Enhanced generation of A:T->T:A transversions in a recA730 lexA51(Def) mutant of Escherichia coli

RecA730 belongs to a class of mutant RecA protein that is often referred to as RecA*, since it is constitutively activated for coprotease functions in the absence of exogenous DNA-damage. Escherichia coli strains carrying recA730 (or other recA* alleles) exhibit dramatic increases in SOS-dependent spontaneous mutator activity. We have analyzed the specificity of this mutator phenotype by employing F'-plasmids carrying a set of mutant lacZ genes that can individually detect two types of transitions, four types of transversions, and four kinds of specific frameshift events. Analysis revealed that most of the spontaneous mutagenesis in a recA730 lexA5I(Def) strain (which expresses derepressed levels of all LexA-regulated proteins) can be attributed to a specific increase in A:T --> T:A, A:T --> C:G and G:C --> T:A transversions, with the A:T --> T:A transversions occurring most frequently. These transversion events were completely abolished in a Delta umuDC strain, indicating that the functionally active UmuD'C proteins are normally required for their generation. The spectrum obtained was similar to that of strains with a defect in the epsilon (3' --> 5' proofreading) subunit of DNA polymerase III. Such an observation raises the possibility that the wild-type epsilon protein is in activated in strains expressing the RecA730 and UmuD'C proteins.