Schematic representation of a suggested DSF signaling model including XAC3673. Schematic representation of a suggested DSF signaling model including XAC3673. At a low cell density, the DSF sensor RpfC forms a complex with the DSF synthase RpfF, which prevents the effective synthesis of the DSF signal. At a high cell density, accumulated extracellular DSF signal interacts with RpfC, which undergoes autophosphorylation and facilitates release of RpfF and phosphorelay from the sensor to its response regulator RpfG. The event boosts DSF biosynthesis and induces the expression of the EPS and extracelular enzymes. In either, low or high cell density, there may be other stimuli (signals), in the extracellular environment from the host or the environment, regardless of the bacterial cellular concentration. The synthesis of Xcc virulence factors only start after the perception of such signals. XAC3673, through a phosphorylation cascade, relays this information to RpfG or to another protein downstream (arrows with yellow lines). A mutation in XAC3673 prevents the transduction of signals from the environment or host, and thus, the virulence factors are not produced, even in the presence of all functional rpf genes and with a high cell concentration. The solid arrow indicates signal flow or signal generation and the dashed arrow indicates basal signal generation or no signal flow. OM = outer membrane; IM = inner membrane.