The Wsp system is responsible for the synthesis of cyclic-di-GMP and the activation of the Wrinkly Spreader (WS) phenotype. The functioning of the P. fluorescens SBW25 Wsp system has been modelled on the Che chemosensory system of E. coli and provides a mechanistic explanation linking adaptive mutations to the WS phenotype. The methyl-accepting chemotaxis protein (WspA), scaffold proteins (WspB and WspD), and histidine kinase (WspE) form a membrane-associated receptor-signaling complex. In the absence of an appropriate environmental signal, the complex is silent. Upon activation by phosphorylation (indicated by the black circles), the diguanylate cyclase (DGC) response regulator (WspR) synthesizes cyclic-di-GMP from GTP. The system is controlled by the opposing activities of a methyltransferase (WspC) and methylesterase (WspF), which add and remove, respectively, methyl (CH₃) groups on the signalling domain of WspA (circles). In wild-type SBW25, the activities of the two are balanced, preventing the activation of WspR and allowing the Wsp complex to oscillate between active and inactive states. Mutations inhibiting WspF function or activating WspE kinase activity result in the activation of WspR and the production of cyclic-di-GMP. Increased levels of cyclic-di-GMP then lead to the expression of the WS phenotype. The Wsp system is shown as a schematic only; the three-dimensional structure of the proteins, their relative placement, numbers, and the positioning of the complex in the inner membrane have not yet been determined.