Roles of recA mutant allele (recA495) in frameshift mutagenesis.

The chemical carcinogen N-acetoxy-N-2-acetylaminofluorene (N-AcO-AAF) induces frameshift mutations located within two types of specific sequences (mutation hot spots): i) contiguous guanine sequences and ii) alternating GC sequences. The genetic requirements of these frameshift events were investigated using specific reversion assays. AAF-induced -2 frameshift mutagenesis at alternating GC sequences is peculiar in that it requires a LexA- controlled function which is not UmuDC and occurs in the absence of RecA protein, provided the SOS regulon is derepressed. Moreover, the non-activated form of the RecA protein was shown to act as an inhibitor in this mutation pathway. As we were interested in elucidating this mutation pathway, we have developed a convenient spot reversion assay specific for the detection of this class of mutations. This assay allowed us to isolate E coli mutants affected either in repair or mutagenesis functions. One particular mutant, recA495, is very sensitive to UV and N-AcO-AAF, and is defective in recombination and UV mutagenesis. The RecA495 protein exhibits very low binding to both single- and double-stranded DNA. We show that when the SOS regulon is derepressed, the recA495 allele has two contrasting roles in frameshift mutagenesis: i) it prevents the induction of -1 frameshift mutations at repetitive sequences and ii) it is permissive for the induction of -2 frameshift mutations within alternating GC sequences.