The Staphylococcus aureus rsbW (orf159) gene encodes an anti-sigma factor of SigB

SigB, a newly discovered alternative sigma factor of Staphylococcus aureus, bas been shown to play an important role in stress responses and the regul ation of virulence factors. The rsbW (orf159) gene is immediately upstream, of sigB. Its gene product is homologous to Bacillus subtilis RsbW which un der appropriate conditions binds to B. subtilis SigB and functions as an an ti-sigma factor or negative posttranslational regulator. To define the func tion of S. aureus RsbW, both the S. aureus SigB and RsbW proteins were expr essed in Escherichia coli and purified. Cross-linking experiments with thes e purified proteins revealed that RsbW was capable of specific binding to S igB. In an in vitro transcription runoff assay, RsbW prevented SigB-directe d transcription from the sar P3 promoter, a known SigB-dependent promoter,a nd the inhibitory activity of RsbW was found to be concentration dependent. We also identified SigB promoter consensus sequences upstream of the genes encoding alkaline shock protein 23 and coagulase and have demonstrated Sig B and RsbW dependence for the promoters in vitro. These results show that R sbW is a protein sequestering anti-sigma factor of S. aureus SigB and sugge st that SigB activity in S. aureus is regulated posttranslationally.