THE MUTATIONAL SPECIFICITY OF 2 ESCHERICHIA-COLI DNAE-ANTIMUTATOR ALLELES AS DETERMINED FROM LACI MUTATION SPECTRA

In a companion study we have described the isolation of a series of mu tants of Escherichia coli that replicate their DNA with increased fide lity. These mutants carry a mutation in the dnaE gene, encoding the al pha (polymerase) subunit of DNA polymerase III holoenzyme, which is re sponsible for the faithful replication of the bacterial chromosome. Th e mutants were detected as suppressors of the high mutability of a mut L strain (defective in postreplicative mismatch correction), in which mutations may be considered to arise predominantly from errors of DNA replication. To investigate the specificity of these antimutator effec ts, we have analyzed spectra of forward mutations in the N-terminal pa rt of the lacI gene (i(-d) mutations) for two of the mutL dnaE derivat ives (dnaE911 and dnaE915), as well as the control mutL strain. DNA se quencing of over 600 mutants revealed that in the mutL background both antimutator alleles reduce specifically transition mutations (A.T --> G.C and G.C --> A.T). However, the two alleles behave differently in this respect. dnaE911 reduces A.T --> G.C more strongly than it does G .C --> A.T, whereas the reverse is true for dnaE915. Second, dnaE911 d oes not appear to affect either transversion or frameshift mutations, whereas dnaE915 displays a distinct mutator effect for both. This muta tor effect of dnaE915 for frameshift mutations was confirmed by the fr equency of reversion of the trpE9777 frameshift mutation. The discover y that dnaE antimutator alleles possess distinct specificities support s the notion that DNA polymerases discriminate against errors along mu ltiple pathways and that these pathways can be influenced independentl y.