ς(B) Activity in Staphylococcus aureus Is Controlled by RsbU and an Additional Factor(s) during Bacterial Growth

Two genes of the sigB operon, rsbU and rsbV, were deleted in an rsbU(+) strain (FDA486) to evaluate the contribution of these two genes to ς(B) activity in Staphylococcus aureus. The ς(B) protein level and the transcription of two ς(B)-dependent promoters (sigB and sarA P3 transcripts) were analyzed in the constructed mutants. A deletion of the first gene (rsbU) within the sigB operon led only to a partial reduction in ς(β) activity. A deletion of the second gene (rsbV) resulted in a more dramatic reduction in the ς(B) protein level and its activity than did the deletion of rsbU, thus indicating that RsbV can be activated independent of RsbU. In the parental strain, the ς(B)-dependent transcript initiated upstream of rsbV was 28-fold higher than the ς(A)-dependent transcript originating from the rsbU promoter. The level of the ς(B)-dependent transcript decreased up to 50% in the rsbU mutant and up to 90% in the rsbV mutant compared with the transcript in the wild type. The yellow pigment of S. aureus colonies, a ς(B)-dependent phenotype, was partially reduced in the rsbU and rsbV mutants, whereas alpha-hemolysin was increased. Additionally, the sarA P3 promoter activity of the parental strain was induced to a higher level in response to pH 5.5 than was that of the rsbU or rsbV mutant, indicating that RsbU is the major activator of the ς(B) response to acid stress. Using a tetracycline-inducible system to modulate the expression of RsbW, we progressively repressed pigment production, presumably by reducing the free ς(B) level. Collectively, our data indicated that RsbU and RsbV in S. aureus contributed to different levels of ς(B) protein expression and varying ς(B) activities. Although RsbV can activate ς(B) independent of RsbU, RsbU remains the major activator of ς(B) during acid stress.