Characterization of a dam mutant of Haemophilus influenzae Rd

The gene encoding Dam methyltransferase ofHaemophilus influenzaewas mutagenized by the insertion of a chloramphenicol-resistance cassette into the middle of the Dam coding sequence. This mutant construct was introduced into theH. influenzaechromosome by transformation and selection for CamRtransformants. The authors have shown that several phenotypic properties, resistance to antibiotics, dyes and detergent as well as efficiency of transformation, depend on the Dam methylation state of the DNA. Although the major role of the methyl-directed mismatch repair (MMR) system is to repair postreplicative errors, it seems that inH. influenzaeits effect is more apparent in repairing DNA damage caused by oxidative compounds. In thedammutant treated with hydrogen peroxide, MMR is not targeted to newly replicated DNA strands and therefore mismatches are converted into single- and double-strand DNA breaks. This is shown by the increased peroxide sensitivity of thedammutant and the finding that the sensitivity can be suppressed by amutHmutation inactivating MMR. In thedammutant treated with nitrofurazone the resulting damage is not converted into DNA breaks but the high sensitivity is also suppressed by amutHmutation.

Characterization of the DNA adenine 5'-GATC-3' methylase HpyIIIM from Helicobacter pylori

The effect of inactivation of the 5'-GATC-3' methylase HpyIIIM in Helicobacter pylori (H. pylori) on mismatch repair, adherence, and in vitro fitness was examined. Chromosomal DNA from 90 H. pylori strains was isolated, and restriction enzyme digestion indicated all strains examined possess HpyIIIM. Wild-type H. pylori and a strain with an inactive HpyIIIM were found to have rifampicin mutation frequencies of 2.93 x 10(-7) and 1.05 x 10(-7) (p > 0.05), respectively, indicating that HpyIIIM does not appear to be important in mismatch repair. Adherence of H. pylori in an in vitro model cell system was also unaffected by inactivation of HpyIIIM. Inactivation of HpyIIIM did not result in a decrease in fitness, as determined by liquid in vitro competition experiments.

Functional characterization of Escherichia coli DNA adenine methyltransferase, a novel target for antibiotics

We have characterized Escherichia coli DNA adenine methyltransferase, a critical regulator of bacterial virulence. Steady-state kinetics, product inhibition, and isotope exchange studies are consistent with a kinetic mechanism in which the cofactor S-adenosylmethionine binds first, followed by sequence-specific DNA binding and catalysis. The enzyme has a fast methyl transfer step followed by slower product release steps, and we directly demonstrate the competence of the enzyme cofactor complex. Methylation of adjacent GATC sites is distributive with DNA derived from a genetic element that controls the transcription of the adjacent genes. This indicates that the first methylation event is followed by enzyme release. The affinity of the enzyme for both DNA and S-adenosylmethionine was determined. Our studies provide a basis for further structural and functional analysis of this important enzyme and for the identification of inhibitors for potential therapeutic applications.

Host response to a dam mutant of Salmonella enterica serovar enteritidis with a temperature-sensitive phenotype

The temperature-sensitive dam mutant strain of Salmonella enterica serovar Enteritidis SD1 is highly attenuated and induces innate and protective immunity in mice. SD1 activates NF-kappaB and induces gamma interferon secretion. Early interaction of the SD1 mutant with intestinal epithelial cells was associated with ruffling of enterocytes. Invading bacteria were found inside Peyer's patches after inoculation.

MutS inhibits RecA-mediated strand exchange with platinated DNA substrates

Human cell lines and Escherichia coli dam mutants are sensitive to the cytotoxic action of the anticancer agent, cisplatin. Introduction of mutations disabling DNA mismatch repair into these cell lines renders them resistant to the action of this drug. We used RecA-mediated strand exchange between homologous phiX174 molecules, one that was platinated and the other that was unmodified, to show that strand transfer is decreased in a dose-dependent manner. Transfer was severely decreased at 10 adducts per molecule (5,386 bp) and abolished with 24 adducts. At low levels of adduction, addition of MutS to the reaction further decreases the rate and yield in a dose-dependent manner. MutL addition was without effect even in the presence of MutS. The results suggest that although mismatch repair is beneficial for mutation avoidance, its antirecombination activity on inappropriate substrates can be lethal to the cell.

Complete genomic nucleotide sequence of the temperate bacteriophage Aa Phi 23 of Actinobacillus actinomycetemcomitans

The entire double-stranded DNA genome of the Actinobacillus actinomycetemcomitans bacteriophage Aa Phi 23 was sequenced. Linear DNA contained in the phage particles is circularly permuted and terminally redundant. Therefore, the physical map of the phage genome is circular. Its size is 43,033 bp with an overall molar G+C content of 42.5 mol%. Sixty-six potential open reading frames (ORFs) were identified, including an ORF resulting from a translational frameshift. A putative function could be assigned to 23 of them. Twenty-three other ORFs share homologies only with hypothetical proteins present in several bacteria or bacteriophages, and 20 ORFs seem to be specific for phage Aa Phi 23. The organization of the phage genome and several genetic functions share extensive similarities to that of the lambdoid phages. However, Aa Phi 23 encodes a DNA adenine methylase, and the DNA packaging strategy is more closely related to the P22 system. The attachment sites of Aa Phi 23 (attP) and several A. actinomycetemcomitans hosts (attB) are 49 bp long.

Stopped-flow and mutational analysis of base flipping by the Escherichia coli Dam DNA-(adenine-N6)-methyltransferase

By stopped-flow kinetics using 2-aminopurine as a probe to detect base flipping, we show here that base flipping by the Escherichia coli Dam DNA-(adenine-N6)-methyltransferase (MTase) is a biphasic process: target base flipping is very fast (k(flip)>240 s(-1)), but binding of the flipped base into the active site pocket of the enzyme is slow (k=0.1-2 s(-1)). Whereas base flipping occurs in the absence of S-adenosyl-l-methionine (AdoMet), binding of the target base in the active site pocket requires AdoMet. Our data suggest that the tyrosine residue in the DPPY motif conserved in the active site of DNA-(adenine-N6)-MTases stacks to the flipped target base. Substitution of the aspartic acid residue of the DPPY motif by alanine abolished base flipping, suggesting that this residue contacts and stabilizes the flipped base. The exchange of Ser188 located in a loop next to the active center by alanine led to a seven- to eightfold reduction of k(flip), which was also reduced with substrates having altered GATC recognition sites and in the absence of AdoMet. These findings provide evidence that the enzyme actively initiates base flipping by stabilizing the transition state of the process. Reduced rates of base flipping in substrates containing the target base in a non-canonical sequence demonstrate that DNA recognition by the MTase starts before base flipping. DNA recognition, cofactor binding and base flipping are correlated and efficient base flipping takes place only if the enzyme has bound to a cognate target site and AdoMet is available.

Dam inactivation in Neisseria meningitidis: prevalence among diverse hyperinvasive lineages

BACKGROUND

DNA adenine methyltransferase (Dam) activity is absent in many, but not all, disease isolates of Neisseria meningitidis, as a consequence of the insertion of a restriction endonuclease-encoding gene, the 'dam replacing gene' (drg) at the dam locus. Here, we report the results of a survey to assess the prevalence of drg in a globally representative panel of disease-associated meningococci.

RESULTS

Of the known meningococcal hyper-invasive lineages investigated, drg was absent in all representatives of the ST-8 and ST-11 clonal complexes tested, but uniformly present in the representatives of the other hyper-invasive lineages present in the isolate collection (the ST-1, ST-4, ST-5, ST-32 and ST-41/44 clonal complexes). The patterns of sequence diversity observed in drg were consistent with acquisition of this gene from a source organism with a different G+C content, at some time prior to the emergence of present-day meningococcal clonal complexes, followed by spread through the meningococcal population by horizontal genetic exchange. During this spread a number of alleles have arisen by mutation and intragenic recombination.

CONCLUSION

These findings are consistent with the idea that possession of the drg gene may contribute to the divergence observed among meningococcal clonal complexes, but does not have a direct mechanistic involvement in virulence.

Stress-dependent regulation of the gene encoding gamma-glutamylcysteine synthetase from the fission yeast

Glutathione (GSH), an important antioxidant involved in stress response, is synthesized in two sequential reactions. Gamma-glutamylcysteine synthetase (GCS) catalyzes the first step in GSH biosynthesis, which is usually known to be rate-limiting. In this work, regulatory patterns of the GCS gene from the fission yeast Schizosaccharomyces pombe have been investigated. The 607 bp upstream region from the translational initiation point was amplified by the two synthetic primers. The amplified DNA was ligated into the BamHI/HindIII site of the shuttle vector YEp367R to generate the fusion plasmid pUGCS101. The GCS-lacZ fusion gene construct was confirmed by restriction mapping and nucleotide sequencing. The GCS-lacZ fusion gene was used to study effects of various agents on the transcription of the GCS gene. The synthesis of beta-galactosidase from the fusion plasmid pUGCS101 was enhanced by metals, oxidative and nitrosative stresses, and glutathione-depleting agents. The GCS mRNA level in the wildtype S. pombe cells was significantly elevated by the treatment with sodium nitroprusside or menadione, which was detected by RT-PCR. It was also induced by low concentrations of glucose and sucrose. These results suggest that the expression of S. pombe GCS gene is regulated by various stresses and carbon sources.

The intricate workings of a bacterial epigenetic switch

Bacteria have developed epigenetic mechanisms to control the reversible Off-to-On switching of cell surface structures such as pyelonephritis-associated pili (PAP). The pap pili switch is primarily controlled by the global regulator leucine-responsive regulatory protein (Lrp), the local regulator PapI, and DNA adenine methylase (Dam). There are two sets of binding sites for Lrp in the pap regulatory region: promoter proximal sites 1,2,3 and promoter distal sites 4,5,6. The pilin promoter proximal (GATCprox) and distal (GATCdist) targets for Dam are located within Lrp binding sites 2 and 5, respectively. In the Off state, Lrp binds cooperatively to sites 1,2,3 overlapping the papBA pilin promoter, shutting off pilin transcription, and blocking methylation of GATCprox. Binding of Lrp at sites 1,2,3, together with methylation of GATCdist, reduces the affinity of Lrp for sites 4,5,6, preventing simultaneous binding of Lrp at sites 4,5,6 upstream. Switching to the phase. On state requires the environmentally regulated PapI co-regulator, which increases the affinity of Lrp for sites 5 and 2. PapI binds specifically to Lrp-pap DNA complexes via binding with Lrp as well as contact with DNA sequences within pap sites 5 and 2. Directionality in switching from Off to On appears to be due to methylation of GATCprox, which prevents formation of the PapI-Lrp-pap site 2 ternary complex. A switch model is presented in which DNA replication is proposed to play a critical role by generating a hemimethylated GATCdist site and displacing Lrp from sites 1,2,3. This facilitates methylation of GATCprox and binding of PapI-Lrp to sites 4,5,6, with subsequent activation of pap transcription. The first gene product of the pap operon, PapB, positively regulates papI transcription, resulting in a positive feedback loop that helps maintain the On state. The pap switch is environmentally regulated by a number of factors including the CpxAR two-component regulatory system, the Histone-like nucleoid structuring protein H-NS, and cAMP-Catabolite Gene Activator Protein (CAP), which all involve binding of regulatory binding proteins to pap DNA sequences with subsequent alteration of PapI and Lrp binding. The Pap switch mechanism, with interesting variations, is conserved among a number of enteric bacteria, controlling expression of many unrelated pili-adhesin complexes.

Re-replication from non-sequesterable origins generates three-nucleoid cells which divide asymmetrically

In rapidly growing Escherichia coli cells replication cycles overlap and initiation occurs at multiple replication origins (oriCs). All origins within a cell are initiated essentially in synchrony and only once per cell cycle. Immediate re-initiation of new origins is avoided by sequestration, a mechanism dependent on the SeqA protein and Dam methylation of GATC sites in oriC. Here, GATC sites in oriC were changed to GTTC. This reduced the sequestration to essentially the level found in SeqA-less cells. The mutant origins underwent re-initiation, showing that the GATC sites in oriC are required for sequestration. Each re-initiation eventually gave rise to a cell containing an extra nucleoid. The three-nucleoid cells displayed one asymmetrically placed FtsZ-ring and divided into a two-nucleoid cell and a one-nucleoid cell. The three nucleoid-cells thus divided into three daughters by two consecutive divisions. The results show that extra rounds of replication cause extra daughter cells to be formed prematurely. The fairly normal mutant growth rate and size distribution show, however, that premature rounds of replication, chromosome segregation, and cell division are flexibly accommodated by the existing cell cycle controls.

A DNA adenine methylase mutant of Shigella flexneri shows no significant attenuation of virulence

Mutants of Salmonella defective in DNA adenine methylase (dam) have been reported to be attenuated for virulence and to provide protective immunity when used as vaccine strains. To determine whether these observations could be extended to Shigella, a dam mutant of Shigella flexneri 2a was characterized and examined for the role of dam in pathogenesis. The Shigella dam mutant showed some unique characteristics; however, it retained virulence in vivo as well as in vitro. The mutant invaded cultured L2 monolayer cells as efficiently as the wild-type parent, but its intracellular growth was suppressed up to 7 h post-invasion. Furthermore, the invading dam mutant formed smaller plaques in cell monolayers compared to the parent strain. However, the mutant produced keratoconjunctivitis in the Sereny test in guinea pigs only slightly more slowly than the wild-type. While the effect of the dam mutation on virulence was modest, the rate of spontaneous mutation in the dam mutant was 1000-fold greater compared with the wild-type. The virulence and high mutability displayed by the dam mutant of Sh. flexneri suggest that a general anti-bacterial pathogen vaccine strategy based on mutations in dam needs to be re-evaluated.

The recognition and modification sites for the bacterial type I restriction systems KpnAI, StySEAI, StySENI and StySGI

Using an in vivo plasmid transformation method, we have determined the DNA sequences recognized by the KpnAI, StySEAI, StySENI and StySGI R-M systems from Klebsiella oxytoca strain M5a1, Salmonella eastbourne, Salmonella enteritidis and Salmonella gelsenkirchen, respectively. These type I restriction-modification systems were originally identified using traditional phage assay, and described here is the plasmid transformation test and computer program used to determine their DNA recognition sequences. For this test, we constructed two sets of plasmids, pL and pE, that contain phage lambda and Escherichia coli K-12 chromosomal DNA fragments, respectively. Further, using the methylation sensitivities of various known type II restriction enzymes, we identified the target adenines for methylation (listed in bold italics below as A or T in case of the complementary strand). The recognition sequence and methylation sites are GAA(6N)TGCC (KpnAI), ACA(6N)TYCA (StySEAI), CGA(6N)TACC (StySENI) and TAAC(7N)RTCG (StySGI). These DNA recognition sequences all have a typical type I bipartite pattern and represent three novel specificities and one isoschizomer (StySENI). For confirmation, oligonucleotides containing each of the predicted sequences were synthesized, cloned into plasmid pMECA and transformed into each strain, resulting in a large reduction in efficiency of transformation (EOT).

GATC-specific restriction and modification systems in treponemes

AIMS

To investigate the presence of GATC-specific modification and restriction activities in rumen isolates of Treponema sp.

METHODS

The presence of N6-methyladenine within GATC (Dam) sequences was analysed using isoschizomeric restriction endonucleases having different sensitivities to the methylation of the target sequence. A fast screening method was used for testing of site-specific endonuclease activities directly in crude cell extracts. Three out of six rumen isolates of Treponema sp. showed restriction activities. Restriction endonucleases were further purified by Heparin-Sepharose chromatography. Using PCR and specific primers, no sequence homologous to the T. pallidum dam gene was found.

CONCLUSIONS

Three rumen treponemal strains were documented to possess MboI isoschizomeric restriction-modification systems.

SIGNIFICANCE

This is the first report on restriction activity in rumen treponemes.

The cholesteryl ester transfer protein (CETP) TaqIB polymorphism in the cholesterol and recurrent events study: no interaction with the response to pravastatin therapy and no effects on cardiovascular outcome

OBJECTIVES

On the basis of quantitative coronary angiography data, the cholesteryl ester transfer protein (CETP) TaqIB gene polymorphism has been postulated to predict the progression of coronary atherosclerosis and response to cholesterol-lowering therapy.

BACKGROUND

Cholesteryl ester transfer protein mediates the exchange of lipids between anti-atherogenic high-density lipoprotein (HDL) and atherogenic apolipoprotein B containing lipoproteins and therefore plays a key role in human lipid metabolism. Hence, CETP gene polymorphisms may alter susceptibility to atherosclerosis.

METHODS

To investigate the significance of the CETP TaqIB gene polymorphism with respect to clinical end points, we used the Cholesterol And Recurrent Events (CARE) cohort. The CARE study was designed to investigate the effect of five years of pravastatin therapy on coronary events.

RESULTS

We found that the odds ratios for the primary end point were not significantly different from unity for the three genetic subgroups after five years of placebo treatment. Furthermore, pravastatin induced similar changes in total cholesterol, low-density lipoprotein cholesterol, and HDL cholesterol among TaqIB genotypes, and both nonfatal myocardial infarction and deaths from coronary heart disease were reduced to the same extent in all three genotypes.

CONCLUSIONS

In the CARE cohort, the CETP TaqIB polymorphism does not predict cardiovascular events or discriminate between those who will or will not benefit from pravastatin treatment.

Vitamin D Receptor Genotype Is Associated With Fat-Free Mass and Sarcopenia in Elderly Men

We investigated the association of vitamin D receptor (VDR) genotype with fat-free mass (FFM) in a cohort of 302 older (aged 58-93 years) Caucasian men who underwent body composition analysis by dual-energy X-ray absorptiometry, and completed questionnaires addressing comorbidities, physical activity, and dietary intake. All participants were genotyped for a VDR translation start site (FokI) polymorphism [FF (37.7%), Ff (48.4%), and ff (13.9%)] and the previously studied BsmI polymorphism [BB (24.9%), Bb (37.7%), and bb (37.4%)]. The BsmI polymorphism was not associated with FFM in any analysis; however, the FokI polymorphism was significantly associated with total FFM, appendicular FFM, and relative (kg/m(2)) appendicular FFM (all p <.05), with the FF group demonstrating significantly lower FFM than the Ff and ff groups (e.g., total FFM: FF = 57.6 +/- 0.4, Ff = 59.4 +/- 0.4, ff = 59.4 +/- 0.7 kg; p <.02). Age-adjusted logistic regression revealed a 2.17-fold higher risk for sarcopenia (defined previously as appendicular FFM <7.26 kg/m(2)) in FF homozygotes (95% CI [confidence interval] = 1.19-3.85; p =.03) compared to men with one or more f alleles. The VDR translation start site (FokI) polymorphism is significantly associated with FFM and sarcopenia in this cohort of older Caucasian men.

The mechanism by which DNA adenine methylase and PapI activate the pap epigenetic switch

The expression of pyelonephritis-associated pili (Pap) in uropathogenic Escherichia coli is epigenetically controlled by a reversible OFF to ON switch. In phase OFF cells, the global regulator Lrp is bound to pap sites proximal to the pilin promoter, whereas in phase ON cells, Lrp is bound to promoter distal sites. We have found that the local regulator PapI increases the affinity of Lrp for the sequence "ACGATC," which contains the target "GATC" site for DNA adenine methylase (Dam) and is present in both promoter proximal and distal sites. Mutational analyses show that methylation of the promoter proximal GATC(prox) site by Dam is required for transition to the phase ON state by specifically blocking PapI-dependent binding of Lrp to promoter proximal sites. Furthermore, our data support the hypothesis that PapI-dependent binding of Lrp to a hemimethylated GATC(dist) site generated by DNA replication is a critical component of the switch mechanism.

Bacteriophage T4-encoded Stp can be replaced as activator of anticodon nuclease by a normal host cell metabolite

The bacterial tRNALys-specific anticodon nuclease is known as a phage T4 exclusion system. In the uninfected host cell anticodon nuclease is kept latent due to the association of its core protein PrrC with the DNA restriction-modification endonuclease EcoprrI. Stp, the T4-encoded peptide inhibitor of EcoprrI activates the latent enzyme. Previous in vitro work indicated that the activation by Stp is sensitive to DNase and requires added nucleotides. Biochemical and mutational data reported here suggest that Stp activates the latent holoenzyme when its EcoprrI component is tethered to a cognate DNA substrate. Moreover, the activation is driven by GTP hydrolysis, possibly mediated by the NTPase domain of PrrC. The data also reveal that Stp can be replaced as the activator of latent anticodon nuclease by certain pyrimidine nucleotides, the most potent of which is dTTP. The activation by dTTP likewise requires an EcoprrI DNA substrate and GTP hydrolysis but involves a different form of the latent holoenzyme/DNA complex. Moreover, whereas Stp relays its activating effect through EcoprrI, dTTP targets PrrC. The activation of the latent enzyme by a normal cell constituent hints that anticodon nuclease plays additional roles, other than warding off phage T4 infection.

A dual lethal system to enhance containment of recombinant micro-organisms

Active containment systems based on the controlled expression of a lethal gene are designed to increase containment of recombinant micro-organisms used for environmental applications. A major drawback in containment is the existence of mutations that generate surviving cells that cease to respond to the toxic effect of the lethal function. In this work the authors have developed for the first time a strategy to reduce the problem of mutations and increase the efficiency of containment based on the combination of two lethal functions acting on different cellular targets of major concern in containment, DNA and RNA, and whose expression is under control of different regulatory signals. To engineer the dual gene containment circuit, two toxin–antitoxin pairs, i.e. the colicin E3–immunity E3 and theEcoRI restriction–modification systems, were combined. The genes encoding the immunity E3 and theEcoRI methyltransferase proteins (antitoxins) were stably inserted into the chromosome of the host cell, whereas the broad-host-range lethal genes encoding the colicin E3 RNase and theEcoRI restriction endonuclease (toxins) were flanking the contained trait in a plasmid. This dual lethal cassette decreased gene transfer frequencies, through killing of the recipient cells, by eight orders of magnitude, which provides experimental evidence that the anticipated containment level due to the combination of single containment systems is generally achieved. Survivors that escaped killing were analysed and the mutational events involved were characterized.

Bacteriophage T4Dam (DNA-(adenine-N6)-methyltransferase): evidence for two distinct stages of methylation under single turnover conditions

We compared the (pre)steady-state and single turnover methylation kinetics of bacteriophage T4Dam (DNA-(adenine-N6)-methyltransferase)-mediated methyl group transfer from S-adenosyl-l-methionine (AdoMet) to oligodeoxynucleotide duplexes containing a single recognition site (palindrome 5'-GATC/5'-GATC) or some modified variant. T4Dam-AdoMet functions as a monomer under steady-state conditions (enzyme/DNA << 1), whereas under single turnover conditions (enzyme/DNA > 1), a catalytically active complex containing two Dam-AdoMet molecules is formed initially, and two methyl groups are transferred per duplex (to produce a methylated duplex and S-adenosyl-l-homocysteine (AdoHcy)). We propose that the single turnover reaction proceeds in two stages. First, two preformed T4Dam-AdoMet complexes bind opposite strands of the unmodified target site, and one enzyme molecule catalyzes the rapid transfer of the AdoMet-methyl group (kmeth1 = 0.21 s-1); this is 2.5-fold slower than the rate observed with monomeric T4Dam-AdoMet bound under pre-steady-state conditions for burst determination. In the second stage, methyl transfer to adenine in GATC on the complementary strand occurs at a rate that is 1 order of magnitude slower (kmeth2 = 0.023 s-1). We suggest that under single turnover conditions, methylation of the second strand is rate-limited by Dam-AdoHcy dissociation or its clearance from the methylated complementary strand. The hemimethylated duplex 5'-GATC/5'-GMTC also interacts with T4Dam-AdoMet complexes in two stages under single turnover reaction conditions. The first stage (kmeth1) reflects methylation by dimeric T4Dam-AdoMet productively oriented to the strand with the adenine residue capable of methylation. The slower second stage (kmeth2) reflects methylation by enzyme molecules non-productively oriented to the GMTC chain, which then have to re-orient to the opposite productive chain. Substitutions of bases and deletions in the recognition site affect the kinetic parameters in different fashions. When the GAT portion of GATC was disrupted, the proportion of the initial productive enzyme-substrate complexes was sharply reduced.

Identification and mutational analysis of Mg2+ binding site in EcoP15I DNA methyltransferase: involvement in target base eversion

EcoP15I DNA methyltransferase catalyzes the transfer of the methyl group of S-adenosyl-l-methionine to the N6 position of the second adenine within the double-stranded DNA sequence 5'-CAGCAG-3'. To achieve catalysis, the enzyme requires a magnesium ion. Binding of magnesium to the enzyme induces significant conformational changes as monitored by circular dichroism spectroscopy. EcoP15I DNA methyltransferase was rapidly inactivated by micromolar concentrations of ferrous sulfate in the presence of ascorbate at pH 8.0. The inactivated enzyme was cleaved into two fragments with molecular masses of 36 and 35 kDa. Using this affinity cleavage assay, we have located the magnesium binding-like motif to amino acids 355-377 of EcoP15I DNA methyltransferase. Sequence homology comparisons between EcoP15I DNA methyltransferase and other restriction endonucleases allowed us to identify a PD(X)n(D/E)XK-like sequence as the putative magnesium ion binding site. Point mutations generated in this region were analyzed for their role in methyltransferase activity, metal coordination, and substrate binding. Although the mutant methyltransferases bind DNA and S-adenosyl-l-methionine as well as the wild-type enzyme does, they are inactive primarily because of their inability to flip the target base. Collectively, these data are consistent with the fact that acidic amino acid residues of the region 355-377 in EcoP15I DNA methyltransferase are important for the critical positioning of magnesium ions for catalysis. This is the first example of metal-dependent function of a DNA methyltransferase. These findings provide impetus for exploring the role(s) of metal ions in the structure and function of DNA methyltransferases.

Crystal structure of MboIIA methyltransferase

DNA methyltransferases (MTases) are sequence-specific enzymes which transfer a methyl group from S-adenosyl-L-methionine (AdoMet) to the amino group of either cytosine or adenine within a recognized DNA sequence. Methylation of a base in a specific DNA sequence protects DNA from nucleolytic cleavage by restriction enzymes recognizing the same DNA sequence. We have determined at 1.74 A resolution the crystal structure of a beta-class DNA MTase MboIIA (M.MboIIA) from the bacterium Moraxella bovis, the smallest DNA MTase determined to date. M.MboIIA methylates the 3' adenine of the pentanucleotide sequence 5'-GAAGA-3'. The protein crystallizes with two molecules in the asymmetric unit which we propose to resemble the dimer when M.MboIIA is not bound to DNA. The overall structure of the enzyme closely resembles that of M.RsrI. However, the cofactor-binding pocket in M.MboIIA forms a closed structure which is in contrast to the open-form structures of other known MTases.

Identification of long intergenic repeat sequences associated with DNA methylation sites in Caulobacter crescentus and other alpha-proteobacteria

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.

Regulated expression of the Escherichia coli dam gene

Regulated expression of the Escherichia coli dam gene has been achieved with the araBAD promoter lacking a ribosome binding site. Cultures of dam mutants containing plasmid pMQ430 show no detectable methylation in the absence of arabinose and complete methylation in its presence. Dam methyltransferase is a substrate for the Lon protease.