Bacterial and mammalian DNA alkyltransferases sensitize Escherichia coli to the lethal and mutagenic effects of dibromoalkanes

Here we confirm and extend our previous studies demonstrating that themutagenic potency of 1,2-dibromoethane (DBE) and dibromomethane (DBM) ismarkedly enhanced (not prevented) in bacteria expressing the O6-alkylguanine-DNA alkyltransferase (ATase) encoded by the Escherichia coliogt gene. We demonstrate that, in close parallel with mutagenesis, the OgtATase sensitizes the bacteria to the lethal effects of these carcinogens,suggesting that one or more of the potentially mutagenic lesions induced byDBE and DBM in the presence of Ogt has additional lethal capacity. Wefurther demonstrate that the sensitization to both lethality andmutagenesis by DBE and DBM is a property shared by other DNAalkyltransferases. This objective was accomplished by quantifying theinduction of mutations and lethal events in ogt- ada- E. coli expressing anexogenous bacterial or mammalian ATase from a multicopy plasmid. Mammalianrecombinant ATases enhanced the lethal and mutagenic actions of DBE andsuppressed the lack of sensitivity of the vector- transformed bacteria toDBM. In most cases the order of effectiveness of the ATases ranked: murine> human > Ogt > rat. Further comparisons included the full-lengthAda ATase from E. coli and a truncated Ada version (T-ada) that retains theO6-methylguanine binding domain of the protein. The full-length Ada ATasewas effective in enhancing the lethality but not the mutagenicity inducedby DBE and DBM. The T-ada ATase provided less sensitization than Ada tolethality by DBE, but of the three bacterial ATases T-ada yielded thehighest sensitization to mutagenesis by this compound. T-ada and Ada ATaseswere in general less effective than the mammalian versions, with theexception of the rat recombinant ATase. The effectiveness of the differentmammalian and bacterial ATases in promoting the deleterious actions ofdibromoalkanes was compared with the effectiveness of these proteins insuppressing the lethal and mutagenic effects induced byN-nitroso-N-methylurea. The ability to sensitize E. coli to the lethal andmutagenic effects of DBE and DBM seems restricted to DNA alkyltransferase,since overexpression of thioredoxin (Trx) or glutaredoxin (Grx1) in ogt-ada- cells showed no effect, in spite of the reported potential ofbioactive dihaloethane- derived species to alkylate Trx.