Structural basis for environmental sensing in Pseudomonas fluorescens.

Over the past decades, it has become clear that bacteria often prefer a sessile over a free-swimming lifestyle. The transition from a mobile to an immobile, social lifestyle, called a biofilm, is induced in response to a variety of environmental stimuli, such as carbon or phosphate sources, linked to signaling via the second messenger cyclic-di-GMP (c-di-GMP). In gammaproteobacteria, c-di-GMP regulates cell adhesion to biotic and abiotic surfaces through the transmembrane Lap system that controls the surface presentation of large adhesin proteins. Signaling specificity is achieved through direct physical interaction between a c-di-GMP-producing diguanylate cyclases (DGCs) and LapD, the c-di-GMP receptor central to the Lap system. To date, ten DGCs have been identified as regulators of LapD, affecting biofilm formation. However, how they differ functionally and the stimuli to which they respond are often not well understood. To shed light on the environmental sensing mechanism for DGC activation, we have determined the structure of the periplasmic domain of two DGCs in P. fluorescens, a model organism for biofilm research. These domains, belonging to the CACHE family of protein domains, protrude into the periplasm where they encounter their respective ligands, which correlates with their activation. By comparing the structures of CACHE domains and their ligand binding profiles, we identify molecular signatures that contribute to c-di-GMP signaling specificity, environmental sensing, and biofilm formation.