title: The ada operon of Mycobacterium tuberculosis encodes two DNA methyltransferases for inducible repair of DNA alkylation damage.
abstract: The ada operon of Mycobacterium tuberculosis, which encodes a composite protein of AdaA and AlkA and a separate AdaB/Ogt protein, was characterized. M. tuberculosis treated with N-methyl-N'-nitro-N-nitrosoguanidine induced transcription of the adaA-alkA and adaB genes, suggesting that M. tuberculosis mount an inducible response to methylating agents. Survival assays of the methyltransferase defective Escherichia coli mutant KT233 (ada ogt), showed that expression of the adaB gene rescued the alkylation sensitivity. Further, adaB but not adaA-alkA complemented the hypermutator phenotype of KT233. Purified AdaA-AlkA and AdaB possessed methyltransferase activity. These data suggested that AdaB counteract the cytotoxic and mutagenic effect of O(6)-methylguanine, while AdaA-AlkA most likely transfers methyl groups from innocuous methylphosphotriesters. AdaA-AlkA did not possess alkylbase DNA glycosylase activity nor rescue the alkylation sensitivity of the E. coli mutant BK2118 (tag alkA). We propose that AdaA-AlkA is a positive regulator of the adaptive response in M. tuberculosis. It thus appears that the ada operon of M. tuberculosis suppresses the mutagenic effect of alkylation but not the cytotoxic effect of lesions such as 3-methylpurines. Collectively, these data indicate that M. tuberculosis hypermutator strains with defective adaptive response genes might sustain robustness to cytotoxic alkylation DNA damage and confer a selective advantage contributing to host adaptation.
authors.affiliation: Department of Microbiology, Oslo University Hospital, Rikshospitalet, PO Box 4950 Nydalen, NO-0424 Oslo, Norway.
authors.forename: Mingyi
authors.initials: M
authors.lastname: Yang
authors.forename: Randi M
authors.initials: RM
authors.lastname: Aamodt
authors.forename: Bjørn
authors.initials: B
authors.lastname: Dalhus
authors.forename: Seetha
authors.initials: S
authors.lastname: Balasingham
authors.forename: Ina
authors.initials: I
authors.lastname: Helle
authors.forename: Pernille
authors.initials: P
authors.lastname: Andersen
authors.forename: Tone
authors.initials: T
authors.lastname: Tønjum
authors.forename: Ingrun
authors.initials: I
authors.lastname: Alseth
authors.forename: Torbjørn
authors.initials: T
authors.lastname: Rognes
authors.forename: Magnar
authors.initials: M
authors.lastname: Bjørås
mesh_terms: D000478:Alkylation; D004249:DNA Damage; D015254:DNA Modification Methylases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004260:DNA Repair; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D008958:Models, Molecular; D009169:Mycobacterium tuberculosis / Q000201:enzymology* / Q000235:genetics; D009690:Nucleic Acid Conformation; D009876:Operon; D017434:Protein Structure, Tertiary; D040681:Structural Homology, Protein
chemical_list: D004269:DNA, Bacterial; D015254:DNA Modification Methylases