Figure 4.

The EsaI/R quorum sensing pathway and the Rcs environmental sensing pathway converge to control stewartan EPS production. At low cell density, AHL ligand‐free EsaR binds to the promoter region of rcsA and represses transcription. There is also low, basal level expression of RcsA. However, degradation by Lon protease prevents significant RcsA accumulation (Gottesman and Stout, 1991) leaving insufficient RcsA to form the RcsA/B activation complex. At high cell density (3‐oxo‐C6‐HSL inducing conditions), the AHL ligand binds to EsaR which relieves EsaR repression of rcsA. RcsA production now exceeds the degradation capability of Lon protease. RcsA recruits RcsB and the RcsA/B heterodimer activates transcription of the wce gene cluster leading to production of stewartan EPS. The role of the remaining components of the Rcs phosphorelay are less clear in Pnss but are likely similar to what occurs in E. coli where rcsC and rcsD encode 2 sensor kinases that participate in a phosphorelay that transfers phosphate to the RcsA/B complex. In E.coli, RcsF is a membrane bound protein that is likely the receiver of an environmental signal (s) that serves as the trigger of the Rcs phosphorelay. The precise environmental signal (s) that stimulate the Rcs phoshorelay are unknown. Figure adapted from Minogue et al. (2005) and Madjalani and Gottesman (2005). Note: Figure not drawn to scale.