Transcriptional regulator that controls expression of some virulence factors in a cell density-dependent manner. Acts as an activator of the gene encoding protein A (spa). Negatively regulates the expression of alpha-hemolysin (hla). Expressed at the early exponential growth phase, decreases through exponential phase and reaches a steady-state level at post-exponential phase. Repressed by MgrA and SarA. Activated by Rot and SarT. Transcription is also dependent on SigA and SigB factors. Is activated by SigB in strains harboring an intact sigB operon (rsbU, rsbV, rsbW, and sigB). Mutational analysis of the spa promoter suggests that SarS might stimulate spa transcription by competing with SarA. The regulatory events observed in strain NCTC 8325 are not representative of the events that occur in clinical isolates of S.aureus. Belongs to the SarA family.