PsiB polypeptide prevents activation of RecA protein in Escherichia coli.

We further characterize a novel plasmid function preventing SOS induction called Psi (Plasmid SOS Inhibition). We show that Psi function is expressed by psiB, a gene located at coordinate 54.9 of plasmid R6-5 and near oriT, the origin of conjugal transfer. Deletions and amber mutations of the psiB gene permitted us to demonstrate that PsiB polypeptide (apparent molecular weight, 12 kDa) is responsible for Psi function. PsiB protein prevents recA730-promoted mutagenesis and intra-chromosomal recombination but not recombination following conjugation. Overproduction of PsiB protein sensitizes the host cell to UV irradiation. We propose that PsiB polypeptide has an anti-SOS action by inhibiting activation of RecA protein, thus preventing the occurrence of LexA-controlled functions.