Searching for small σB-regulated genes in Staphylococcus aureus

In recent years, small RNAs (sRNAs) have been identified as important regulators of gene expression in bacteria. Most sRNAs are encoded from intergenic regions and are only expressed under highly specific growth conditions. In Staphylococcus aureus, the alternative sigma factor, σB, is known to contribute to the overall stress response, antibiotic resistance, and virulence. The σB regulon in S. aureus is well described and comprises approximately 200 annotated genes, including several genes encoding virulence factors. In the present study, we have identified three novel σB-dependent transcripts encoded from genomic regions previously annotated as intergenic. All three transcripts, named SbrA, SbrB, and SbrC, are highly conserved in S. aureus, and we confirmed their presence in four different isolates (SH1000, Newman, COL, and UAMS-1). Curiously, two of these genes (sbrA and sbrB) were found to contain open reading frames encoding small, highly basic peptides that are conserved among Staphylococci. The third transcript (SbrC) did not contain any likely open reading frame and thus constitute a genuine non-coding sRNA. The functions of these genes are currently unknown but are likely to be important for the σB-mediated response of S. aureus to adverse conditions.