MsaB activates capsule production at the transcription level in Staphylococcus aureus

Abstract

Staphylococcus aureus produces several virulence factors that allow it to cause a variety of infections. One of the major virulence factors is the capsule, which contributes to the survival of the pathogen within the host as a way to escape phagocytosis. The production of the capsular polysaccharide is encoded in a 16 gene operon, which is regulated in response to several environmental stimuli including nutrient availability. For instance, the capsule is produced in the late-and post-exponential growth phases, but not in the early-or mid-exponential growth phase. Several regulators are involved in capsule production, but the regulation of the cap operon is still poorly understood. In this study, we show that MsaB activates the cap operon by binding directly to a 10 bp repeat in the promoter region. We show that despite the fact that MsaB is expressed throughout four growth phases, it only activates capsule production in the late-and post-exponential growth phases. Furthermore, we find that MsaB does not bind to its target site in the early and mid-exponential growth phases. This correlates with decreased nutrient availability and capsule production. These data suggest either that MsaB binding ability changes in response to nutrients or that other cap operon regulators interfere with the binding of MsaB to its target site. This study increases our understanding of the regulation of capsule production and the mechanism of action of MsaB.

Revisiting the regulation of the capsular polysaccharide biosynthesis gene cluster in Staphylococcus aureus

“…We reproduced the activating effect of MsaB/CspA on cap expression, but demonstrated that it is mediated by modulation of SigB activity: msa deletion negatively affected P cap and P asp23 activities and constitutive expression of sigB rendered both promoters insensitive to MsaB/CspA. Moreover, in contrast to Batte et al (2016), no detectable MsaB/CspA binding to P cap was observed, challenging the role of MsaB/CspA as a classical transcription factor. Indeed, it was recently shown that MsaB/ CspA binds rsbVWsigB mRNA, thereby increasing transcript stability (Caballero et al, 2018).…”

“…It was previously shown that the SigB −35 consensus sequence functions as binding site for two cap activators RbsR and MsaB (Lei and Lee, ; Batte et al , ). We did not observe an effect of RbsR on P cap activity, neither in wild‐type nor in a sae ‐negative background.…”

Revisiting the regulation of the capsular polysaccharide biosynthesis gene cluster inStaphylococcus aureus

“…Interestingly, the crucial IR structure that in fact constitutes the SigB −35 motif was shown to be the binding site for two cap activators, RbsR and MsaB (35, 36). To elucidate whether and how these regulators interfere with SigB-dependent promoter activity, full-length P cap promoter fusions were introduced into msa and rbsR knockout mutants.…”

“…The identified transcriptional start site (TSS) is not preceded by a classical Sigma factor consensus sequence (12); instead, several inverted and direct repeats were identified further upstream, amongst which a 10 bp inverted repeat (IR) was shown to be crucial for promoter activity (12). It has been proposed that this IR functions as an operator site for the cap activators RbsR and MsaB (35, 36). RbsR also functions as a repressor of the rbsUDK operon involved in ribose uptake.…”

“…While the presence of ribose relieves repression of the rbsUDK operon by RbsR, the presence and absence of ribose had no effect on cap expression (35). MsaB is described as a transcriptional factor with DNA binding capacity (36) but is also annotated as cold-shock protein CspA, which exerts regulatory effects via RNA binding (37). In addition to RbsR and MsaB/CspA, there are several other transcription factors (MgrA, CcpA, RpiR, SpoVG, CcpE, XdrA, CodY, Rot), two-component regulatory systems (Agr, ArlRS, KdpDE, AirRS, SaeRS) and the alternative Sigma factor B (SigB) known to be involved in regulation of cap expression (3, 4).…”