Identification of psiB genes of plasmids F and R6-5. Molecular basis for psiB enhanced expression in plasmid R6-5

Abstract

PsiB protein of plasmid R6–5 inhibits the induction of the SOS pathway. The F sex factor also carries a psiB gene homologous to that of R6–5. Yet, it fails to inhibit SOS induction. In order to solve this difference, we characterized the psiB genes of R6–5 and F. We found that (i) the sequences of the two psiB genes share extensive homology the predicted amino acid sequences of the two proteins differing by 5 residues, (ii) the expression of R6–5 psiB is 4 times higher than F psiB gene, (iii) in plasmid R6–5, a TnlO transposon upstream from the psiB gene enhances psiB expression. Hence, the F sex factor may be unable to prevent SOS induction for two non-exclusive reasons: (i) F PsiB protein, being slightly different from R6–5, may be less active, (ii) the level of synthesis of F PsiB protein may be insufficient to prevent SOS induction.