The σB regulon in Staphylococcus aureus and its regulation

The Staphylococcus aureus genome codes for a sigma factor that shows close sequence similarity to the alternative sigma factor σB of Bacillus subtilis. However, of the proteins controlling the activity of σB in B. subtilis only RsbU, RsbV, and RsbW are encoded in the staphylococcal genome. Therefore, the regulation of the σB activity must differ between these two bacterial species. The present study was designed (i) to describe the σB regulon and (ii) to identify stimuli leading to an activation of σB-dependent transcription. All conditions under which σB was activated in S. aureus (heat shock, addition of MnCl2 or NaCl, alkaline shock) required the presence of RsbU, a positive regulator of σB. 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 σB in S. aureus. Moreover, ethanol, a strong inductor of σB activity in B. subtilis, also failed to induce σB in S. aureus. Expression of sigB and σB-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 σB under alkaline stress conditions. Interestingly, only 12% of these genes have an orthologue in the B. subtilis σB regulon, suggesting that the function of the σB regulon in S. aureus is different from that in B. subtilis. We could show that σB 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. σB-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 σB-dependent genes.