Chapter 28 : Intercellular Communication in Marine Vibrio Species: Density-Dependent Regulation of the Expression of Bioluminescence

Model for regulation of bioluminescence in V. fischeri. Synthesis of the enzymes for light production in V. fischeri is dependent on transcription of operon R, which contains luxICDABEG. The product of the luxG gene is not required for luminescence. Transcription of operon R is regulated by the interaction of autoinducer (the production of which is a function of the luxI gene) and the protein LuxR, encoded by the luxR gene. In dilute cell suspensions, weak constitutive transcription of operon R occurs, resulting in a low concentration of autoinducer. As the cell density increases, a concentration of autoinducer is reached that is sufficient to induce transcription of operon R. Because luxI is in operon R, autoinducer synthesis is also increased on induction of operon R. The functions regulating operon R form a positive feedback circuit, and induction of operon R results in an exponential increase in light production per cell. A compensatory negative feedback circuit operates to indirectly modulate the expression of operon R (and light production) after induction. Negative regulation is achieved by limiting the expression of operon L containing luxR and requires the interaction of autoinducer and LuxR. VAI, V. fischeri autoinducer; PL and PR, left- and rightward promoters, respectively.

Model for genetic control of luminescence in V. harveyi. Expression of the operon (luxCDABEGH) encoding luminescence enzymes is controlled by two signalling channels, signalling systems 1 and 2. Each signalling system consists of functions that produce an extracellular signal substance (Signal 1 or 2) and functions that regulate the response to the corresponding signal substance (Sensor 1 or 2). Each signal substance (autoinducer 1 or 2 denoted by HAI-1 or HAI-2) interacts with the corresponding sensor (O). Signal 1 function is encoded by luxL and luxM, and Sensor 1 function is encoded by luxN. The Sensor 2 function is the product of the luxP and luxQ genes, and the gene(s) encoding Signal 2 synthesis function has not been identified. Sensor proteins regulate expression of luminescence indirectly by inactivating LuxO, a repressor of the luxCDABEGH operon. Expression of luxCDABEGH also requires LuxR, a positive transcription factor. The interactions and activities of the sensor and repressor proteins are probably controlled by phosphorylation reactions (see text).

Response phenotypes of LuxN and LuxQ transposon insertion mutants of V. harveyi. The wild-type V. harveyi (top panel) is a control strain with a Tn5lac insertion outside the lux regulatory region. Response curves for transposon insertion mutants with the LuxN (middle panel) and LuxQ (bottom panel) phenotypes are also shown. Bright cultures of these strains were diluted 1:5,000, and light production was then measured during growth of the cultures. Cell-free supernatant (10%) or synthetic autoinducer (1 µg ml-1) was added at the time of the first measurement. No addition (○), wild-type supernatant (●), LuxM supernatant (i.e., HAI-2 [■]), and synthetic autoinducer (i.e., HAI-1 [□]). Relative light units are denned as light emission per cell (i.e., counts min-1 ml-1 X 103/colony-forming units ml-1) and are plotted as a function of cell density (colony-forming units/ml). Data were taken from Bassler et al. (1994b) .

Amino acid sequence comparisons between Lux regulatory proteins and members of Arc bacterial two-component sensory transduction system. The protein comparisons were aligned to LuxQ. Amino acid sequence similarities between the histidine protein kinase and response regulator domains of V. harveyi LuxQ, and those domains in V. harveyi LuxN and LuxO and E. coli ArcB and ArcA are shown. The symbols “.” and “:” denote amino acid similarities and identities, respectively. The highly conserved H, N, G1, F, and G2 blocks in the histidine protein kinase domain and the Asp-12,13, Asp-57, and Lys-109 blocks in the response regulator domain are boxed in LuxQ. Numbers next to protein designations refer to the position in the protein of the first amino acid residue used in each line of the sequence comparison. The full lengths of the proteins are LuxN, 849; LuxQ, 859; LuxO, 453; ArcB, 778; and ArcA, 238 amino acids.

Two-component signal relay in density-sensing. Density-dependent regulation of luminescence in V. harveyi begins with binding of the signalling system 1 and 2 autoinducers (HAI-1 and HAI-2) by their cognate sensors LuxN and LuxP, Q, respectively. The LuxP protein is similar to the periplasmic ribose binding protein of E. coli and S. typhimurium. LuxN and LuxQ are members of the bacterial “two-component” family of adaptive response regulators, and each contains a histidine-protein kinase sensor domain and a response regulator domain. Subsequent signal transduction is proposed to occur via a series of phosphorelay reactions (see text). Coupling and integration of the signals from the two pathways could be accomplished by LuxO, a negative regulator of luminescence. LuxO is similar to the response regulator class of two-component regulatory proteins, and it contains a DNA binding domain so it could directly interact with lux DNA. H, histidine; D, aspartate; and H-T-H, helix-turn-helix DNA binding domain.