HbtR, a heterofunctional homolog of the virulence regulator TcpP, facilitates the transition between symbiotic and planktonic lifestyles inVibrio fischeri

AbstractThe bioluminescent bacteriumVibrio fischeriforms a mutually beneficial symbiosis with the Hawaiian bobtail squid,Euprymna scolopes, in which the bacteria, housed inside a specialized light organ, produce light used by the squid in its nocturnal activities. Upon hatching,E. scolopesjuveniles acquireV. fischerifrom the seawater through a complex process that requires, among other factors, chemotaxis by the bacteria along a gradient ofN-acetylated sugars into the crypts of the light organ, the niche in which the bacteria reside. Once inside the light organ,V. fischeritransitions into a symbiotic, sessile state in which the quorum-signaling regulator LitR induces luminescence. In this work we show that expression oflitRand luminescence are repressed by a homolog of theV. choleraevirulence 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.ImportanceTcpP homologs are widespread throughout theVibriogenus; however, the only protein in this family described thus far is aV. choleraevirulence regulator. Here we show that HbtR, the TcpP homolog inV. fischeri, has both a biological role and regulatory pathway completely unlike that inV. cholerae. Through its repression of the quorum-signaling regulator LitR, HbtR affects the expression of genes important for colonization of theE. scolopeslight 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 ofV. fischericells into seawater, reverses this process to aid the switch from a symbiotic to a planktonic state. The possible importance of HbtR to the survival ofV. fischerioutside of its animal host may have broader implications for the ways in which bacteria transition between often vastly different environmental niches.