The staphylococcal accessory regulator (sar) represses transcription of the Staphylococcus aureus collagen adhesin gene (cna) in an agr-independent manner

Comparison of Staphylococcus aureus strains carrying mutations inactivating the staphylococcal accessory regulator (sar) and/or the accessory gene reg ulator (agr) suggests that sar is the primary regulatory element controllin g transcription of the collagen adhesin gene (cna) and that the regulatory effect of sar is independent of the interaction between SarA and agr. To te st this hypothesis, we cloned the regions encoding each of the overlapping sar transcripts, all of which include the sarA open reading frame (ORF), an d introduced each clone into cna-positive sar and agr mutants. The introduc tion of each clone restored the expected sar transcripts and the temporal p attern of sar transcription. The introduction of each clone also complement ed the defect in cna transcription and restored collagen binding to wild-ty pe levels. This was true even when the clones were introduced into a sar/ag r double mutant. These results confirm the hypothesis that the sar-mediated regulation of cna transcription occurs via an agr-independent pathway. Dir ect evidence supporting this hypothesis comes from electrophoretic mobility shift assays demonstrating that SarA exhibits high-affinity binding to cis elements upstream of the cna structural gene. We also examined the correla tion between sar transcription and the production of SarA. Western blot ana lysis of two wild-type strains indicated that SarA was produced in indistin guishable amounts during both the exponential and the post-exponential grow th phases.