σΒ 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 sigma (B) activity in Staphylococcus aureus. The sigma (B) protein level and the transcription of two sigma (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 sigma (beta) activity. A deletion of the second gene (rsbV) resulted in a more dramatic reduction in the sigma (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 sigma (B)-dependent transcript initiated upstream of rsbV was 28-fold higher than the (sigma (A)-dependent transcript originating from the rsbU promoter. The level of the sigma (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 sigma (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 sigma (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 sigma (B) level. Collectively, our data indicated that RsbU and RsbV in S. aureus contributed to different levels of sigma (B) protein expression and varying sigma (B) activities. Although RsbV can activate sigma (B) independent of RsbU, RsbU remains the major activator of sigma (B) during acid stress.