We examined the role of the O6-alkylguanine-DNA alkyltransferase encoded by ogt 
gene in the sensitivity of Escherichia coli to the mutagenic 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 system, the adaptive response, mucAB and umuDC 
mutagenic processing, and glutathione bioactivation on the differential 
responses of ogt- and ogt+ bacteria were also studied. Mutation induction was 
monitored by measuring 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 dibromoethane) 
dependent on the alkyltransferase (ATase) encoded by the ogt gene. An increased 
mutagenic response to both dibromoalkanes was also seen in ogt- bacteria that 
overexpressed the ogt protein from a multicopy plasmid, indicating that the 
differences in mutability between ogt+ and ogt- bacteria were not dependent on 
the ogt- null allele carried by the defective strain. The ATase encoded by the 
constitutive ogt gene was more effective in promoting dibromoalkane mutagenicity 
than the ada ATase induced by exposure to low doses of a methylating agent. The 
mutagenicity promoted by the ogt ATase was dependent on both glutathione 
bioactivation and SOS mutagenic processing. To our knowledge, this paper 
presents for the first time evidence that DNA ATases, in particular the ATase 
encoded by the ogt gene, can increase the mutagenic effects of a DNA-damaging 
agent. The mechanism of this effect has yet to be established.