Staphylococcus aureus SigS induces expression of a regulatory protein pair that modulate its mRNA stability

SigS is the sole extracytoplasmic function sigma in S. aureus and is necessary for virulence, immune evasion, as well as surviving exposure to toxic chemicals and environmental stressors. Despite the contribution of SigS to a myriad of critical phenotypes, the downstream effectors of the SigS-dependent S. aureus pathogenesis, immune evasion, and stress response remain elusive. To address this knowledge gap, we analyzed the S. aureus transcriptome following transient over-expression of SigS. We identified a bi-cistronic transcript, up-regulated by 1000-fold, containing two mid-sized genes each containing single domains of unknown function (DUF). We renamed these genes sroA (SigS regulated orfA) and sroB (SigS regulated orfB). We demonstrated that the SigS regulation of the sroAB operon is direct using in vitro transcription analysis. Using northern blot analysis, we also demonstrated that SroA and SroB have opposing auto- regulatory functions on the transcriptional architecture of the sigS locus; with SroA stimulated SigS mRNA levels and SroB stimulating s750 (SigS antisense) levels. We hypothesized that these this opposing regulatory effects were due to a direct interaction. We demonstrated an interaction between SroA and SroB using an in-vivo surrogate genetics approach via Bacterial Two Hybrid. We demonstrated that the SroA effect on SigS is at the post-transcriptional level of mRNA stability, highlighting a mechanism likely used by S. aureus to tightly control SigS levels. Finally, we demonstrate that the sroAB locus promotes virulence in a female murine pneumonia model of infection.