The Characterization of the Transcription Factor Msab and Its Role in Staphylococcal Virulence

Staphylococcus aureus is a common human pathogen that is responsible for a wide range of infections, ranging from relative minor skin infections to life-threatening disease such as bacteremia, septicemia, and endocarditis. S. aureus possesses many different virulent factors that aid in its ability to cause this wide array of infections. One major virulence factor includes the production of capsular polysaccharide (CP). The production of CP plays a major role in the virulence response during infection specifically by providing S. aureus an antiphagocytic mechanism that allows the pathogen to evade phagocytosis during an infection. S. aureus has developed complex genetic regulatory systems responsible for controlling virulence factors including CP. The production of CP is encoded by a 16-gene operon, which is regulated in response to several environmental stimuli. In detail, CP is produced in the late- and post-exponential growth phases in vitro, but not in the early- or mid-exponential growth phases. Several genetic regulators have been identified to be involved in controlling CP production, but the mechanisms involved in this regulation of cap are still poorly understood. In this work, we describe that the msaABCR operon significantly regulates CP production. We found that a protein product of the msaABCR transcript, MsaB, directly binds to a nucleotide repeat upstream of the promoter region of the cap operon, termed MsaB binding site or (MBS), as a transcriptional activator of CP production. Furthermore, we show that even though MsaB is expressed throughout all four exponential growth phases, it only activates CP production in the late- and post-exponential growth phases. Several other regulators have also been shown to directly bind within the cap promoter region. We examined potential interactions between MsaB and two other nutrient-sensing regulators, CodY and CcpE, binding the cap promoter. From the findings of this work, we have concluded that MsaB (activator) and CodY (repressor) have nutrient-dependent competitive interactions binding to the cap promoter. Findings from this work has led us to explore targeting and inhibiting MsaB transcriptional regulation as a potential novel therapeutic target in combating S. aureus