Characterization of sarR, a modulator of sar expression in Staphylococcus aureus

The expression of virulence determinants in Staphylococcus aureus is contro lled by global regulatory loci (e.g., sar and agr). The sar locus is compos ed of three overlapping transcripts (sar P1, P3, and P2 transcripts from P1 , P3, and P2 promoters, respectively), all encoding the 372-bp sarA gene. T he level of SarA, the major regulatory protein, is partially controlled by the differential activation of sar promoters. We previously partially purif ied a similar to 12 kDa protein with a DNA-specific column containing a sar P2 promoter fragment. In this study, the putative gene, designated sarR, w as identified and found to encode a 13.6-kDa protein with homology to SarA, Transcriptional and immunoblot studies revealed the sarR gene to be expres sed in other staphylococcal strains. Recombinant SarR protein bound sar P1, P2, and P3 promoter fragments in gel shift and footprinting assays. A sarR mutant expressed a higher level of P1 transcript than the parent, as confi rmed by promoter green fluorescent protein fusion assays. As the pi transcr ipt is the predominant sar transcript, we confirmed that the sarR mutant ex pressed more SarA than the parental strain. We thus proposed that SarR is a regulatory protein that binds to the sar promoters to down-regulate P1 tra nscription and the ensuing SarA protein expression.