Opposing roles of σB and σB-controlled SpoVG in the global regulation of esxA in Staphylococcus aureus

Background: The production of virulence factors in Staphylococcus aureus is tightly controlled by a complex web of interacting regulators. EsxA is one of the virulence factors that are excreted by the specialized, type VII-like Ess secretion system of S. aureus. The esxA gene is part of the sigma(B)-dependent SpoVG subregulon. However, the mode of action of SpoVG and its impact on other global regulators acting on esxA transcription is as yet unknown. Results: We demonstrate that the transcription of esxA is controlled by a regulatory cascade involving downstream sigma(B)-dependent regulatory elements, including the staphylococcal accessory regulator SarA, the ArlRS two-component system and SpoVG. The esxA gene, preceding the ess gene cluster, was shown to form a monocistronic transcript that is driven by a sigma(A) promoter, whereas a putative sigma(B) promoter identified upstream of the sigma(A) promoter was shown to be inactive. Transcription of esxA was strongly upregulated upon either sarA or sigB inactivation, but decreased in agr, arlR and spoVG single mutants, suggesting that agr, ArlR and SpoVG are able to increase esxA transcription and relieve the repressing effect of the sigma(B)-controlled SarA on esxA. Conclusion: SpoVG is a sigma(B)-dependent element that fine-tunes the expression of esxA by counteracting the sigma(B)-induced repressing activity of the transcriptional regulator SarA and activates esxA transcription.