Spx is a global regulator that is widespread among the low G+C Gram-positive bacteria. Spx has been extensively studied in Bacillus subtilis, where it acts as an activator and a repressor of transcription in response to disulfide stress. Under non-stress conditions, Spx is rapidly degraded by the ClpXP protease. This degradation is enhanced by the YjbH adaptor protein. Upon disulfide stress, the amount of Spx rapidly increases due to a decrease in degradation. In the opportunistic pathogen Staphylococcus aureus, Spx is a global regulator influencing growth, biofilm formation and general stress protection, and cells lacking the spx gene exhibit poor growth also under non-stress conditions. To investigate the mechanism by which the activity of Spx is regulated we identified a homolog in S. aureus of the B. subtilis yjbH gene. The gene encodes a protein that shows approximately 30% sequence identity to YjbH of B. subtilis. Heterologous expression of S. aureus yjbH in a B. subtilis yjbH mutant restored Spx to wild type levels both under non-stress conditions and under conditions of disulfide stress. From these studies we conclude that the two YjbH homologues have a conserved physiological function. Accordingly, inactivation of yjbH in S. aureus increased the level of Spx protein and transcription of the Spx-regulated gene trxB. Notably, the yjbH mutant exhibited reduced growth and increased pigmentation, and both phenotypes were reversed by complementation of the yjbH gene.
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