OGT ALKYLTRANSFERASE ENHANCES DIBROMOALKANE MUTAGENICITY IN EXCISION REPAIR-DEFICIENT ESCHERICHIA-COLI K-12

We examined the role of the O-6-alkylguanine-DNA alkyltransferase enco ded by ogt gene in the sensitivity of Escherichia coli to the mutageni c effects of the dibromoalkanes, dibromoethane and dibromomethane, by comparing responses in ogt bacteria to those in their isogenic ogt(+) parental counterparts. The effects of the uvrABC excision-repair syste m, the adaptive response, mucAB and umuDC mutagenic processing, and gl utathione bioactivation on the differential responses of ogt(-) and og t(+) bacteria were also studied. Mutation induction was monitored by m easuring the frequency of forward mutations to L-arabinose resistance. Induced mutations occurred only in excision repair-defective strains and were totally (with dibromomethane) or substantially (with dibromoe thane) dependent on the alkyltransferase (ATase) encoded by the ogt ge ne. An increased mutagenic response to both dibromoalkanes was also se en in ogt(-) bacteria that overexpressed the ogt protein from a multic opy plasmid, indicating that the differences in mutability between ogt (+) and ogt(-) bacteria were not dependent on the ogt(-) null allele c arried by the defective strain. The ATase encoded by the constitutive ogt gene was more effective in promoting dibromoalkane mutagenicity th an the ada ATase induced by exposure to low doses of a methylating age nt. The mutagenicity promoted by the ogt ATase was dependent on both g lutathione bioactivation and SOS mutagenic processing. To our knowledg e, this paper presents for the first time evidence that DNA ATases, in particular the ATase encoded by the ogt gene, can increase the mutage nic effects of a DNA-damaging agent. The mechanism of this effect has yet to be established. (C) 1995 Wiley-Liss, Inc.