HbtR, a Heterofunctional Homolog of the Virulence Regulator TcpP, Facilitates the Transition between Symbiotic and Planktonic Lifestyles in Vibrio fischeri.

The bioluminescent bacterium forms a mutually beneficial symbiosis with the Hawaiian bobtail squid, , in which the bacteria, housed inside a specialized light organ, produce light used by the squid in its nocturnal activities. Upon hatching, juveniles acquire from the seawater through a complex process that requires, among other factors, chemotaxis by the bacteria along a gradient of -acetylated sugars into the crypts of the light organ, the niche in which the bacteria reside. Once inside the light organ, transitions into a symbiotic, sessile state in which the quorum-signaling regulator LitR induces luminescence. In this work we show that expression of and luminescence are repressed by a homolog of the virulence factor TcpP, which we have named HbtR. Further, we demonstrate that LitR represses genes involved in motility and chemotaxis into the light organ and activates genes required for exopolysaccharide production. TcpP homologs are widespread throughout the genus; however, the only protein in this family described thus far is a virulence regulator. Here, we show that HbtR, the TcpP homolog in , has both a biological role and regulatory pathway completely unlike those in Through its repression of the quorum-signaling regulator LitR, HbtR affects the expression of genes important for colonization of the light organ. While LitR becomes activated within the crypts and upregulates luminescence and exopolysaccharide genes and downregulates chemotaxis and motility genes, it appears that HbtR, upon expulsion of cells into seawater, reverses this process to aid the switch from a symbiotic to a planktonic state. The possible importance of HbtR to the survival of outside its animal host may have broader implications for the ways in which bacteria transition between often vastly different environmental niches.