CONSTRUCTION AND CHARACTERIZATION OF MUTANTS OF SALMONELLA-TYPHIMURIUM DEFICIENT IN DNA-REPAIR OF O-6-METHYLGUANINE

Escherichia coli has two O-6-methylguanine DNA methyltransferases that repair alkylation damage in DNA and are encoded by the ada and ogt ge nes. The ada gene of E. coli also regulates the adaptive response to a lkylation damage. The closely related species Salmonella typhimurium p ossesses methyltransferase activities but does not exhibit an adaptive response conferring detectable resistance to mutagenic methylating ag ents. We have previously cloned the ada-like gene of S. typhimurium (a da(ST)) and constructed an ada(ST)-deletion derivative of S. typhimuri um TA1535. Unexpectedly, the sensitivity of the resulting strain to th e mutagenic action of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was similar to that of the parent strain. In this study, we have cloned an d sequenced the ogt-like gene of S. typhimurium (ogt(ST)) and characte rized ogt(ST)-deletion derivatives of TA1535. The ogt(ST) mutant was m ore sensitive than the parent strain to the mutagenicity of MNNG and o ther simple alkylating agents with longer alkyl groups (ethyl, propyl, and butyl). The ada(ST)-ogt(ST) double mutant had a level of hypersen sitivity to these agents similar to that of the ogt(ST) single mutant. The ogt(ST) and the ada(ST)-ogt(ST) mutants also displayed a two to t hree times higher spontaneous mutation frequency than the parent strai n and the ada(ST) mutant. These results indicate that the Ogt(ST) prot ein, but not the Ada(ST) protein, plays a major role in protecting S. typhimurium from the mutagenic action of endogenous as well as exogeno us alkylating agents.