Constitutive expression of SOS functions and modulation of mutagenesis resulting from resolution of genetic instability at or near the recA locus of Escherichia coli

Summary Cellular activities normally inducible by DNA damage (SOS functions) are expressed, without DNA damage, in recA441 (formerly tif-1) mutants of Escherichia coli at 42° C but not at 30° C. We describe a strain (SC30) that expresses SOS functions (including mutator activity, prophage induction and copious synthesis of recA protein) constitutively at both temperatures. SC30 is one of four stable subclones (SC strains) derived from an unstable recombinant obtained in a conjugation between a recA441 K12 donor and a recA⁺ B/r-derived recipient. SC30 does not owe its SOS-constitutive phenotype to a mutation in the lexA gene (which codes the repressor of recA and other DNA damage-inducible genes), since it is lexA⁺. Each of the SC strains expresses SOS functions in a distinctively anomalous way. We show that the genetic basis for the differences in SOS expression among the SC strains is located at or very near the recA locus. We propose that resolution of genetic instability in this region, in the original recombinant, has altered the pattern of expression of SOS functions in the SC strains.