Staphylococcus aureus capsule is an important virulence factor that is regulated by a large number of regulators. Capsule genes are expressed from a major promoter upstream of the cap operon. A 10-bp inverted repeat (IR) located 13 bp upstream of the ؊35 region of the promoter was previously shown to affect capsule gene transcription. However, little is known about transcriptional activation of the cap promoter. To search for potential proteins which directly interact with the cap promoter region (Pcap), we directly analyzed the proteins interacting with the Pcap DNA fragment from shifted gel bands identified by electrophoretic mobility shift assay. One of these regulators, RbsR, was further characterized and found to positively regulate cap gene expression by specifically binding to the cap promoter region. Footprinting analyses showed that RbsR protected a DNA region encompassing the 10-bp IR. Our results further showed that rbsR was directly controlled by SigB and that RbsR was a repressor of the rbsUDK operon, involved in ribose uptake and phosphorylation. The repression of rbsUDK by RbsR could be derepressed by D-ribose. However, D-ribose did not affect RbsR activation of capsule.
IMPORTANCEStaphylococcus aureus is an important human pathogen which produces a large number of virulence factors. We have been using capsule as a model virulence factor to study virulence regulation. Although many capsule regulators have been identified, the mechanism of regulation of most of these regulators is unknown. We show here that RbsR activates capsule by direct promoter binding and that SigB is required for the expression of rbsR. These results define a new pathway wherein SigB activates capsule through RbsR. Our results further demonstrate that RbsR inhibits the rbs operon involved in ribose utilization, thereby providing an example of coregulation of metabolism and virulence in S. aureus. Thus, this study further advances our understanding of staphylococcal virulence regulation.
Staphylococcus aureus produces a large number of virulence factors that endow the organism with the ability to cause a wide range of diseases in humans and animals. The expression of virulence genes is controlled by an equally impressive number of regulators forming a complex regulatory network (1-3). Although the regulation of virulence genes has been the subject of extensive studies recently, our knowledge of the virulence regulatory network in S. aureus is still fragmented. To further understand virulence regulation, we have been studying the S. aureus virulence regulatory network by employing capsule as a model virulence factor (4-7). Capsule is an antiphagocytic virulence factor, and the majority of S. aureus strains produce either type 5 or type 8 capsule (8, 9). Sixteen cap genes, which are organized as a long operon, are required for the biosynthesis of either type of capsule (10). The genetic loci for the type 5 and type 8 capsules (cap5 and cap8) are allelic, with the four genes in the middle of the operon being type specific...