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 incl uding alpha-hemolysin-, protein A-, and fibronectin-binding proteins is con trolled by global regulatory loci such as sar and agr. In addition to contr olling 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 enco ded within sar, binds to a conserved sequence, homologous to the SarA-bindi ng site on the agr promoter, upstream of the -35 promoter boxes of several target genes including hla (alpha-hemolysin gene), spa (protein A gene), fn b (fibronectin-binding protein genes), and sec (enterotoxin C gene). Deleti on 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 fusi on assay, became derepressed in a wild type strain containing a shuttle pla smid in which the SarA recognition site had been deleted from the spa promo ter region. However, DNase I footprinting assays demonstrated that the SarA -binding region on the spa and hla promoter is more extensive than the pred icted consensus sequence, thus raising the possibility that the consensus s equence is an activation site within a larger binding region. Because the s ar and agr regulate an assortment of virulence factors in S. aureus, we pro pose, based on our data, a unifying hypothesis for virulence gene activatio n in S. aureus whereby SarA is a regulatory protein that binds to its conse nsus SarA recognition motif to activate (e.g. hla) or repress (e.g. spa) th e transcription of sar target genes, thus accounting for both agr-dependent and agr-independent mode of regulation.