CroSR391, an ortholog of the λ Cro repressor, plays a major role in suppressing polVR391-dependent mutagenesis

When subcloned into low-copy-number expression vectors, rumAB, encoding polV(R391) (RumA ' B-2), is best characterized as a potent mutator giving rise to high levels of spontaneous mutagenesis in vivo. This is in dramatic contrast to the poorly mutable phenotype when polV(R391) is expressed from the native 88.5 kb R391, suggesting that R391 expresses trans-acting factors that suppress the expression and/or the activity of polV(R391). Indeed, we recently discovered that SetR(R391), an ortholog of lambda cI repressor, is a transcriptional repressor of rumAB. Here, we report that CroS(R391), an ortholog of lambda Cro, also serves as a potent transcriptional repressor of rumAB. Levels of RumA are dependent upon an interplay between SetR(R391) and CroS(R391), with the greatest reduction of RumA protein levels observed in the absence of SetR(R391) and the presence of CroS(R391). Under these conditions, CroS(R391) completely abolishes the high levels of mutagenesis promoted by polV(R391) expressed from low-copy-number plasmids. Furthermore, deletion of croS(R391) on the native R391 results in a dramatic increase in mutagenesis, indicating that CroS(R391) plays a major role in suppressing polV(R391) mutagenesis in vivo. Inactivating mutations in CroS(R391) therefore have the distinct possibility of increasing cellular mutagenesis that could lead to the evolution of antibiotic resistance of pathogenic bacteria harboring R391.

Automated summary

RumAB is a potent mutator in low-copy-number expression vectors but poorly mutable in the native 88.5 kb R391, with SetR(R391) and CroS(R391) playing crucial roles in regulating RumA levels and mutagenesis. Inactivating mutations in CroS(R391) have the potential to increase cellular mutagenesis and evolution of antibiotic resistance in pathogenic bacteria harboring R391.