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1.  Mechanical constraints on Hin subunit rotation imposed by the Fis-enhancer system and DNA supercoiling during site-specific recombination 
Molecular cell  2009;34(6):746-759.
Hin, a member of the serine family of site-specific recombinases, regulates gene expression by inverting a DNA segment. DNA inversion requires assembly of an invertasome complex in which a recombinational enhancer DNA segment bound by the Fis protein associates with the Hin synaptic complex at the base of a supercoiled DNA branch. Each of the four Hin subunits becomes covalently joined to the cleaved DNA ends, and DNA exchange occurs by translocation of a Hin subunit pair within the tetramer. We show here that although the Hin tetramer forms a bidirectional molecular swivel, the Fis/enhancer system determines both the direction and number of subunit rotations. The chirality of supercoiling directs rotational direction, and the short DNA loop stabilized by Fis-Hin contacts limit rotational processivity, thereby ensuring that the DNA strands re-ligate in the recombinant configuration. We identify multiple rotational conformers that are formed under different supercoiling and solution conditions.
PMCID: PMC2752211  PMID: 19560425
2.  Multiple interfaces between a serine recombinase and an enhancer control site-specific DNA inversion 
eLife  2013;2:e01211.
Serine recombinases are often tightly controlled by elaborate, topologically-defined, nucleoprotein complexes. Hin is a member of the DNA invertase subclass of serine recombinases that are regulated by a remote recombinational enhancer element containing two binding sites for the protein Fis. Two Hin dimers bound to specific recombination sites associate with the Fis-bound enhancer by DNA looping where they are remodeled into a synaptic tetramer competent for DNA chemistry and exchange. Here we show that the flexible beta-hairpin arms of the Fis dimers contact the DNA binding domain of one subunit of each Hin dimer. These contacts sandwich the Hin dimers to promote remodeling into the tetramer. A basic region on the Hin catalytic domain then contacts enhancer DNA to complete assembly of the active Hin tetramer. Our results reveal how the enhancer generates the recombination complex that specifies DNA inversion and regulates DNA exchange by the subunit rotation mechanism.
eLife digest
Many processes in biology rely on enzymes that break both the strands in a DNA molecule, then rearrange the strands, and finally join them back together in a new configuration. These recombination reactions can, for example, change the positions of genetic elements such as enhancers and promoters within the DNA molecule and, therefore, influence how a given gene is expressed as a protein. Cells need to be able to control recombination reactions because they can lead to leukemia and lymphomas if they go wrong.
The enzymes that catalyze these recombination reactions are called recombinases. One type of recombinase binds to specific sequences of DNA bases and uses an amino acid in the enzyme–usually serine or tyrosine–to break and rejoin the DNA strands. Recombination reactions require the assembly of complexes containing many proteins bound to DNA. Tyrosine recombinases form relatively simple protein-DNA complexes, and these have been studied in detail. Serine recombinases, on the other hand, form more elaborate protein-DNA complexes, and much less is known about these.
Now McLean et al. have unraveled the mechanism that a serine recombinase called Hin uses to reverse the direction of a stretch of chromosomal DNA in the bacteria Salmonella enterica. Inverting this stretch of DNA–which contains about 1000 base pairs–changes the position of a gene promoter that is responsible for the production of flagellin, which is the protein that enables the bacterium to move. This is one of the tricks that Salmonella uses to evade the immune system of its host.
Previous research has established that four Hin subunits and two copies of a protein called Fis are needed to invert this stretch of DNA: two Hin subunits bind to each of the two hix recombination sites, and the Fis proteins (which are dimers) bind to each end of an enhancer that is located between the hix sites. A protein called HU then causes the DNA to bend and form a loop, and the four Hin subunits and the two Fis dimers all come together at the enhancer to form a structure called the invertasome where the recombination reaction occurs. All four DNA strands at the crossover point are broken as a result of a near simultaneous attack by the catalytic serine amino acids in the Hin subunits. One pair of Hin subunits–and the two DNA strands attached to them–then rotate by 180 degrees around the other pair of Hin subunits. This means that the stretch of DNA between the hix sites is inverted when the DNA strands are rejoined at the end of the reaction.
Enhancers often regulate transcription and other reactions from a distance. McLean et al. reveal how an enhancer of a DNA recombination reaction works. The pairs of Hin subunits that initially bind to the DNA are not catalytically active, but when they are brought together by the enhancer and form a tetramer, they become active. Two of the Hin subunits are clamped onto the enhancer by the Fis dimers and by directly interacting with the enhancer DNA, but the other two (and the DNA strands attached to them) are free to rotate within the tetramer. In the Salmonella chromosome the enhancer is located close to one of the hix sites (∼100 base pairs away from it), so the length of the DNA between the enhancer and hix site physically limits the number of Hin subunit rotations to just one.
PMCID: PMC3798978  PMID: 24151546
Salmonella enterica; site-specific DNA recombination; serine recombinase; recombinational enhancer; synaptic complex; DNA strand exchange; E. coli
3.  Two DNA Invertases Contribute to Flagellar Phase Variation in Salmonella enterica Serovar Typhimurium Strain LT2 
Journal of Bacteriology  2006;188(3):950-957.
Salmonella enterica serovar Typhimurium strain LT2 possesses two nonallelic structural genes, fliC and fljB, for flagellin, the component protein of flagellar filaments. Flagellar phase variation occurs by alternative expression of these two genes. This is controlled by the inversion of a DNA segment, called the H segment, containing the fljB promoter. H inversion occurs by site-specific recombination between inverted repetitious sequences flanking the H segment. This recombination has been shown in vivo and in vitro to be mediated by a DNA invertase, Hin, whose gene is located within the H segment. However, a search of the complete genomic sequence revealed that LT2 possesses another DNA invertase gene that is located adjacent to another invertible DNA segment within a resident prophage, Fels-2. Here, we named this gene fin. We constructed hin and fin disruption mutants from LT2 and examined their phase variation abilities. The hin disruption mutant could still undergo flagellar phase variation, indicating that Hin is not the sole DNA invertase responsible for phase variation. Although the fin disruption mutant could undergo phase variation, fin hin double mutants could not. These results clearly indicate that both Hin and Fin contribute to flagellar phase variation in LT2. We further showed that a phase-stable serovar, serovar Abortusequi, which is known to possess a naturally occurring hin mutation, lacks Fels-2, which ensures the phase stability in this serovar.
PMCID: PMC1347348  PMID: 16428399
4.  DNA sequence adjacent to flagellar genes and evolution of flagellar-phase variation. 
Journal of Bacteriology  1983;155(1):74-81.
A variety of factors, including phase variation, are involved in the regulation of flagellin gene expression in Salmonella sp. Flagellar-phase variation refers to the alternate expression of two different flagellin genes, H1 and H2. Site-specific inversion of a DNA segment adjacent to the H2 gene is responsible for switching expression. The segment includes the H2 promoter as well as the hin gene, which is required to mediate the inversion. Sequences in this region have homology with the corresponding sequences adjacent to the H1 flagellin gene in Salmonella sp. and the hag flagellin gene in Escherichia coli. The hin gene has also been shown to be homologous to the gin gene, which is found on bacteriophage Mu. To understand gene expression and the origin of these relationships, we have compared the DNA sequence adjacent to all three flagellin genes. The sequence data suggest a mechanism for the evolution of the hin-H2 locus.
PMCID: PMC217654  PMID: 6305924
5.  Site-specific recombinase genes in three Shigella subgroups and nucleotide sequences of a pinB gene and an invertible B segment from Shigella boydii. 
Journal of Bacteriology  1991;173(13):4079-4087.
Inversional switching systems in procaryotes are composed of an invertible DNA segment and a site-specific recombinase gene adjacent to or contained in the segment. Four related but functionally distinct systems have previously been characterized in detail: the Salmonella typhimurium H segment-hin gene (H-hin), phage Mu G-gin, phage P1 C-cin, and Escherichia coli e14 P-pin. In this article we report the isolation and characterization of three new recombinase genes: pinB, pinD, and defective pinF from Shigella boydii, Shigella dysenteriae, and Shigella flexneri, respectively. The genes pinB and pinD were detected by the complementation of a hin mutation of Salmonella and were able to mediate inversion of the H, P, and C segments. pinB mediated H inversion as efficiently as the hin gene did and mediated C inversion with a frequency three orders of magnitude lower than that of the cin gene. pinD mediated inversion of H and P segments with frequencies ten times as high as those for the genes intrinsic to each segment and mediated C inversion with a frequency ten times lower than that for cin. Therefore, the pinB and pinD genes were inferred to be different from each other. The invertible B segment-pinB gene cloned from S. boydii is highly homologous to the G-gin in size, organization, and nucleotide sequence of open reading frames, but the 5' constant region outside the segment is quite different in size and predicted amino acid sequence. The B segment underwent inversion in the presence of hin, pin, or cin. The defective pinF gene is suggested to hae the same origin as P-pin on e14 by the restriction map of the fragment cloned from a Pin+ transductant that was obtained in transduction from S. flexneri to E. coli delta pin.
PMCID: PMC208056  PMID: 2061288
6.  In Vivo Assay of Protein-Protein Interactions in Hin-Mediated DNA Inversion 
Journal of Bacteriology  1998;180(22):5954-5960.
In order to form the catalytic nucleoprotein complex called the invertasome in the Hin-mediated DNA inversion reaction, interactions of the DNA-binding proteins Hin and Fis are required. Assays for these protein-protein interactions have been exploited with protein cross-linkers in vitro. In this study, an in vivo assay system that probes protein-protein interactions was developed. The formation of a DNA loop generated by protein interactions resulted in transcriptional repression of an artificially designed operon, which in turn increased the chance of survival of Escherichia coli host cells in a streptomycin-containing medium. Using this system, we were able to assay the Hin-Hin interaction that results in the pairing of the two recombination sites and protein interactions that result in the formation of the invertasome. This assay system also led us to find that an individual Hin dimer bound on a recombination site can form a stable complex with Fis bound on the recombinational enhancer; this finding has never been observed in in vitro studies. Possible pathways toward the formation of the invertasome are discussed based on the assay results for a previously reported Hin mutant.
PMCID: PMC107670  PMID: 9811654
7.  Pilin-gene phase variation of Moraxella bovis is caused by an inversion of the pilin genes. 
Journal of Bacteriology  1988;170(7):3032-3039.
Moraxella bovis Epp63 can express either of two different pilin proteins, called alpha and beta. We have previously cloned and sequenced the beta-pilin gene and now report that DNAs isolated from bacteria expressing alpha pilin have hybridization patterns consistently different from those of bacteria expressing beta pilin. The phase variation between alpha- and beta-pilin gene expression appears to be associated with an inversion of about 2 kilobases of DNA, whose endpoints occur within the coding region of the expressed pilin gene. Comparisons of the beta-pilin gene sequence with those of well-studied bacterial inversion systems revealed a stretch of 58% sequence similarity (21 of 36 base pairs) between the left inverted repeat of the Salmonella typhimurium flagellar hin control region and the amino-terminal portion of the beta-pilin gene.
PMCID: PMC211245  PMID: 2898471
8.  Novel non-specific DNA adenine methyltransferases 
Nucleic Acids Research  2011;40(5):2119-2130.
The mom gene of bacteriophage Mu encodes an enzyme that converts adenine to N6-(1-acetamido)-adenine in the phage DNA and thereby protects the viral genome from cleavage by a wide variety of restriction endonucleases. Mu-like prophage sequences present in Haemophilus influenzae Rd (FluMu), Neisseria meningitidis type A strain Z2491 (Pnme1) and H. influenzae biotype aegyptius ATCC 11116 do not possess a Mom-encoding gene. Instead, at the position occupied by mom in Mu they carry an unrelated gene that encodes a protein with homology to DNA adenine N6-methyltransferases (hin1523, nma1821, hia5, respectively). Products of the hin1523, hia5 and nma1821 genes modify adenine residues to N6-methyladenine, both in vitro and in vivo. All of these enzymes catalyzed extensive DNA methylation; most notably the Hia5 protein caused the methylation of 61% of the adenines in λ DNA. Kinetic analysis of oligonucleotide methylation suggests that all adenine residues in DNA, with the possible exception of poly(A)-tracts, constitute substrates for the Hia5 and Hin1523 enzymes. Their potential ‘sequence specificity’ could be summarized as AB or BA (where B = C, G or T). Plasmid DNA isolated from Escherichia coli cells overexpressing these novel DNA methyltransferases was resistant to cleavage by many restriction enzymes sensitive to adenine methylation.
PMCID: PMC3299994  PMID: 22102579
9.  DNA binding and bending are necessary but not sufficient for Fis-dependent activation of rrnB P1. 
Journal of Bacteriology  1993;175(6):1580-1589.
The Escherichia coli Fis protein binds to three sites in the upstream activation region of the rrnB P1 promoter and enhances transcription 5- to 10-fold in vivo. In this report, we investigate the mechanism of Fis-dependent activation of transcription. We show that stimulation of rrnB P1 transcription by Fis can occur on linear DNA templates and does not require DNA upstream of the promoter-proximal Fis site I. Mutants of Fis defective for Hin-mediated recombination have been isolated previously and have defined an N-terminal domain required for DNA inversion by Hin in addition to the C-terminal domain which is required for DNA binding. Several of these mutants were found to be defective in stimulation of rrnB P1 transcription in vivo and in vitro. Activation-defective mutants fall into three classes: those that fail to bind to the upstream activation region, those that bind but fail to bend the DNA normally, and those that bind and bend but still fail to activate transcription. We conclude that it is unlikely that Fis functions by simply bringing upstream sequences or bound factors into the proximity of RNA polymerase to activate transcription. Rather, the data are most easily interpreted in terms of transcription activation by direct interactions between Fis and RNA polymerase, requiring precise positioning of the two proteins facilitated by bending of the DNA binding site.
PMCID: PMC203950  PMID: 8449867
10.  Interferon-Inducible Protein 16: Insight into the Interaction with Tumor Suppressor p53 
IFI16 is a member of the interferon-inducible HIN-200 family of nuclear proteins. It has been implicated in transcriptional regulation by modulating protein-protein interactions with p53 tumor suppressor protein and other transcription factors. However, the mechanisms of interaction remain unknown. Here, we report the crystal structures of both HIN-A and HIN-B domains of IFI16 determined at 2.0 and 2.35 Å resolution, respectively. Each HIN domain comprises a pair of tightly packed OB-fold subdomains that appear to act as a single unit. We show that both HIN domains of IFI16 are capable of enhancing p53-DNA complex formation and transcriptional activation via distinctive means. HIN-A domain binds to the basic C terminus of p53, whereas the HIN-B domain binds to the core DNA-binding region of p53. Both interactions are compatible with the DNA-bound state of p53 and together contribute to the effect of full-length IFI16 on p53-DNA complex formation and transcriptional activation.
PMCID: PMC3760383  PMID: 21397192
11.  Sequence analysis of the inversion region containing the pilin genes of Moraxella bovis. 
Journal of Bacteriology  1990;172(1):310-316.
Moraxella bovis EPP63 is able to produce two antigenically distinct pili called Q and I pili (previously called beta and alpha pili). Hybridization studies have shown that the transition between the types is due to inversion of a 2.1-kilobase segment of chromosomal DNA. We present the sequence of a 4.1-kilobase region of cloned DNA spanning the entire inversion region in orientation 1 (Q pilin expressed). Comparison of this sequence with the sequence of the polymerase chain reaction-amplified genomic DNA from orientation 2 (I pilin expressed) allows the site-specific region of recombination to be localized to a 26-base-pair region in which sequence similarity to the left inverted repeat of the Salmonella typhimurium hin system was previously noted. In addition, 50% sequence similarity was seen in a 60-base-pair segment of our sequence to the recombinational enhancer of bacteriophage P1, an inversion system related to the hin system of S. typhimurium. Finally, two open reading frames representing potential genes were identified.
PMCID: PMC208434  PMID: 2403542
12.  The Nul subunit of bacteriophage lambda terminase binds to specific sites in cos DNA. 
Journal of Virology  1988;62(2):387-392.
The maturation and packaging of bacteriophage lambda DNA are under the control of the multifunctional viral terminase enzyme, which is composed of the protein products of Nu1 and A, the two most leftward genes of the phage chromosome. Terminase binds selectively to the cohesive end site (cos) of multimeric replicating lambda DNA and introduces staggered nicks to regenerate the 12-base single-stranded cohesive ends of the mature phage genome. The purified gpNu1 subunit of terminase forms specific complexes with cos lambda DNA. DNase I footprinting experiments showed that gpNu1 bound to three distinct regions near the extreme left end of the lambda chromosome. These regions coincided with two 16-base-pair sequences (CTGTCGTTTCCTTTCT) that were in inverted orientation, as well as a truncated version of this sequence. Bear et al. (J. Virol. 52:966-972,1984) isolated a mutant phage which contained a CG to TA transition at the 10th position of the rightmost 16-base-pair sequence, and this phage (termed lambda cos 154) exhibits a defect in DNA maturation when it replicates in Escherichia coli which is deficient in integration host factor. Footprinting experiments with cos 154 DNA showed that gpNu1 could not bind to the site which contained the mutation but could protect the other two sites. Since the DNA-packaging specificity of terminase resides in the gpNu1 subunit, these studies suggest that terminase uses these three sites as recognition sequences for specific binding to cos lambda.
PMCID: PMC250547  PMID: 2826803
13.  Simian Virus 40 DNA Segment of the Adenovirus 7-Simian Virus 40 Hybrid, E46+, and Its Transcription During Permissive Infection of Monkey Kidney Cells 
Journal of Virology  1975;15(5):1214-1221.
Nucleic acid hybridization methodology has been used to investigate the span of the simian virus 40 (SV40) DNA segment in the adenovirus 7-SV40 hybrid, E46+, and the extent of its transcription in lytically infected monkey kidney cells. The SV40 segment of E46+ comprises approximately 62% of the SV40 genome; it originates in the proximal region of Hin-G (the G fragment derived by cleavage of intact SV40 DNA with Haemophilus influenzae restriction endonuclease), extends sequentially through approximately 80% of this fragment, all of fragments Hin-B, -I, -H, and -A, and terminates approximately 70% of the distance through Hin-C. During E46+ lytic infection of permissive cells, the vast majority of stable cytoplasmic SV40-specific RNA is transcribed from the minus (E) strand of the fragments Hin-A, -H, -I, and -B, comprising the early template region. Transcripts of the minus strand of the Hin-G and -C fragments are detected in much lower concentrations, especially in the total lytic cellular RNA, whereas RNA complementary to the plus (L) strand is not detected. The transcriptional pattern of the SV40 segment within E46+ is thus very similar to that in a number of transformed cell lines and in some respects to the transcriptional pattern in a series of nondefective adenovirus 2-SV40 hybrid viruses. These results suggest a common transcriptional mechanism for integrated SV40 DNA.
PMCID: PMC354576  PMID: 16789154
14.  Identification of a positive regulator of the Mu middle operon. 
Journal of Bacteriology  1990;172(12):6641-6650.
Transcription of bacteriophage Mu occurs in a regulatory cascade consisting of three phases: early, middle, and late. The 1.2-kb middle transcript is initiated at Pm and encodes the C protein, the activator of late transcription. A plasmid containing a Pm-lacZ operon fusion was constructed. beta-Galactosidase expression from the plasmid increased 23-fold after Mu prophage induction. Infection of plasmid-containing cells with lambda phages carrying different segment of the Mu early region localized the Pm-lacZ transactivation function to the region containing open reading frames E16 and E17. Deletion and linker insertion analyses of plasmids containing this region identified E17 as the transactivator; therefore we call this gene mor, for middle operon regulator. Expression of mor under the control of a T7 promoter and T7 RNA polymerase resulted in the production of a single polypeptide of 17 kDa as detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Insertion of a linker into mor substantially reduced the ability of Mu to form plaques. When growth of the mor mutant was assayed in liquid, lysis was delayed by about 50 min and the burst size was approximately one-fifth that of wild-type Mu. The mor requirement for plaque formation and normal growth kinetics was abolished when C protein was provided in trans, indicating that the primary function of Mor is to provide sufficient C for late gene expression. Comparison of the predicted amino acid sequence of Mor with other proteins revealed that Mor and C share substantial amino acid sequence homology.
PMCID: PMC210775  PMID: 2147678
15.  Structure of HinP1I endonuclease reveals a striking similarity to the monomeric restriction enzyme MspI 
Nucleic Acids Research  2005;33(6):1892-1901.
HinP1I, a type II restriction endonuclease, recognizes and cleaves a palindromic tetranucleotide sequence (G↓CGC) in double-stranded DNA, producing 2 nt 5′ overhanging ends. Here, we report the structure of HinP1I crystallized as one protein monomer in the crystallographic asymmetric unit. HinP1I displays an elongated shape, with a conserved catalytic core domain containing an active-site motif of SDX18QXK and a putative DNA-binding domain. Without significant sequence homology, HinP1I displays striking structural similarity to MspI, an endonuclease that cleaves a similar palindromic DNA sequence (C↓CGG) and binds to that sequence crystallographically as a monomer. Almost all the structural elements of MspI can be matched in HinP1I, including both the DNA recognition and catalytic elements. Examining the protein–protein interactions in the crystal lattice, HinP1I could be dimerized through two helices located on the opposite side of the protein to the active site, generating a molecule with two active sites and two DNA-binding surfaces opposite one another on the outer surfaces of the dimer. A possible functional link between this unusual dimerization mode and the tetrameric restriction enzymes is discussed.
PMCID: PMC1074309  PMID: 15805123
16.  Intra- and intersubunit interactions controlling assembly of active synaptic complexes during Hin-catalyzed DNA recombination 
Journal of molecular biology  2011;411(4):744-764.
Serine recombinases, which generate double strand breaks in DNA, must be carefully regulated to ensure that chemically-active DNA complexes are assembled correctly. In the Hin-catalyzed site-specific DNA inversion reaction, two inversely-oriented recombination sites on the same DNA molecule assemble into a synaptic complex that uniquely generates inversion products. The Fis-bound recombinational enhancer, together with topological constraints directed by DNA supercoiling, function to regulate Hin synaptic complex formation and activity. We have isolated a collection of gain-of-function mutants in 22 positions within the catalytic and oligomerization domains of Hin using two genetic screens and by site-directed mutagenesis. One genetic screen measured recombination in the absence of Fis and the other assessed SOS induction as a readout of increased DNA cleavage. These mutations, together with molecular modeling, identify important sites of dynamic intrasubunit and intersubunit interactions that regulate assembly of the active tetrameric recombination complex. Of particular interest are interactions between the oligomerization helix-E and the catalytic domain of the same subunit that function to hold the dimer in an inactive state in the absence of the Fis-enhancer system. Among these is a relay involving a triad of phenylalanines that are proposed to switch positions during the transition from dimers to the catalytically-active tetramer. Novel Hin mutants also are described that generate synaptic complexes that are blocked at step(s) prior to DNA cleavage.
PMCID: PMC3156330  PMID: 21708172
site-specific DNA recombination; serine recombinase; Fis; hyperactive mutants; DNA invertase
17.  Identification and characterization of the in vitro synthesized gene products of bacteriophage M13. 
Journal of Virology  1975;15(3):570-584.
Bacteriophage M13 replicative form (RF) DNA was used to direct coupled transcription and translation in cell-free extracts prepared from Escherichia coli. By using RF DNA, isolated from cells infected with appropriate amber mutants of this phage, it has been possible to identify the products of genes I through IV. By using the same methods no gene-product relationship could be demonstrated for genes VI and VII. Coupled in vitro protein synthesis studies on RF-III DNA, a linear double-stranded DNA molecule, obtained after cleavage of either RF-I or RF-II DNA with the restriction endonuclease R.Hin11 from Haemophilus influenzae, indicated that the cleavage site for this enzyme is located in gene II. The in vitro products of both gene III and gene VIII are about 30 and six amino acids longer, respectively, than their native counterparts present within the virion. These results suggest that the latter proteins arise in vivo by cleavage of precursor molecules. Coupled transcription and translation studies on a DNA fragment which only contained the genetic information coding for gene IV protein, obtained after cleavage of RF DNA with the restriction endonuclease R.Hap11 from Haemophilus aphirophilus, indicated that a large number of the in vitro synthesized polypeptides are the result of premature chain termination.
PMCID: PMC354494  PMID: 1089807
18.  Identification and Characterization of the fis Operon in Enteric Bacteria 
Journal of Bacteriology  1998;180(22):5932-5946.
The small DNA binding protein Fis is involved in several different biological processes in Escherichia coli. It has been shown to stimulate DNA inversion reactions mediated by the Hin family of recombinases, stimulate integration and excision of phage λ genome, regulate the transcription of several different genes including those of stable RNA operons, and regulate the initiation of DNA replication at oriC. fis has also been isolated from Salmonella typhimurium, and the genomic sequence of Haemophilus influenzae reveals its presence in this bacteria. This work extends the characterization of fis to other organisms. Very similar fis operon structures were identified in the enteric bacteria Klebsiella pneumoniae, Serratia marcescens, Erwinia carotovora, and Proteus vulgaris but not in several nonenteric bacteria. We found that the deduced amino acid sequences for Fis are 100% identical in K. pneumoniae, S. marcescens, E. coli, and S. typhimurium and 96 to 98% identical when E. carotovora and P. vulgaris Fis are considered. The deduced amino acid sequence for H. influenzae Fis is about 80% identical and 90% similar to Fis in enteric bacteria. However, in spite of these similarities, the E. carotovora, P. vulgaris, and H. influenzae Fis proteins are not functionally identical. An open reading frame (ORF1) preceding fis in E. coli is also found in all these bacteria, and their deduced amino acid sequences are also very similar. The sequence preceding ORF1 in the enteric bacteria showed a very strong similarity to the E. coli fis P region from −53 to +27 and the region around −116 containing an ihf binding site. Both β-galactosidase assays and primer extension assays showed that these regions function as promoters in vivo and are subject to growth phase-dependent regulation. However, their promoter strengths vary, as do their responses to Fis autoregulation and integration host factor stimulation.
PMCID: PMC107668  PMID: 9811652
19.  Location, degree, and direction of DNA bending associated with the Hin recombinational enhancer sequence and Fis-enhancer complex. 
Journal of Bacteriology  1997;179(15):4747-4753.
The Fis protein of Escherichia coli and Salmonella typhimurium stimulates several site-specific DNA recombination reactions, as well as transcription of a number of genes. Fis binds to a 15-bp core recognition sequence and induces DNA bending. Mutations in Fis which alter its ability to bend DNA have been shown to reduce the stimulatory activity of Fis in both site-specific recombination and transcription systems. To examine the role of DNA bending in the activity of the Fis-recombinational enhancer complex in Hin-mediated site-specific DNA inversion, we have determined the locations, degrees, and directions of DNA bends associated with the recombinational enhancer and the Fis-enhancer complex. Circular-permutation assays demonstrated that a sequence-directed DNA bend is associated with the Fis binding sites in the proximal and distal domains of the recombinational enhancer. Binding of Fis to its core recognition sequence significantly increases the degree of DNA bending associated with the proximal and distal domains. The degree of DNA bending induced by Fis binding depended on the DNA sequences flanking the core Fis binding site, with angles ranging from 42 to 69 degrees. Phasing analyses indicate that both the sequence-directed and the Fis-induced DNA bends associated with the proximal and distal domains face the minor groove of the DNA helix at the center of the Fis binding site. The positions and directions of DNA bends associated with the Fis-recombinational complex support a direct role for Fis-induced DNA bending in assembly of the active invertasome.
PMCID: PMC179320  PMID: 9244261
20.  Specific detection of Salmonella spp. by multiplex polymerase chain reaction. 
Three sets of oligonucleotide primers were used in the polymerase chain reaction (PCR) assay to detect Salmonella species. phoP primers specific to the phoP/phoQ loci of coliform pathogenic bacteria such as Salmonella, Shigella, Escherichia coli, and Citrobacter species served as presumptive indicators of enteric bacteria. In addition to the phoP primers, the Hin and the H-1i primers, which targeted a 236-bp region of hin/H2 and a 173-bp region of the H-1i flagellin gene, respectively, were used. Both Hin and H-1i primers are specific to motile Salmonella species and are not present in Shigella, E. coli, or Citrobacter species. Thus, by multiplex PCR amplification, Salmonella species including Salmonella typhi, Salmonella typhimurium, Salmonella paratyphi A, and Salmonella enteritidis can be specifically detected. Optimal reaction conditions have been described to demonstrate this specific, sensitive detection of Salmonella species. By using agarose gel electrophoresis for detection of the PCR-amplified products, the sensitivity of detection was 10(2) CFU after 25 cycles of PCR and 1 (10(0)) CFU after a 50-cycle double PCR. The efficacy of these primers was demonstrated on environmental isolates which had previously been confirmed as Salmonella species by the use of conventional cultural techniques. In addition, positive amplifications resulted from Salmonella species in environmental samples including soil and water.
PMCID: PMC182106  PMID: 8517741
21.  Sequence and deletion analysis of the recombination enhancement gene (ref) of bacteriophage P1: evidence for promoter-operator and attenuator-antiterminator control. 
Journal of Bacteriology  1988;170(10):4881-4889.
The ref gene of bacteriophage P1 stimulates recombination between two defective lacZ genes in the Escherichia coli chromosome (lac x lac recombination) and certain other RecA-dependent recombination processes. We determined the DNA sequence of the 5' portion of the ref gene and tested various regions for functionality by inserting DNA fragments lacking increasing amounts of 5' sequence into plasmid and lambda phage vectors and measuring the ability of the constructs to stimulate lac x lac recombination. The region found essential for Ref activity in the absence of external heterologous promoters encodes two presumptive promoters, pref-1 and pref-2, whose -10 regions fall in a nearly perfect 13-base-pair (bp) tandem repeat. The -10 region of the putative pref-1 is part of a phage P1 c1 repressor recognition sequence. The first two ATG codons in the ref reading frame are, respectively, 90 and 216 bp downstream from the putative promoter-operator region. Deletion analysis indicated that translation can initiate at either ATG (although neither is associated with a canonical ribosome-binding sequence) and that the 42 amino acids in between are not indispensable for Ref stimulation of lac x lac recombination. However, the shorter reading frame appears to encode a less active polypeptide. The 91-bp leader region between the putative promoter-operator and the first ATG contains 30 codons in frame with the ref structural sequence, but its frame can be shifted without affecting Ref activity. The leader region ends with an apparent rho-independent termination sequence (attenuator). Deletion of 18 bp of early leader sequence drastically reduced Ref activity, even when ref was driven by a heterologous promoter (plac). An 8-bp internal deletion in the putative attenuator sequence relieved this requirement for the early leader sequence. This latter observation, along with nucleotide complementarity between portions of the early leader and attenuator sequences, are consistent with preemption of attenuation by the early leader.
PMCID: PMC211533  PMID: 3170487
22.  The mammalian PYHIN gene family: Phylogeny, evolution and expression 
Proteins of the mammalian PYHIN (IFI200/HIN-200) family are involved in defence against infection through recognition of foreign DNA. The family member absent in melanoma 2 (AIM2) binds cytosolic DNA via its HIN domain and initiates inflammasome formation via its pyrin domain. AIM2 lies within a cluster of related genes, many of which are uncharacterised in mouse. To better understand the evolution, orthology and function of these genes, we have documented the range of PYHIN genes present in representative mammalian species, and undertaken phylogenetic and expression analyses.
No PYHIN genes are evident in non-mammals or monotremes, with a single member found in each of three marsupial genomes. Placental mammals show variable family expansions, from one gene in cow to four in human and 14 in mouse. A single HIN domain appears to have evolved in the common ancestor of marsupials and placental mammals, and duplicated to give rise to three distinct forms (HIN-A, -B and -C) in the placental mammal ancestor. Phylogenetic analyses showed that AIM2 HIN-C and pyrin domains clearly diverge from the rest of the family, and it is the only PYHIN protein with orthology across many species. Interestingly, although AIM2 is important in defence against some bacteria and viruses in mice, AIM2 is a pseudogene in cow, sheep, llama, dolphin, dog and elephant. The other 13 mouse genes have arisen by duplication and rearrangement within the lineage, which has allowed some diversification in expression patterns.
The role of AIM2 in forming the inflammasome is relatively well understood, but molecular interactions of other PYHIN proteins involved in defence against foreign DNA remain to be defined. The non-AIM2 PYHIN protein sequences are very distinct from AIM2, suggesting they vary in effector mechanism in response to foreign DNA, and may bind different DNA structures. The PYHIN family has highly varied gene composition between mammalian species due to lineage-specific duplication and loss, which probably indicates different adaptations for fighting infectious disease. Non-genomic DNA can indicate infection, or a mutagenic threat. We hypothesise that defence of the genome against endogenous retroelements has been an additional evolutionary driver for PYHIN proteins.
PMCID: PMC3458909  PMID: 22871040
PYHIN; HIN-200; cytosolic DNA; ALR; IFI16; AIM2
23.  DNA nicking by HinP1I endonuclease: bending, base flipping and minor groove expansion 
Nucleic Acids Research  2006;34(3):939-948.
HinP1I recognizes and cleaves the palindromic tetranucleotide sequence G↓CGC in DNA. We report three structures of HinP1I–DNA complexes: in the presence of Ca2+ (pre-reactive complex), in the absence of metal ion (binary complex) and in the presence of Mg2+ (post-reactive complex). HinP1I forms a back-to-back dimer with two active sites and two DNA duplexes bound on the outer surfaces of the dimer facing away from each other. The 10 bp DNA duplexes undergo protein-induced distortions exhibiting features of A-, B- and Z-conformations: bending on one side (by intercalation of a phenylalanine side chain into the major groove), base flipping on the other side of the recognition site (by expanding the step rise distance of the local base pair to Z-form) and a local A-form conformation between the two central C:G base pairs of the recognition site (by binding of the N-terminal helix in the minor groove). In the pre- and post-reactive complexes, two metals (Ca2+ or Mg2+) are found in the active site. The enzyme appears to cleave DNA sequentially, hydrolyzing first one DNA strand, as seen in the post-reactive complex in the crystalline state, and then the other, as supported by the observation that, in solution, a nicked DNA intermediate accumulates before linearization.
PMCID: PMC1363774  PMID: 16473850
24.  E. coli Histidine Triad Nucleotide Binding Protein 1 (ecHinT) Is a Catalytic Regulator of D-Alanine Dehydrogenase (DadA) Activity In Vivo 
PLoS ONE  2011;6(7):e20897.
Histidine triad nucleotide binding proteins (Hints) are highly conserved members of the histidine triad (HIT) protein superfamily. Hints comprise the most ancient branch of this superfamily and can be found in Archaea, Bacteria, and Eukaryota. Prokaryotic genomes, including a wide diversity of both Gram-negative and Gram-positive bacteria, typically have one Hint gene encoded by hinT (ycfF in E. coli). Despite their ubiquity, the foundational reason for the wide-spread conservation of Hints across all kingdoms of life remains a mystery. In this study, we used a combination of phenotypic screening and complementation analyses with wild-type and hinT knock-out Escherichia coli strains to show that catalytically active ecHinT is required in E. coli for growth on D-alanine as a sole carbon source. We demonstrate that the expression of catalytically active ecHinT is essential for the activity of the enzyme D-alanine dehydrogenase (DadA) (equivalent to D-amino acid oxidase in eukaryotes), a necessary component of the D-alanine catabolic pathway. Site-directed mutagenesis studies revealed that catalytically active C-terminal mutants of ecHinT are unable to activate DadA activity. In addition, we have designed and synthesized the first cell-permeable inhibitor of ecHinT and demonstrated that the wild-type E. coli treated with the inhibitor exhibited the same phenotype observed for the hinT knock-out strain. These results reveal that the catalytic activity and structure of ecHinT is essential for DadA function and therefore alanine metabolism in E. coli. Moreover, they provide the first biochemical evidence linking the catalytic activity of this ubiquitous protein to the biological function of Hints in Escherichia coli.
PMCID: PMC3130732  PMID: 21754980
25.  Structural bases of a long-stretched deletion: completing the lambda plac5 DNA primary structure. 
Nucleic Acids Research  1988;16(21):10199-10212.
In studying molecular mechanisms of specialised transduction, the lacI (E. coli)-Ea47 (lambda) DNA junction in transducing bacteriophage lambda plac 5 has been structurally elucidated, thus yielding the complete sequence of lambda plac 5 DNA including the lac5 substitution, a well-known segment of lambdoid vectors. The lambda plac5 DNA is shown to consist of 19368 bp (lambda left arm) + 3924 bp (lac5 substitution) + 25353 bp (lambda right arm), totally amounting to 48645 bp. The presence of the phage rho bL promoter near to the right end of the lac5 insert is shown. The lacI gene distal end in lambda plac5 proved to be much longer than it was postulated earlier, coding for 224 C-terminal amino acid residues of lac repressor. Both the recombination studied in this paper and the earlier studied abnormal prophage excision (2, 3) occur near to Chi-like structures (chi*lacI and chi*lom, respectively). On the basis of the data obtained, a key role of the E. coli RecBCD system and Chi-like sequences in the formation of deletions in bacterial cells is suggested.
PMCID: PMC338846  PMID: 2973573

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