PsiB, an anti‐SOS protein, is transiently expressed by the F sex factor during its transmission to an Escherichia coli K‐12 recipient

PsiB, an anti‐SOS protein, shown previously to prevent activation of RecA protein, was purified from the crude extract of PsiB overproducing cells. PsiB is probably a tetrameric protein, whose subunit has a sequence‐deduced molecular mass of 15741 daltons. Using an immuno‐assay with anti‐PsiB antibodies, we have monitored PsiB cell concentrations produced by F and R6‐5 piasmids: the latter type produces a detectable level of PsiB protein while the former does not. The discrepancy can be assigned to a Tn 10 outgoing promoter located upstream of psiB. When we inserted a Tn 10 promoter upstream of F psiB, the F PsiB protein concentration reached the level of R6‐5 PsiB. We describe here the physiological role that PsiB protein may have in the cell and how it causes an anti‐SOS function. We observed that PsiB protein was transiently expressed by a wild‐type F sex factor during its transmission to an Escherichia coli K‐12 recipient, In an F× F cross, PsiB concentration increased at least 10‐fold in F recipient bacteria after 90 minutes and declined thereafter; the psiB gene may be repressed when F plasm id replicates vegetatively. PsiB protein may be induced zygotically so as to protect F single‐stranded DNA transferred upon conjugation. PsiB protein, when overproduced, may interfere with RecA protein at chromosomal single‐stranded DNA sites generated by discontinuous DNA replication, thus causing an SOS inhibitory pheno‐type. Copyright © 1992, Wiley Blackwell. All rights reserved