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 regulator (agr ) suggests that sar is the primary regulatory element controlling transcription of the collagen adhesin gene (cna ) and that the regulatory effect of sar is independent of the interaction between SarA and agr. To test this hypothesis, we cloned the regions encoding each of the overlapping sar transcripts, all of which include the sarA open reading frame (ORF), and introduced each clone into cna‐positive sar and agr mutants. The introduction of each clone restored the expected sar transcripts and the temporal pattern of sar transcription. The introduction of each clone also complemented the defect in cna transcription and restored collagen binding to wild‐type levels. This was true even when the clones were introduced into a sar/agr double mutant. These results confirm the hypothesis that the sar‐mediated regulation of cna transcription occurs via an agr‐independent pathway. Direct 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 correlation between sar transcription and the production of SarA. Western blot analysis of two wild‐type strains indicated that SarA was produced in indistinguishable amounts during both the exponential and the post‐exponential growth phases.