Multiple species of Bacillus subtilis DNA alkyltransferase involved in the adaptive response to simple alkylating agents

Genomic Insights into the Radiation-Resistant Capability of Sphingomonas qomolangmaensis S5-59T and Sphingomonas glaciei S8-45T, Two Novel Bacteria from the North Slope of Mount Everest

Mount Everest provides natural advantages to finding radiation-resistant extremophiles that are functionally mechanistic and possess commercial significance. (1) Background: Two bacterial strains, designated S5-59T and S8-45T, were isolated from moraine samples collected from the north slope of Mount Everest at altitudes of 5700m and 5100m above sea level. (2) Methods: The present study investigated the polyphasic features and genomic characteristics of S5-59^T and S8-45^T. (3) Results: The major fatty acids and the predominant respiratory menaquinone of S5-59^T and S8-45^T were summed as feature 3 (comprising C16:1 ω6c and/or C16:1 ω7c) and ubiquinone-10 (Q-10). Phylogenetic analyses based on 16S rRNA sequences and average nucleotide identity values among these two strains and their reference type strains were below the species demarcation thresholds of 98.65% and 95%. Strains S5-59^T and S8-45^T harbored great radiation resistance. The genomic analyses showed that DNA damage repair genes, such as mutL, mutS, radA, radC, recF, recN, etc., were present in the S5-59^T and S8-45^T strains. Additionally, strain S5-59^T possessed more genes related to DNA protection proteins. The pan-genome analysis and horizontal gene transfers revealed that strains of Sphingomonas had a consistently homologous genetic evolutionary radiation resistance. Moreover, enzymatic antioxidative proteins also served critical roles in converting ROS into harmless molecules that resulted in resistance to radiation. Further, pigments and carotenoids such as zeaxanthin and alkylresorcinols of the non-enzymatic antioxidative system were also predicted to protect them from radiation. (4) Conclusions: Type strains S5-59^T (=JCM 35564T =GDMCC 1.3193T) and S8-45^T (=JCM 34749T =GDMCC 1.2715T) represent two novel species of the genus Sphingomonas with the proposed name Sphingomonas qomolangmaensis sp. nov. and Sphingomonas glaciei sp. nov. The type strains, S5-59^T and S8-45^T, were assessed in a deeply genomic study of their radiation-resistant mechanisms and this thus resulted in a further understanding of their greater potential application for the development of anti-radiation protective drugs.

The role of radiation induced oxidative stress as a regulator of radio-adaptive responses

Purpose: Various sources of radiation including radiofrequency, electromagnetic radiation (EMR), low- dose X-radiation, low-level microwave radiation and ionizing radiation (IR) are indispensable parts of modern life. In the current review, we discussed the adaptive responses of biological systems to radiation with a focus on the impacts of radiation-induced oxidative stress (RIOS) and its molecular downstream signaling pathways.Materials and methods: A comprehensive search was conducted in Web of Sciences, PubMed, Scopus, Google Scholar, Embase, and Cochrane Library. Keywords included Mesh terms of "radiation," "electromagnetic radiation," "adaptive immunity," "oxidative stress," and "immune checkpoints." Manuscripts published up until December 2019 were included.Results: RIOS induces various molecular adaptors connected with adaptive responses in radiation exposed cells. One of these adaptors includes p53 which promotes various cellular signaling pathways. RIOS also activates the intrinsic apoptotic pathway by depolarization of the mitochondrial membrane potential and activating the caspase apoptotic cascade. RIOS is also involved in radiation-induced proliferative responses through interaction with mitogen-activated protein kinases (MAPks) including p38 MAPK, ERK, and c-Jun N-terminal kinase (JNK). Protein kinase B (Akt)/phosphoinositide 3-kinase (PI3K) signaling pathway has also been reported to be involved in RIOS-induced proliferative responses. Furthermore, RIOS promotes genetic instability by introducing DNA structural and epigenetic alterations, as well as attenuating DNA repair mechanisms. Inflammatory transcription factors including macrophage migration inhibitory factor (MIF), nuclear factor κB (NF-κB), and signal transducer and activator of transcription-3 (STAT-3) paly major role in RIOS-induced inflammation.Conclusion: In conclusion, RIOS considerably contributes to radiation induced adaptive responses. Other possible molecular adaptors modulating RIOS-induced responses are yet to be divulged in future studies.

Bacteriophages To Sensitize a Pathogenic New Delhi Metallo β-Lactamase-Positive Escherichia coli to Solar Disinfection

Bacteriophages active against a New Delhi metallo beta lactamase (NDM)-positive E. coli PI-7 were isolated from municipal wastewater and tested for their lytic effect against the bacterial host. Bacteriophages were highly specific to E. coli PI-7 when tested for host-range. After determining host-specificity, bacteriophages were tested for their ability to sensitize E. coli PI-7 to solar irradiation. Solar irradiation coupled with bacteriophages successfully reduced the length of the lag-phase for E. coli PI-7 from 4 h to 2 h in buffer solution. The reduction of lag-phase length was also observed in filtered wastewater effluent and chlorinated effluent. Previously, we found through gene expression analysis that cell wall, oxidative stress, and DNA repair functions played a large role in protecting E. coli PI-7 against solar damage. Here, gene expression analysis of bacteriophage-supplemented solar-irradiated E. coli PI-7 revealed downregulation of cell wall functions. Downregulation of functions implicated in scavenging and detoxifying reactive oxygen species, as well as DNA repair genes, was also observed in bacteriophage-supplemented solar-irradiated E. coli PI-7. Moreover, solar irradiation activates recA, which can induce lytic activity of bacteriophages. Overall, the combined treatment led to gene responses that appeared to make E. coli PI-7 more susceptible to solar disinfection and bacteriophage infection. Our findings suggest that bacteriophages show good potential to be used as a biocontrol tool to complement solar irradiation in mitigating the persistence of antibiotic-resistant bacteria in reuse waters.

DNA repair and genome maintenance in Bacillus subtilis

From microbes to multicellular eukaryotic organisms, all cells contain pathways responsible for genome maintenance. DNA replication allows for the faithful duplication of the genome, whereas DNA repair pathways preserve DNA integrity in response to damage originating from endogenous and exogenous sources. The basic pathways important for DNA replication and repair are often conserved throughout biology. In bacteria, high-fidelity repair is balanced with low-fidelity repair and mutagenesis. Such a balance is important for maintaining viability while providing an opportunity for the advantageous selection of mutations when faced with a changing environment. Over the last decade, studies of DNA repair pathways in bacteria have demonstrated considerable differences between Gram-positive and Gram-negative organisms. Here we review and discuss the DNA repair, genome maintenance, and DNA damage checkpoint pathways of the Gram-positive bacterium Bacillus subtilis. We present their molecular mechanisms and compare the functions and regulation of several pathways with known information on other organisms. We also discuss DNA repair during different growth phases and the developmental program of sporulation. In summary, we present a review of the function, regulation, and molecular mechanisms of DNA repair and mutagenesis in Gram-positive bacteria, with a strong emphasis on B. subtilis.

Diverse capacities for the adaptive response to DNA alkylation in Bacillus species and strains

Our previous studies of Bacillus subtilis showed that the genes responsible for the adaptive response to DNA alkylation were organized as a divergent regulon, in contrast to scattered operons in Escherichia coli ada regulon. To study the generality and diversity of gene organization, several species and strains of Bacillus were examined for the responsiveness to DNA alkylation. B. cereus cells exhibited the highest resistance to MNNG treatment. When the cells were grown in the presence of MNNG, 3-methyladenine DNA glycosylase and two species of DNA methyltransferase were induced as in B. subtilis 168 cells. B. licheniformis 749 and B. amyloliquefaciens H cells exhibited a partial response that manifested itself as the induction of one species of DNA methyltransferase. On the other hand, B. thuringiensis var. Tohokuensis, B. megaterium KMT, and B. subtilis W23 cells were totally deficient in this response, and were hypersensitive to alkylating agents. To determine the cause of this deficiency in strain W23, we examined the genomic structure of the corresponding region where three genes (alkA, adaA, and adaB) were located in 168. No homologues for the three genes were detected in W23 DNA by Southern hybridization. Two genes (glmS and ndhF) flanking the adaptive response regulon in 168 were also present in W23. A sequence of about 2750 bp that carried the entire regulon in 168 was replaced with a sequence of about 250 bp that was unique to W23. At the ends of the conserved segments, palindromic sequences corresponding to the transcriptional termination sites of the adaB and glmS genes were observed. The regulon in 168 could be artificially replaced by the W23 sequence, and be regained through DNA-mediated transformation.

Evidence for a weak adaptive response to alkylation damage in Vibrio cholerae

Wild-type Vibrio cholerae cells, when adapted by a stepwise treatment with sub-lethal concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), acquired resistance to killing and mutagenesis by subsequent challenges with higher concentrations of MNNG. This was also seen in the rec isogenic strain indicating that the observed phenomenon was not due to the induction of SOS functions. Further, the adapted cells of both the wild-type and rec strains could reactivate lethally alkylated phages with equal efficiency. Increased resistance of adapted cells correlated with the induction of a 17-kDa DNA methyltransferase, capable of repairing O6-methylguanine lesions in DNA. This induced methyltransferase was found to be antigenically unrelated to the Escherichia coli methyltransferase (Ada protein) as determined by Western blotting with polyclonal antiserum raised against the E. coli protein. Even though no counterpart of the constitutively expressed methyltransferase (Ogt) of E. coli could be detected in V. cholerae, several lines of evidence pointed towards the presence of an E. coli alk A-like gene in the organism.

Bacillus subtilis alkA gene encoding inducible 3-methyladenine DNA glycosylase is adjacent to the ada operon

In Bacillus subtilis, the adaptive response to DNA alkylation depends on the ada operon, which consists of the adaA and adaB genes, which encode methylphosphotriester DNA methyltransferase (AdaA protein) and O6-methylguanine DNA methyltransferase (AdaB protein), respectively. A structural gene (alkA) that encodes 3-methyladenine DNA glycosylase was found upstream of the ada operon, but in the opposite orientation. This cluster of genes was mapped at about 235 kb from the SfiI recognition site near the origin of replication in the physical map of the B. subtilis chromosome. Disruption of the alkA gene sensitized cells to N-propyl-N'-nitro-N-nitrosoguanidine, while its overproduction rendered cells highly resistant to N-propyl-N'-nitro-N-nitrosoguanidine, indicating that lethal DNA damage produced by bulky alkylating agents was effectively counteracted by AlkA glycosylase. Transcription of the alkA gene was induced by treating adaA+ cells with methylating agents concurrent with transcription of the ada operon. This was accomplished by using methylated AdaA protein bound to a 30-bp segment in the middle of the 100-bp sequence between the transcriptional start sites of the alkA gene and ada operon. Thus, in this organism, the adaptive response to DNA alkylation is achieved by autologous activation of a divergent regulon composed of the genes for a DNA glycosylase and two species of DNA alkyltransferase.

Inducible alkyltransferase DNA repair proteins in the filamentous fungus Aspergillus nidulans

We have investigated the response of the filamentous fungus Aspergillus nidulans to low, non-killing, doses of the alkylating agent MNNG (N-methyl-N'-nitro-N-nitrosoguanidine). Such treatment causes a substantial induction of DNA alkyltransferase activity, with the specific activity in treated cells increasing up to one hundred-fold. Fluorography reveals the two main inducible species as proteins of 18.5 kDa and 21 kDa, both of which have activity primarily against O6-methylguanine (O6-MeG) lesions. In addition, two other alkyltransferase proteins can also be detected. One, of MW 16 kDa, is expressed in non-treated cells, but is not induced to the same extent as the 18.5 and 21 kDa proteins. The other, a protein of 19.5 kDa, is highly inducible and can only be detected in treated cells. Unlike the other three proteins, it acts primarily against methyl-phosphotriester (Me-PT) lesions. This is the first instance in which an MePT alkyltransferase has been detected in a eukaryotic organism and, coupled with the high level of induction of the O6-MeG alkyltransferase enzymes, this indicates that a control system similar to the bacterial adaptive response may be present in filamentous fungi.

Molecular analysis of Bacillus subtilis ada mutants deficient in the adaptive response to simple alkylating agents

Previously, we isolated and characterized six Bacillus subtilis ada mutants that were hypersensitive to methylnitroso compounds and deficient in the adaptive response to alkylation. Cloning of the DNA complementing the defects revealed the presence of an ada operon consisting of two tandem and partially overlapping genes, adaA and adaB. The two genes encoded proteins with methylphosphotriester-DNA methyltransferase and O6-methylguanine-DNA methyltransferase activities, respectively. To locate the six mutations, the ada operon was divided into five overlapping regions of about 350 bp. The fragments of each region were amplified by polymerase chain reaction and analyzed by gel electrophoresis to detect single-strand conformation polymorphism. Nucleotide sequences of the fragments exhibiting mobility shifts were determined. Three of the mutants carried sequence alterations in the adaA gene: the adaA1 and adaA2 mutants had a one-base deletion and insertion, respectively, and the adaA5 mutant had a substitution of two consecutive bases causing changes of two amino acid residues next to the presumptive alkyl-accepting Cys-85 residue. Three mutants carried sequence alterations in the adaB gene: the adaB3 mutant contained a rearrangement, the adaB6 mutant contained a base substitution causing a change of the presumptive alkyl-accepting Cys-141 to Tyr, and the adaB4 mutant contained a base substitution changing Leu-167 to Pro. The adaB mutants produced ada transcripts upon treatment with low doses of alkylating agents, whereas the adaA mutant did not. We conclude that the AdaA protein functions as the transcriptional activator of this operon, while the AdaB protein specializes in repair of alkylated residues in DNA.

Drosophila methyltransferase activity and the repair of alkylated DNA

The biochemical mechanism and developmental expression for the repair of alkylated DNA has been characterized from Drosophila. As in other organisms, the correction of O6-methylguanine in Drosophila was found to involve the transfer of a methyl group from DNA to a protein cysteine residue. Two methylated proteins with subunit molecular weights of 30 kDa and 19 kDa were identified following incubation with [3H]-methylated substrate DNA and denaturing polyacrylamide gel electrophoresis. Identical molecular weights were found for the unmethylated forms of protein through their reaction to an antibody prepared against the 19 kDa Escherichia coli methyltransferase. Both Drosophila proteins are serologically reactive in adult males and females and most of the other developmental stages tested, with embryos representing the possible exception. The Drosophila proteins do not appear to be induced by sublethal exposures to alkylating agent.

Increased spontaneous mutation and alkylation sensitivity of Escherichia coli strains lacking the ogt O6-methylguanine DNA repair methyltransferase

Escherichia coli expresses two DNA repair methyltransferases (MTases) that repair the mutagenic O6-methylguanine (O6MeG) and O4-methylthymine (O4MeT) DNA lesions; one is the product of the inducible ada gene, and here we confirm that the other is the product of the constitutive ogt gene. We have generated various ogt disruption mutants. Double mutants (ada ogt) do not express any O6MeG/O4MeT DNA MTases, indicating that Ada and Ogt are probably the only two O6MeG/O4MeT DNA MTases in E. coli. ogt mutants were more sensitive to alkylation-induced mutation, and mutants arose linearly with dose, unlike ogt+ cells, which had a threshold dose below which no mutants accumulated; this ogt(+)-dependent threshold was seen in both ada+ and ada strains. ogt mutants were also more sensitive to alkylation-induced killing (in an ada background), and overexpression of the Ogt MTase from a plasmid provided ada, but not ada+, cells with increased resistance to killing by alkylating agents. The induction of the adaptive response was normal in ogt mutants. We infer from these results that the Ogt MTase prevents mutagenesis by low levels of alkylating agents and that, in ada cells, the Ogt MTase also protects cells from killing by alkylating agents. We also found that ada ogt E. coli had a higher rate of spontaneous mutation than wild-type, ada, and ogt cells and that this increased mutation occurred in nondividing cells. We infer that there is an endogenous source of O6MeG or O4MeT DNA damage in E. coli that is prevalent in nondividing cells.

Bacillus subtilis ada operon encodes two DNA alkyltransferases

By prophage transformation and subcloning, we have obtained Bacillus subtilis DNA fragments that could complement the hypersensitivity of ada (adaptive response deficient) mutants to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The nucleotide sequence contained two open reading frames that were assigned to the genes adaA and adaB, encoding methylphosphotriester-DNA methyltransferase and O6-methylguanine-DNA methyltransferase, respectively. These two genes overlap by 11 bp and comprise a small operon. The 1.6 Kb transcripts derived from the operon were detected in ada+ cells cultured in the presence of MNNG but not in control ada+ cells. From analysis of the syntheses of DNA alkyltransferases in the ada mutant cells harboring the plasmid carrying the complete or partial fragment, we conclude that the adaA gene product functions as a transcriptional activator of the ada operon, while the adaB gene product specializes in repair of mutagenic O6-methylguanine residues. Comparison with Escherichia coli ada operon showed that the two genes correspond to portions of the E. coli ada gene, implicating gene fusion or splitting as the origin of the difference in the organizations of the genes.

A constitutive O6-methylguanine-DNA methyltransferase of Rhizobium meliloti

We have identified a DNA methyltransferase activity of the nitrogen-fixing bacterium, Rhizobium meliloti, that repairs O6-methylguanine lesions. Repair of the O6-methylguanine residue results in transfer of the methyl group to a cysteine residue of a 28,000-dalton protein. The O6-methyltransferase activity is expressed constitutively and R. meliloti does not exhibit an adaptive response to alkylating agents.

Purification to homogeneity and partial amino acid sequence of a fragment which includes the methyl acceptor site of the human DNA repair protein for O6-methylguanine

DNA repair by O6-methylguanine-DNA methyltransferase (O6-MT) is accomplished by removal by the enzyme of the methyl group from premutagenic O6-methylguanine-DNA, thereby restoring native guanine in DNA. The methyl group is transferred to an acceptor site cysteine thiol group in the enzyme, which causes the irreversible inactivation of O6-MT. We detected a variety of different forms of the methylated, inactivated enzyme in crude extracts of human spleen of molecular weights higher and lower than the usually observed 21-24kDa for the human O6-MT. Several apparent fragments of the methylated form of the protein were purified to homogeneity following reaction of partially-purified extract enzyme with O6-[3H-CH3]methylguanine-DNA substrate. One of these fragments yielded amino acid sequence information spanning fifteen residues, which was identified as probably belonging to human methyltransferase by virtue of both its significant sequence homology to three procaryote forms of O6-MT encoded by the ada, ogt (both from E. coli) and dat (B. subtilis) genes, and sequence position of the radiolabelled methyl group which matched the position of the conserved procaryote methyl acceptor site cysteine residue. Statistical prediction of secondary structure indicated good homologies between the human fragment and corresponding regions of the constitutive form of O6-MT in procaryotes (ogt and dat gene products), but not with the inducible ada protein, indicating the possibility that we had obtained partial amino acid sequence for a non-inducible form of the human enzyme. The identity of the fragment sequence as belonging to human methyltransferase was more recently confirmed by comparison with cDNA-derived amino acid sequence from the cloned human O6-MT gene from HeLa cells (1). The two sequences compared well, with only three out of fifteen amino acids being different (and two of them by only one nucleotide in each codon).

Active site amino acid sequence of the bovine O6-methylguanine-DNA methyltransferase

An O6-methylguanine-DNA methyltransferase has been partially purified from calf thymus by conventional biochemical techniques. The enzyme was specifically radioactively labelled at the cysteine residue of the active site and further purified by attachment to a solid support. Following digestion with trypsin, a radioactive peptide containing the active site region of the protein was purified by size fractionation, ion exchange chromatography and reverse phase HPLC. The technique yielded an essentially homogeneous oligopeptide which was subjected to amino acid sequencing. The sequence adjacent to the acceptor cysteine residue of the bovine protein exhibits striking homology to the C-terminal methyl acceptor site of the E. coli Ada protein and the proposed acceptor sites of the E. coli Ogt and the B. subtilis Dat1 proteins.

Isolation of a Bacillus subtilis mutant defective in constitutive O6-alkylguanine-DNA alkyltransferase

A mutant of Bacillus subtilis defective in the constitutive activity of O6-alkylguanine-DNA alkyltransferase was isolated from a strain (ada-1) deficient in the adaptive response to DNA alkylation. Cells carrying the mutation dat-1 which was responsible for the defect in constitutive activity exhibited hypersensitivity for lethality and mutagenesis when challenged with methyl-nitroso compounds. The constitutive activity is independent of the adaptive response, and seems to function as a basal defense against environmental alkylating agents.

Cloning and expression of the Bacillus subtilis methyltransferase gene in Escherichia coli ada- cells

DNA fragments of Bacillus subtilis were inserted into a plasmid vector that can multiply in Escherichia coli cells, and foreign genes were expressed under the control of the lac promoter. By selecting hybrid plasmids that confer an increased resistance to alkylating agents on E. coli ada- mutant cells, the B. subtilis gene dat, which encodes O6-methylguanine-DNA methyltransferase, was cloned. The Dat protein, with a molecular weight of about 20,000, could transfer the methyl group from methylated DNA to its own protein molecule. Based on the nucleotide sequence of the gene, it was deduced that the protein comprises 165 amino acids and that the molecular weight is 18,779. The presumptive amino acid sequence of Dat protein is homologous to the sequences of the E. coli Ogt protein and the C-terminal half of the Ada protein, both of which carry O6-methylguanine-DNA methyltransferase activity. The pentaamino acid sequence Pro-Cys-His-Arg-Val, the cysteine residue of which is the methyl acceptor site in Ada protein, was conserved in the 3 methyltransferase proteins. The structural similarity of these methyltransferases suggests possible evolution from a single ancestral gene.

Characterization of the major DNA repair methyltransferase activity in unadapted Escherichia coli and identification of a similar activity in Salmonella typhimurium

Escherichia coli has two DNA repair methyltransferases (MTases): the 39-kilodalton (kDa) Ada protein, which can undergo proteolysis to an active 19-kDa fragment, and the 19-kDa DNA MTase II. We characterized DNA MTase II in cell extracts of an ada deletion mutant and compared it with the purified 19-kDa Ada fragment. Like Ada, DNA MTase II repaired O6-methylguanine (O6MeG) lesions via transfer of the methyl group from DNA to a cysteine residue in the MTase. Substrate competition experiments indicated that DNA MTase II repaired O4-methylthymine lesions by transfer of the methyl group to the same active site within the DNA MTase II molecule. The repair kinetics of DNA MTase II were similar to those of Ada; both repaired O6MeG in double-stranded DNA much more efficiently than O6MeG in single-stranded DNA. Chronic pretreatment of ada deletion mutants with sublethal (adapting) levels of two alkylating agents resulted in the depletion of DNA MTase II. Thus, unlike Ada, DNA MTase II did not appear to be induced in response to chronic DNA alkylation at least in this ada deletion strain. DNA MTase II was much more heat labile than Ada. Heat lability studies indicated that more than 95% of the MTase in unadapted E. coli was DNA MTase II. We discuss the possible implications of these results for the mechanism of induction of the adaptive response. A similarly active 19-kDa O6MeG-O4-methylthymine DNA MTase was identified in Salmonella typhimurium.

Bacillus subtilis gene coding for constitutive O6-methylguanine-DNA alkyltransferase

We have cloned a Bacillus subtilis DNA fragment that could correct the defect in a constitutive O6-methylguanine-DNA alkyltransferase (Dat1). This fragment also corrected the hypersensitivity of the strain TKJ6951(ada-1 dat-1) to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In the fragment, the gene activity resides in a region of about 850 bp which contains an open reading frame capable of coding for a protein of 165 amino acid residues. The amino acid sequence of this protein exhibits striking similarity to those of E. coli O6-methylguanine-DNA alkyltransferases (Ogt and Ada proteins). We conclude that this is a structural gene for the Dat1 protein, which is distinct from inducible DNA alkyltransferases involved in the adaptive response. The dat-1 mutation was shown to be caused by a structural rearrangement affecting the coding region, and the 0.8 kb transcripts of this gene were detected in dat+ cells but not in dat mutant cells.

Polyclonal antibodies against O6-methylguanine-DNA methyltransferase in adapted bacteria

The similarity of the adaptive response and the methyltransferase component in bacterial strains from different phylogenic groups was investigated. An adaptive response with induction of transferase activity was found for the first time in the soil bacteria P. aeruginosa and X. maltophilia. Polyclonal antibodies against the E. coli ada protein were used to investigate the structural similarity of the transferases from several bacterial strains with adaptive responses and inducible transferase activity. These antibodies cross-reacted with transferase from M. luteus and P. aeruginosa but not with proteins from other related bacteria, and not with human transferase. The phylogenic relationships of bacteria with adaptive responses suggest that the response likely was present in the common ancestor of eubacteria. The restricted antibody cross-reactivity may reflect the dual role of the E. coli ada protein not only in DNA repair but in positive gene regulation.

Characterisation and nucleotide sequence of ogt, the O6-alkylguanine-DNA-alkyltransferase gene of E. coli

The plasmid pO61 that was isolated from an E. coli genomic DNA library and codes for O6-alkylguanine (O6AG) DNA alkyltransferase (ATase) activity (1) has been further characterised. Subclones of the 9 Kb insert of pO61 showed that the ATase activity was encoded in a 2Kb Pst1 fragment but a partial restriction endonuclease map of this was different to that of the E. coli ada gene that codes for O6-AG and alkylphosphotriester dual ATase protein. Fluorographic analyses confirmed that the molecular weight of the pO61-encoded ATase was 19KDa i.e. similar to that of the O6AG ATase function that is cleaved from the 39KDa ada protein but rabbit polyclonal antibodies to the latter reacted only very weakly with the pO61-encoded protein. A different set of hybridisation signals was produced when E. coli DNA, which had been digested with a variety of restriction endonucleases was probed with 2Kb Pst 1 fragment or the ada gene. These results provided evidence for the existence of a second ATase gene in E. coli. The 2Kb Pst-1 fragment of pO61 was therefore sequenced and an open reading frame (ORF) that would give rise to a 19KDa protein was identified. The derived amino acid sequence of this showed a 93 residue region with 49% homology with the O6AG ATase region of the ada protein and had a pentamer and a heptamer of identical sequence separated by 34 amino acids in both proteins. The pentamer included the alkyl accepting cysteine residue of the ada O6AG ATase. The hydrophobic domains were similarly distributed in both proteins. Shine-Dalgarno, -10 and -35 sequences were identified and the origin of transcription was located by primer extension and S1 nuclease mapping. The amino-terminal amino acid sequence of the protein was as predicted from the ORF.