RESPONSE OF REPAIR-COMPETENT AND REPAIR-DEFICIENT ESCHERICHIA-COLI TO3 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 in itiation codon of the lacZ' gene were used to transform Escherichia co li that were repair proficient or deficient in one or both of the E. c oli O-6-alkylguanine-DNA alkyltransferases, the uvr(ABC) excision repa ir system, the recA-mediated recombination system, or the methylation- directed mismatch repair system. Colonies were scored phenotypically f or adduct-induced mutations. With plasmids containing either e(6)G or b(6)G, the frequency of adduct-induced mutation was low and independen t of the repair proficiency of the strain transformed. Plasmids contai ning an m(6)G residue elicited similar responses in all but the mismat ch repair-deficient strain. The generally low mutagenicity of all the O-6-substituted guanines was interpreted as reflecting an adduct-induc ed arrest of replication of the modified strand while the unmodified c omplementary strand was replicated normally. Studies of the involvemen t of mismatch repair in m(6)G mutagenesis showed that m(6)G:T base pai rs were more readily processed than m(6)G:C base pairs, indicating tha t mismatch repair involving m(6)G residues occurs after replication. T hese data support a model in which the E. coil methylation-directed mi smatch 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-a lkylguanine-DNA alkyltransferase repair and excision repair mechanisms .