GENETIC REQUIREMENTS AND MUTATIONAL SPECIFICITY OF THE ESCHERICHIA-COLI SOS MUTATOR ACTIVITY

To better understand the mechanisms of SOS mutagenesis in the bacteriu m Escherichia coli, we have undertaken a genetic analysis of the SOS m utator activity. The SOS mutator activity results from constitutive ex pression of the SOS system in strains carrying a constitutively activa ted RecA protein (RecA730). We show that the SOS mutator activity is n ot enhanced in strains containing deficiencies in the uvr ABC nucleoti de excision-repair system or the xth and nfo base excision-repair syst ems. Further, recA730-induced errors are shown to be corrected by the MutHLS-dependent mismatch-repair system as efficiently as the correspo nding errors in the rec(+) background. These results suggest that the SOS mutator activity does not reflect mutagenesis at so-called cryptic lesions but instead represents an amplification of normally occurring DNA polymerase errors. Analysis of the base-pair-substitution mutatio ns induced by recA730 in a mismatch repair-deficient background shows that both transition and transversion errors are amplified, although t he effect is much larger for transversions than for transitions. Analy sis of the mutator effect in various dnaE strains, including dnaE anti mutators, as well as in proofreading-deficient dnaQ (mutD) strains sug gests that in recA730 strains, two types of replication errors occur i n parallel: (i) normal replication errors that are subject to both exo nucleolytic proofreading and dnaE antimutator effects and (ii) recA730 -specific errors that are not susceptible to either proofreading or dn aE antimutator effects. The combined data are consistent with a model suggesting that in recA730 cells error-prone replication complexes are assembled at sites where DNA polymerization is temporarily stalled, m ost likely when a normal polymerase insertion error has created a poor ly extendable terminal mismatch. The modified complex forces extension of the mismatch largely at the exclusion of proofreading and polymera se dissociation pathways. SOS mutagenesis targeted at replication-bloc king DNA lesions likely proceeds in the same manner.