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.