RESPONSE OF REPAIR-COMPETENT AND REPAIR-DEFICIENT ESCHERICHIA-COLI TO 3 O-6-SUBSTITUTED GUANINES AND INVOLVEMENT OF METHYL-DIRECTED MISMATCH REPAIR IN THE PROCESSING OF O-6-METHYLGUANINE RESIDUES

Plasmids containing a site-specifically incorporated O-6-methyl- (m(6)G), O-6-ethyl- (e(6)G), or O-6-benzylguanine (b(6)G) within the ATG initiation codon of the lacZ' gene were used to transform Escherichia coli that were repair proficient or deficient in one or both of the E. coli O-6-alkylguanine-DNA alkyltransferases, the uvr(ABC) excision repair system, the recA-mediated recombination system, or the methylation-directed mismatch repair system. Colonies were scored phenotypically for adduct-induced mutations. With plasmids containing either e(6)G or b(6)G, the frequency of adduct-induced mutation was low and independent of the repair proficiency of the strain transformed. Plasmids containing an m(6)G residue elicited similar responses in all but the mismatch repair-deficient strain. The generally low mutagenicity of all the O-6-substituted guanines was interpreted as reflecting an adduct-induced arrest of replication of the modified strand while the unmodified complementary strand was replicated normally. Studies of the involvement of mismatch repair in m(6)G mutagenesis showed that m(6)G:T base pairs were more readily processed than m(6)G:C base pairs, indicating that mismatch repair involving m(6)G residues occurs after replication. These data support a model in which the E. coli methylation-directed mismatch repair system diverts plasmids containing promutagenic m(6)G:T base pairs into replication-arrested complexes providing another line of defense against O-6-methylguanine mutagenicity in addition to O-6-alkylguanine-DNA alkyltransferase repair and excision repair mechanisms.