Identification and characterization of SarH1, a new global regulator of virulence gene expression in Staphylococcus aureus

The global regulators agr (accessory gene regulator) and sarA (staphylococcal accessory regulator) have been reported to be both activators and repressors of virulence gene expression in Staphylococcus aureus. How the effector of the agr system, RNAIII, interacts with target gene promoters is unknown. SarA, on the other hand, is a DNA-binding protein, which binds to conserved DNA motifs immediately upstream of both positively and negatively regulated promoters. Here, we searched for additional regulators that could explain the differential effects of RNAIII and SarA. Four differently regulated genes (hla, alpha-toxin; hld, RNAIII; spa, protein A; ssp, serine protease) were analysed for binding of potential regulatory proteins to the corresponding promoter DNA fragments, linked to magnetic beads. One protein (29 kDa), with affinity for all four promoters, showed a high degree of similarity to SarA and was named SarH1 (Sar homologue 1). Expression of sarH1 was strongly repressed by sarA and agr. Analysis of hla, hld, ssp and spa mRNAs in sarH1, sarA and agr mutants, and in sarA/sarH1 and agr/sarH1 double mutants, revealed that sarH1 has a strong repressive effect on hla and an activating effect on spa transcription. SDS-PAGE analysis of secreted proteins from the different mutants showed that the production of several other exoproteins was affected by sarH1. In conclusion, we show that both the agr-dependent suppression of protein A production and the sarA-dependent stimulation of alpha-toxin production is mediated via a new regulator, SarH1, which belongs to a family of Sar homologues.