Figure 1. Model for control of biofilm formation in V. fischeri.

Depicted here is a model based on current data regarding the potential roles of the syp regulators. The sensor kinase RscS acts upstream of SypG, presumably serving as a phosphodonor in response to some as-yet unidentified environmental signal, perhaps from the squid. Once phosphorylated, SypG is predicted to directly bind to sequences upstream of each of 4 syp operons to activate syp transcription by σ⁵⁴-containing RNA polymerase. The Syp proteins contribute to biofilm formation in culture, including the formation of wrinkled colonies and pellicles, as well as in situ biofilm formation and colonization. RscS overproduction also appears to inactivate SypE, which inhibits biofilm formation induced by overproduction of SypG at a level downstream from syp transcription; how RscS inactivates SypE is as yet unknown. Biofilm formation can be also induced by overexpression of a sypF allele with increased activity (SypF1) in a manner that depends in part on SypG and SypE, and in part on VpsR, which promotes cellulose biosynthesis.