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Two classes of Bacillus subtilis mutants deficient in the adaptive response to simple alkylating agents
Abstract:
Six mutant strains of Bacillus subtilis hypersensitive to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were shown to be deficient in the adaptive response to MNNG and termed ada mutants (Morohoshi and Munakata 1985). All the mutations mapped between the attSPO2 and lin loci on the chromosome. The mutant and wild-type (ada+) cells contained similar constitutive levels of O6-methylguanine-DNA methyltransferase activity. Pretreatment with low concentrations of MNNG increased the activity about nine-fold in the ada+ cells, while it uniformly decreased the activity in the ada cells. The pretreatment of three mutants (ada-3, ada-4, and ada-6) as well as ada+, augmented the activity of methylpurine-DNA glycosylase and rendered the cells resistant to the lethal and mutagenic effects of N-propyl- or N-butyl-N'-nitro-N-nitrosoguanidine. With the rest of the mutant strains (ada-1, ada-2, and ada-5), neither of such responses was elicited by the pretreatment. Thus, the former ada strains seem to have a defect in the gene specifically involved in the induction of the methyltransferase, while the latter ada strains have a defect in the gene controlling the adaptive response as a whole.
Insights
Six Bacillus subtilis mutant strains, termed ada mutants, showed deficiencies in their adaptive response to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Some mutants were defective in methyltransferase induction, while others had broader adaptive response defects.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Bacillus subtilis possesses adaptive responses to DNA-damaging agents like N-methyl-N'-nitro-N-nitrosoguanidine (MNNG).
- O6-methylguanine-DNA methyltransferase and methylpurine-DNA glycosylase are key enzymes in DNA repair pathways.
Purpose of the Study:
- To investigate the genetic basis of the adaptive response to MNNG in Bacillus subtilis.
- To identify and characterize mutants deficient in MNNG adaptation.
Main Methods:
- Isolation and genetic mapping of MNNG-hypersensitive Bacillus subtilis mutants (ada mutants).
- Assay of O6-methylguanine-DNA methyltransferase and methylpurine-DNA glycosylase activities in wild-type and mutant strains.
- Assessment of cellular resistance to lethal and mutagenic effects of MNNG and related compounds after pretreatment.
Main Results:
- Six MNNG-hypersensitive mutants (ada mutants) were identified and mapped.
- Mutant and wild-type cells had similar constitutive O6-methylguanine-DNA methyltransferase levels.
- Pretreatment with MNNG induced O6-methylguanine-DNA methyltransferase in wild-type but decreased it in all mutants.
- Three mutants (ada-3, ada-4, ada-6) showed induced methylpurine-DNA glycosylase activity and resistance to other alkylating agents upon pretreatment, unlike mutants ada-1, ada-2, and ada-5.
Conclusions:
- The ada mutants exhibit defects in the adaptive response to MNNG.
- Mutants ada-3, ada-4, and ada-6 appear defective in a gene specifically inducing methyltransferase.
- Mutants ada-1, ada-2, and ada-5 seem to have defects in a gene controlling the overall adaptive response pathway.