UNLABELLED: Staphylococcus aureus is capable of causing a remarkable spectrum of disease, ranging from mild skin eruptions to life-threatening infections. The survival and pathogenic potential of S. aureus depend partly on its ability to sense and respond to changes in its environment. Spx is a thiol/oxidative stress sensor that interacts with the C-terminal domain of the RNA polymerase RpoA subunit, leading to changes in gene expression that help sustain viability under various conditions. Using genetic and deep-sequencing methods, we show that spx is essential in S. aureus and that a previously reported Δspx strain harbored suppressor mutations that allowed it to grow without spx One of these mutations is a single missense mutation in rpoB (a P-to-L change at position 519 encoded by rpoB [rpoB-P519L]) that conferred high-level resistance to rifampin. This mutation alone was found to be sufficient to bypass the requirement for spx The generation of rifampin resistance libraries led to the discovery of an additional rpoB mutation, R484H, which supported strains with the spx disruption. Other rifampin resistance mutations either failed to support the Δspx mutant or were recovered at unexpectedly low frequencies in genetic transduction experiments. The amino acid residues encoded by rpoB-P519L and -R484H map in close spatial proximity and comprise a highly conserved region of RpoB. We also discovered that multicopy expression of either trxA (encoding thioredoxin) or trxB (encoding thioredoxin reductase) supports strains with the deletion of spx Our results reveal intriguing properties, especially of RNA polymerase, that compensate for the loss of an essential gene that is a key mediator of diverse processes in S. aureus, including redox and thiol homeostasis, antibiotic resistance, growth, and metabolism.

IMPORTANCE: The survival and pathogenicity of S. aureus depend on complex genetic programs. An objective for combating this insidious organism entails dissecting genetic regulatory circuits and discovering promising new targets for therapeutic intervention. In this study, we discovered that Spx, an RNA polymerase-interacting stress regulator implicated in many stress responses in S. aureus, including responses to oxidative and cell wall antibiotics, is essential. We describe two mechanisms that suppress the lethality of spx disruption. One mechanism highlights how only certain rifampin resistance-encoding alleles of RpoB confer new properties on RNA polymerase, with important mechanistic implications. We describe additional stress conditions where the loss of spx is deleterious, thereby highlighting Spx as a multifaceted regulator and attractive drug discovery target.