The σB regulon in Staphylococcus aureus and its regulation

Abstract

The Staphylococcus aureus genome codes for a sigma factor that shows close sequence similarity to the alternative sigma factor σ B of Bacillus subtilis. However, of the proteins controlling the activity of σ B in B. subtilis only RsbU, RsbV, and RsbW are encoded in the staphylococcal genome. Therefore, the regulation of the σ B activity must differ between these two bacterial species. The present study was designed (i) to describe the σ B regulon and (ii) to identify stimuli leading to an activation of σ B-dependent transcription. All conditions under which σ B was activated in S. aureus (heat shock, addition of MnCl 2 or NaCl, alkaline shock) required the presence of RsbU, a positive regulator of σ B. In contrast to B. subtilis, a drop in the cellular ATP level caused by the addition of carbonyl cyanide m-chlorophenylhydrazone did not lead to an activation of σ B in S. aureus. Moreover, ethanol, a strong inductor of σ B activity in B. subtilis, also failed to induce σ B in S. aureus. Expression of sigB and σ B-dependent genes was enhanced following entry into stationary phase of cells grown in complex medium (LB medium). Our DNA microarray data indicated that 122 genes are positively regulated by σ B under alkaline stress conditions. Interestingly, only 12% of these genes have an orthologue in the B. subtilis σ B regulon, suggesting that the function of the σ B regulon in S. aureus is different from that in B. subtilis. We could show that σ B of S. aureus, in contrast to B. subtilis, may have a function in more basic cellular processes such as cell envelope composition, membrane transport processes and intermediary metabolism. σ B-dependent genes identified by the DNA microarray approach were subjected to detailed transcriptional analyses using primer extension and Northern blot techniques. These analyses confirmed our DNA microarray data and furthermore revealed different regulatory groups of σ B-dependent genes.