Global regulator with both positive and negative effects that controls expression of several virulence factors and biofilm formation process in a cell density-dependent manner. In a strain-dependent manner plays a role in multidrug resistance mechanism. Is required for transcription of primary transcripts RNAII and RNAIII generated by agr (virulence accessory gene regulator) locus. Acts as a transcriptional activator of the genes encoding, among others, for fibronectin binding proteins (fnbA and fnbB), hemolysins (hla, hld, hlgB and hlgC), serine proteases (splA, splB, splD and splF) and of the bap gene, which is essential for biofilm development in some strains. Negatively regulates the expression of the genes for protein A (spa), lipase (lip), thermonuclease (nuc), immunodominant staphylococcal antigen B (isaB), staphylococcal serine and cysteine proteases (sspA and sspB), staphostatin B (sspC), metalloprotease aureolysin (aur) and collagen adhesin (cna). Probably activates the development of biofilm by both enhancing the ica operon transcription and suppressing the transcription of either a protein involved in the turnover of PIA/PNAG or a repressor of its synthesis, whose expression would be sigma-B-dependent. Homodimer. Expressed at the exponential growth phase and maximally induced during the transition from late exponential phase to stationary phase. Repressed by SarR and activated by two-component regulatory system ArlS/ArlR. Activated by SigB in strains harboring an intact sigB operon (rsbU, rsbV, rsbW, and sigB). Transcription is also attenuated by MsrR. Can regulate transcription of some virulence factors (hemolysins, staphylococcal serine and cysteine proteases, lipase, protein A and metalloprotease aureolysin) via either agr-dependent and/or agr-independent pathway. Agr and SarA have opposing effects on the expression of sspA, sspB, sspC, lip and aur. The 11 C-terminal amino acids are absent in substrain NCTC 8325 / RN6390 (AAB05396) due to a nonsense mutation (UGA). However, this truncated SarA protein is functional. The regulatory events observed in strain NCTC 8325 / RN6390 are not representative of the events that occur in clinical isolates of S.aureus. Strains NCTC 8325 and NCTC 8325 / RN6390 are able to infect and kill Arabidopsis thaliana. This effect is attenuated in sarA and agr mutant backgrounds. Belongs to the SarA family.