture of the Lia response of Bacillus subtilis. In addition to the known targets liaIH and yhcYZ-yhdA, we identified ydhE as a novel gene affected by LiaR-dependent regulation. The results of detailed follow-up gene expression studies, together with proteomic analysis, demonstrate that the liaIH operon represents the only relevant LiaR target locus in vivo. It encodes a small membrane protein (LiaI) and a phage shock protein homolog (LiaH). LiaH forms large oligomeric rings reminiscent of those described for Escherichia coli PspA or Arabidopsis thaliana Vipp1. The results of comprehensive phenotype studies demonstrated that the gene products of the liaIH operon are involved in protecting the cell against oxidative stress and some cell wall antibiotics. Our data suggest that the LiaFSR system of B. subtilis and, presumably, other Firmicutes bacilli coordinates a phage shock protein-like response.
PMCID: PMC2937411  PMID: 20639339
24.  The Caulobacter crescentus DNA-(adenine-N6)-methyltransferase CcrM methylates DNA in a distributive manner 
Nucleic Acids Research  2011;40(4):1708-1716.
The specificity and processivity of DNA methyltransferases have important implications regarding their biological functions. We have investigated the sequence specificity of CcrM and show here that the enzyme has a high specificity for GANTC sites, with only minor preferences at the central position. It slightly prefers hemimethylated DNA, which represents the physiological substrate. In a previous work, CcrM was reported to be highly processive [Berdis et al. (1998) Proc. Natl Acad. Sci. USA 95: 2874–2879]. However upon review of this work, we identified a technical error in the setup of a crucial experiment in this publication, which prohibits making any statement about the processivity of CcrM. In this study, we performed a series of in vitro experiments to study CcrM processivity. We show that it distributively methylates six target sites on the pUC19 plasmid as well as two target sites located on a 129-mer DNA fragment both in unmethylated and hemimethylated state. Reaction quenching experiments confirmed the lack of processivity. We conclude that the original statement that CcrM is processive is no longer valid.
PMCID: PMC3287173  PMID: 21926159
25.  The essential genome of a bacterium 
This study reports the essential Caulobacter genome at 8 bp resolution determined by saturated transposon mutagenesis and high-throughput sequencing. This strategy is applicable to full genome essentiality studies in a broad class of bacterial species.
The essential Caulobacter genome was determined at 8 bp resolution using hyper-saturated transposon mutagenesis coupled with high-throughput sequencing.Essential protein-coding sequences comprise 90% of the essential genome; the remaining 10% comprising essential non-coding RNA sequences, gene regulatory elements and essential genome replication features.Of the 3876 annotated open reading frames (ORFs), 480 (12.4%) were essential ORFs, 3240 (83.6%) were non-essential ORFs and 156 (4.0%) were ORFs that severely impacted fitness when mutated.The essential elements are preferentially positioned near the origin and terminus of the Caulobacter chromosome.This high-resolution strategy is applicable to high-throughput, full genome essentiality studies and large-scale genetic perturbation experiments in a broad class of bacterial species.
The regulatory events that control polar differentiation and cell-cycle progression in the bacterium Caulobacter crescentus are highly integrated, and they have to occur in the proper order (McAdams and Shapiro, 2011). Components of the core regulatory circuit are largely known. Full discovery of its essential genome, including non-coding, regulatory and coding elements, is a prerequisite for understanding the complete regulatory network of this bacterial cell. We have identified all the essential coding and non-coding elements of the Caulobacter chromosome using a hyper-saturated transposon mutagenesis strategy that is scalable and can be readily extended to obtain rapid and accurate identification of the essential genome elements of any sequenced bacterial species at a resolution of a few base pairs.
We engineered a Tn5 derivative transposon (Tn5Pxyl) that carries at one end an inducible outward pointing Pxyl promoter (Christen et al, 2010). We showed that this transposon construct inserts into the genome randomly where it can activate or disrupt transcription at the site of integration, depending on the insertion orientation. DNA from hundred of thousands of transposon insertion sites reading outward into flanking genomic regions was parallel PCR amplified and sequenced by Illumina paired-end sequencing to locate the insertion site in each mutant strain (Figure 1). A single sequencing run on DNA from a mutagenized cell population yielded 118 million raw sequencing reads. Of these, >90 million (>80%) read outward from the transposon element into adjacent genomic DNA regions and the insertion site could be mapped with single nucleotide resolution. This yielded the location and orientation of 428 735 independent transposon insertions in the 4-Mbp Caulobacter genome.
Within non-coding sequences of the Caulobacter genome, we detected 130 non-disruptable DNA segments between 90 and 393 bp long in addition to all essential promoter elements. Among 27 previously identified and validated sRNAs (Landt et al, 2008), three were contained within non-disruptable DNA segments and another three were partially disruptable, that is, insertions caused a notable growth defect. Two additional small RNAs found to be essential are the transfer-messenger RNA (tmRNA) and the ribozyme RNAseP (Landt et al, 2008). In addition to the 8 non-disruptable sRNAs, 29 out of the 130 intergenic essential non-coding sequences contained non-redundant tRNA genes; duplicated tRNA genes were non-essential. We also identified two non-disruptable DNA segments within the chromosomal origin of replication. Thus, we resolved essential non-coding RNAs, tRNAs and essential replication elements within the origin region of the chromosome. An additional 90 non-disruptable small genome elements of currently unknown function were identified. Eighteen of these are conserved in at least one closely related species. Only 2 could encode a protein of over 50 amino acids.
For each of the 3876 annotated open reading frames (ORFs), we analyzed the distribution, orientation, and genetic context of transposon insertions. There are 480 essential ORFs and 3240 non-essential ORFs. In addition, there were 156 ORFs that severely impacted fitness when mutated. The 8-bp resolution allowed a dissection of the essential and non-essential regions of the coding sequences. Sixty ORFs had transposon insertions within a significant portion of their 3′ region but lacked insertions in the essential 5′ coding region, allowing the identification of non-essential protein segments. For example, transposon insertions in the essential cell-cycle regulatory gene divL, a tyrosine kinase, showed that the last 204 C-terminal amino acids did not impact viability, confirming previous reports that the C-terminal ATPase domain of DivL is dispensable for viability (Reisinger et al, 2007; Iniesta et al, 2010). In addition, we found that 30 out of 480 (6.3%) of the essential ORFs appear to be shorter than the annotated ORF, suggesting that these are probably mis-annotated.
Among the 480 ORFs essential for growth on rich media, there were 10 essential transcriptional regulatory proteins, including 5 previously identified cell-cycle regulators (McAdams and Shapiro, 2003; Holtzendorff et al, 2004; Collier and Shapiro, 2007; Gora et al, 2010; Tan et al, 2010) and 5 uncharacterized predicted transcription factors. In addition, two RNA polymerase sigma factors RpoH and RpoD, as well as the anti-sigma factor ChrR, which mitigates rpoE-dependent stress response under physiological growth conditions (Lourenco and Gomes, 2009), were also found to be essential. Thus, a set of 10 transcription factors, 2 RNA polymerase sigma factors and 1 anti-sigma factor are the core essential transcriptional regulators for growth on rich media. To further characterize the core components of the Caulobacter cell-cycle control network, we identified all essential regulatory sequences and operon transcripts. Altogether, the 480 essential protein-coding and 37 essential RNA-coding Caulobacter genes are organized into operons such that 402 individual promoter regions are sufficient to regulate their expression. Of these 402 essential promoters, the transcription start sites (TSSs) of 105 were previously identified (McGrath et al, 2007).
The essential genome features are non-uniformly distributed on the Caulobacter genome and enriched near the origin and the terminus regions. In contrast, the chromosomal positions of the published E. coli essential coding sequences (Rocha, 2004) are preferentially located at either side of the origin (Figure 4A). This indicates that there are selective pressures on chromosomal positioning of some essential elements (Figure 4A).
The strategy described in this report could be readily extended to quickly determine the essential genome for a large class of bacterial species.
Caulobacter crescentus is a model organism for the integrated circuitry that runs a bacterial cell cycle. Full discovery of its essential genome, including non-coding, regulatory and coding elements, is a prerequisite for understanding the complete regulatory network of a bacterial cell. Using hyper-saturated transposon mutagenesis coupled with high-throughput sequencing, we determined the essential Caulobacter genome at 8 bp resolution, including 1012 essential genome features: 480 ORFs, 402 regulatory sequences and 130 non-coding elements, including 90 intergenic segments of unknown function. The essential transcriptional circuitry for growth on rich media includes 10 transcription factors, 2 RNA polymerase sigma factors and 1 anti-sigma factor. We identified all essential promoter elements for the cell cycle-regulated genes. The essential elements are preferentially positioned near the origin and terminus of the chromosome. The high-resolution strategy used here is applicable to high-throughput, full genome essentiality studies and large-scale genetic perturbation experiments in a broad class of bacterial species.
PMCID: PMC3202797  PMID: 21878915
functional genomics; next-generation sequencing; systems biology; transposon mutagenesis

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