SetR a negative transcriptional regulator of the integrating conjugative element 391 mutagenic response.

The integrating conjugative element ICE391 (formerly known as IncJ R391) harbors an error-prone DNA polymerase V ortholog, polV, encoded by the ICE391 rumAB operon. polV and its orthologs have previously been shown to be major contributors to spontaneous and DNA damage-induced mutagenesis in vivo. As a result, multiple levels of regulation are imposed on the polymerases so as to avoid aberrant mutagenesis. We report here, that the mutagenesis-promoting activity of polV is additionally regulated by a transcriptional repressor encoded by SetR, since Escherichia coli expressing SetR demonstrated reduced levels of polV-mediated spontaneous mutagenesis relative to cells lacking SetR. SetR regulation was shown to be specific for the rumAB operon and in vitro studies with highly purified SetR revealed that under alkaline conditions, as well as in the presence of activated RecA, SetR undergoes a self-mediated cleavage reaction that inactivates repressor functions. Conversely, a non-cleavable SetR mutant capable of maintaining repressor activity, even in the presence of activated RecA, exhibited low levels of polV-dependent mutagenesis. Electrophoretic mobility shift assays revealed that SetR acts as a transcriptional repressor by binding to a site overlapping the -35 region of the rumAB operon promoter. Our study therefore provides evidence indicating that SetR acts as a transcriptional repressor of the ICE391-encoded mutagenic response.