SarA, a global regulator of virulence determinants in Staphylococcus aureus, binds to a conserved motif essential for sar-dependent gene regulation

The expression of many virulence determinants in Staphylococcus aureus including alpha-hemolysin-, protein A-, and fibronectin-binding proteins is controlled by global regulatory loci such as sar and agr. In addition to controlling target gene expression via agr (e.g. alpha-hemolysin), the sar locus can also regulate target gene transcription via agr-independent mechanisms. In particular, we have found that SarA, the major regulatory protein encoded within sar, binds to a conserved sequence, homologous to the SarA-binding site on the agr promoter, upstream of the -35 promoter boxes of several target genes including hla (alpha-hemolysin gene), spa (protein A gene), fnb (fibronectin-binding protein genes), and sec (enterotoxin C gene). Deletion of the SarA recognition motif in the promoter regions of agr and hla in shuttle plasmids rendered the transcription of these genes undetectable in agr and hla mutants, respectively. Likewise, the transcription activity of spa (a gene normally repressed by sar), as measured by a XylE reporter fusion assay, became derepressed in a wild type strain containing a shuttle plasmid in which the SarA recognition site had been deleted from the spa promoter region. However, DNase I footprinting assays demonstrated that the SarA-binding region on the spa and hla promoter is more extensive than the predicted consensus sequence, thus raising the possibility that the consensus sequence is an activation site within a larger binding region. Because the sar and agr regulate an assortment of virulence factors in S. aureus, we propose, based on our data, a unifying hypothesis for virulence gene activation in S. aureus whereby SarA is a regulatory protein that binds to its consensus SarA recognition motif to activate (e.g. hla) or repress (e.g. spa) the transcription of sar target genes, thus accounting for both agr-dependent and agr-independent mode of regulation.