The Caulobacter crescentus DNA adenine methyltransferase CcrM and its homologs in the α-Proteobacteria are essential for viability. CcrM is 34% identical to the yhdJ gene products of Escherichia coli and Salmonella enterica. This study provides evidence that the E. coli yhdJ gene encodes a DNA adenine methyltransferase. In contrast to an earlier report, however, we show that yhdJ is not an essential gene in either E. coli or S. enterica.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
From the characterization of enzyme activities and the analysis
of genomic sequences, the complement of DNA methyltransferases (MTases)
possessed by the cyanobacterium Anabaena PCC 7120
has been deduced. Anabaena has nine DNA MTases.
Four are associated with Type II restriction enzymes (AvaI, AvaII, AvaIII and the newly recognized
inactive AvaIV), and five are not. Of the latter,
four may be classified as solitary MTases, those whose function lies
outside of a restriction/modification system. The group
is defined here based on biochemical and genetic characteristics.
The four solitary MTases, DmtA/M.AvaVI,
and DmtD/M.AvaIX, methylate at GATC, GGCC,
CGATCG and rCCGGy, respectively. DmtB methylates cytosines at the
N4 position, but its sequence is more similar to N6-adenine MTases
than to cytosine-specific enzymes, indicating that it may have evolved from
the former. The solitary MTases, appear to be of ancient origin
within cyanobacteria, while the restriction MTases appear to have
arrived by recent horizontal transfer as did five now inactive Type
I restriction systems. One Mtase, M.AvaV, cannot
reliably be classified as either a solitary or restriction MTase.
It is structurally unusual and along with a few proteins of prokaryotic
and eukaryotic origin defines a structural class of MTases distinct
from all previously described.
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.
The dam gene of Escherichia coli encodes a DNA methyltransferase that methylates the N6 position of adenine in the sequence GATC. It was stably expressed from a shuttle vector in a repair- and recombination-proficient strain of Bacillus subtilis. In this strain the majority of plasmid DNA molecules was modified at dam sites whereas most chromosomal DNA remained unmethylated during exponential growth. During stationary phase the amount of unmethylated DNA increased, suggesting that methylated bases were being removed. An ultraviolet damage repair-deficient mutant (uvrB) contained highly methylated chromosomal and plasmid DNA. High levels of Dam methylation were detrimental to growth and viability of this mutant strain and some features of the SOS response were also induced. A mutant defective in the synthesis of adaptive DNA alkyltransferases and induction of the adaptive response (ada) also showed high methylation and properties similar to that of the dam gene expressing uvrB strain. When protein extracts from B. subtilis expressing the Dam methyltransferase or treated with N-methyl-N'-nitro-N-nitroso-guanidine were incubated with [3H]-labelled Dam methylated DNA, the methyl label was bound to two proteins of 14 and 9 kD. Some free N6-methyladenine was also detected in the supernatant of the incubation mixture. We propose that N6-methyladenine residues are excised by proteins involved in both excision (uvrB) and the adaptive response (ada) DNA repair pathways in B. subtilis.
The genomic region encoding the type IIS restriction-modification (R-M) system HphI (enzymes recognizing the asymmetric sequence 5'-GGTGA-3'/5'-TCACC-3') from Haemophilus parahaemolyticus were cloned into Escherichia coli and sequenced. Sequence analysis of the R-M HphI system revealed three adjacent genes aligned in the same orientation: a cytosine 5 methyltransferase (gene hphIMC), an adenine N6 methyltransferase (hphIMA) and the HphI restriction endonuclease (gene hphIR). Either methyltransferase is capable of protecting plasmid DNA in vivo against the action of the cognate restriction endonuclease. hphIMA methylation renders plasmid DNA resistant to R.Hindill at overlapping sites, suggesting that the adenine methyltransferase modifies the 3'-terminal A residue on the GGTGA strand. Strong homology was found between the N-terminal part of the m6A methyltransferasease and an unidentified reading frame interrupted by an incomplete gaIE gene of Neisseria meningitidis. The HphI R-M genes are flanked by a copy of a 56 bp direct nucleotide repeat on each side. Similar sequences have also been identified in the non-coding regions of H.influenzae Rd DNA. Possible involvement of the repeat sequences in the mobility of the HphI R-M system is discussed.
DNA adenine methylation by DNA adenine methyltransferase (Dam) in Escherichia coli plays an important role in processes such as DNA replication initiation, gene expression regulation, and mismatch repair. In addition, E. coli strains deficient in Dam are hypersensitive to DNA-damaging agents. We used genome microarrays to compare the transcriptional profiles of E. coli strains deficient in Dam and mismatch repair (dam, dam mutS, and mutS mutants). Our results show that >200 genes are expressed at a higher level in the dam strain, while an additional mutation in mutS suppresses the induction of many of the same genes. We also show by microarray and semiquantitative real-time reverse transcription-PCR that both dam and dam mutS strains show derepression of LexA-regulated SOS genes as well as the up-regulation of other non-SOS genes involved in DNA repair. To correlate the level of SOS induction and the up-regulation of genes involved in recombinational repair with the level of DNA damage, we used neutral single-cell electrophoresis to determine the number of double-strand breaks per cell in each of the strains. We find that dam mutant E. coli strains have a significantly higher level of double-strand breaks than the other strains. We also observe a broad range in the number of double-strand breaks in dam mutant cells, with a minority of cells showing as many as 10 or more double-strand breaks. We propose that the up-regulation of recombinational repair in dam mutants allows for the efficient repair of double-strand breaks whose formation is dependent on functional mismatch repair.
The activities of DNA methyltransferases are important for a variety of cellular functions in bacteria. In this study, we developed a modified high-throughput technique called methyl
homopolymer tail mediated sequencing (methyl HTM-seq) to identify the undermethylated sites in the Vibrio cholerae genome for the two DNA methyltransferases, Dam, an adenine methyltransferase, and VchM, a cytosine methyltransferase, during growth in rich medium in vitro. Many of the undermethylated sites occurred in intergenic regions, and for most of these sites, we identified the transcription factors responsible for undermethylation. This confirmed the presence of previously hypothesized DNA-protein interactions for these transcription factors and provided insight into the biological state of these cells during growth in vitro. DNA adenine methylation has previously been shown to mediate heritable epigenetic switches in gene regulation. However, none of the undermethylated Dam sites tested showed evidence of regulation by this mechanism. This study is the first to identify undermethylated adenines and cytosines genomewide in a bacterium using second-generation sequencing technology.
The gene encoding the DNA methyltransferase M.CviRI from Chlorella virus XZ-6E was cloned and expressed in Escherichia coli. M.CviRI methylates adenine in TGCA sequences. DNA containing the M.CviRI gene was sequenced and a single open reading frame of 1137 bp was identified which could code for a polypeptide of 379 amino acids with a predicted molecular weight of 42,814. Comparison of the M.CviRI predicted amino acid sequence with another Chlorella virus and 14 bacterial adenine methyltransferases revealed extensive similarity to the other Chlorella virus enzyme.
DNA methyltransferases methylate target bases within specific nucleotide sequences. Three structures are described for bacteriophage T4 DNA-adenine methyltransferase (T4Dam) in ternary complexes with partially and fully specific DNA and a methyl-donor analog. We also report the effects of substitutions in the related Escherichia coli DNA methyltransferase (EcoDam), altering residues corresponding to those involved in specific interaction with the canonical GATC target sequence in T4Dam. We have identified two types of protein-DNA interactions: discriminatory contacts, which stabilize the transition state and accelerate methylation of the cognate site, and anti-discriminatory contacts, which do not significantly affect methylation of the cognate site but disfavor activity at noncognate sites. These structures illustrate the transition in enzyme-DNA interaction from nonspecific to specific interaction, suggesting that there is a temporal order for formation of specific contacts.
A systematic search for motifs associated with CcrM DNA methylation sites revealed four long (>100-bp) motifs (CIR sequences) present in up to 21 copies in Caulobacter crescentus. The CIR1 and CIR2 motifs exhibit a conserved inverted repeat organization, with a CcrM site in the center of one of the repeats.
Bacteriophage T2 codes for a DNA-(adenine-N6)methyltransferase (Dam), which is able to methylate both cytosine- and hydroxymethylcytosine-containing DNAs to a greater extent than the corresponding methyltransferase encoded by bacteriophage T4. We have cloned and sequenced the T2 dam gene and compared it with the T4 dam gene. In the Dam coding region, there are 22 nucleotide differences, 4 of which result in three coding differences (2 are in the same codon). Two of the amino acid alterations are located in a region of homology that is shared by T2 and T4 Dam, Escherichia coli Dam, and the modification enzyme of Streptococcus pneumoniae, all of which methylate the sequence 5' GATC 3'. The T2 dam and T4 dam promoters are not identical and appear to have slightly different efficiencies; when fused to the E. coli lacZ gene, the T4 promoter produces about twofold more beta-galactosidase activity than does the T2 promoter. In our first attempt to isolate T2 dam, a truncated gene was cloned on a 1.67-kilobase XbaI fragment. This construct produces a chimeric protein composed of the first 163 amino acids of T2 Dam followed by 83 amino acids coded by the pUC18 vector. Surprisingly, the chimera has Dam activity, but only on cytosine-containing DNA. Genetic and physical analyses place the T2 dam gene at the same respective map location as the T4 dam gene. However, relative to T4, T2 contains an insertion of 536 base pairs 5' to the dam gene. Southern blot hybridization and computer analysis failed to reveal any homology between this insert and either T4 or E. coli DNA.
HP0593 DNA-(N6-adenine)-methyltransferase (HP0593 MTase) is a member of a Type III restriction-modification system in Helicobacter pylori strain 26695. HP0593 MTase has been cloned, overexpressed and purified heterologously in Escherichia coli. The recognition sequence of the purified MTase was determined as 5′-GCAG-3′and the site of methylation was found to be adenine. The activity of HP0593 MTase was found to be optimal at pH 5.5. This is a unique property in context of natural adaptation of H. pylori in its acidic niche. Dot-blot assay using antibodies that react specifically with DNA containing m6A modification confirmed that HP0593 MTase is an adenine-specific MTase. HP0593 MTase occurred as both monomer and dimer in solution as determined by gel-filtration chromatography and chemical-crosslinking studies. The nonlinear dependence of methylation activity on enzyme concentration indicated that more than one molecule of enzyme was required for its activity. Analysis of initial velocity with AdoMet as a substrate showed that two molecules of AdoMet bind to HP0593 MTase, which is the first example in case of Type III MTases. Interestingly, metal ion cofactors such as Co2+, Mn2+, and also Mg2+ stimulated the HP0593 MTase activity. Preincubation and isotope partitioning analyses clearly indicated that HP0593 MTase-DNA complex is catalytically competent, and suggested that DNA binds to the MTase first followed by AdoMet. HP0593 MTase shows a distributive mechanism of methylation on DNA having more than one recognition site. Considering the occurrence of GCAG sequence in the potential promoter regions of physiologically important genes in H. pylori, our results provide impetus for exploring the role of this DNA MTase in the cellular processes of H. pylori.
Most of the adenine residues in GATC sequences in the Escherichia coli chromosome are methylated by the enzyme deoxyadenosine methyltransferase (Dam). However, at least 20 GATC sequences remain nonmethylated throughout the cell cycle. Here we examined how the DNA methylation patterns of GATC sequences within the regulatory regions of the pyelonephritis-associated pilus (pap) operon and the glucitol utilization (gut) operon were formed. The results obtained with an in vitro methylation protection assay showed that the addition of the leucine-responsive regulatory protein (Lrp) to pap DNA was sufficient to protect the two GATC sequences in the pap regulatory region, GATC-I and GATC-II, from methylation by Dam. This finding was consistent with previously published data showing that Lrp was essential for methylation protection of these DNA sites in vivo. Methylation protection also occurred at a GATC site (GATC-44.5) centered 44.5 bp upstream of the transcription start site of the gutABD operon. Two proteins, GutR and the catabolite gene activator protein (CAP), bound to DNA sites overlapping the GATC-44.5-containing region of the gutABD operon. GutR, an operon-specific repressor, was essential for methylation protection in vivo, and binding of GutR protected GATC-44.5 from methylation in vitro. In contrast, binding of CAP at a site overlapping GATC-44.5 did not protect this site from methylation. Mutational analyses indicated that gutABD gene regulation was not controlled by methylation of GATC-44.5, in contrast to regulation of Pap pilus expression, which is directly controlled by methylation of the pap GATC-I and GATC-II sites.
Bacteriophage T4 codes for a DNA-[N6-adenine] methyltransferase (Dam) which recognizes primarily the sequence GATC in both cytosine- and hydroxymethylcytosine-containing DNA. Hypermethylating mutants, damh, exhibit a relaxation in sequence specificity, that is, they are readily able to methylate non-canonical sites. We have determined that the damh mutation produces a single amino acid change (Pro126 to Ser126) in a region of homology (III) shared by three DNA-adenine methyltransferases; viz, T4 Dam, Escherichia coli Dam, and the DpnII modification enzyme of Streptococcus pneumoniae. We also describe another mutant, damc, which methylates GATC in cytosine-containing DNA, but not in hydroxymethylcytosine-containing DNA. This mutation also alters a single amino acid (Phe127 to Val127). These results implicate homology region III as a domain involved in DNA sequence recognition. The effect of several different amino acids at residue 126 was examined by creating a polypeptide chain terminating codon at that position and comparing the methylation capability of partially purified enzymes produced in the presence of various suppressors. No enzyme activity is detected when phenylalanine, glutamic acid, or histidine is inserted at position 126. However, insertion of alanine, cysteine, or glycine at residue 126 produces enzymatic activity similar to Damh.
The DNA of Serratia marcescens has N6-adenine methylation in GATC sequences. Among 2-aminopurine-sensitive mutants isolated from S. marcescens Sr41, one was identified which lacked GATC methylation. The mutant showed up to 30-fold increased spontaneous mutability and enhanced mutability after treatment with 2-aminopurine, ethyl methanesulfonate, or UV light. The gene (dam) coding for the adenine methyltransferase (Dam enzyme) of S. marcescens was identified on a gene bank plasmid which alleviated the 2-aminopurine sensitivity and the higher mutability of a dam-13::Tn9 mutant of Escherichia coli. Nucleotide sequencing revealed that the deduced amino acid sequence of Dam (270 amino acids; molecular mass, 31.3 kDa) has 72% identity to the Dam enzyme of E. coli. The dam gene is located between flanking genes which are similar to those found to the sides of the E. coli dam gene. The results of complementation studies indicated that like Dam of E. coli and unlike Dam of Vibrio cholerae, the Dam enzyme of S. marcescens plays an important role in mutation avoidance by allowing the mismatch repair enzymes to discriminate between the parental and newly synthesized strands during correction of replication errors.