Quorum Sensing Regulatory Systems

First described in V. fischeri, quorum sensing is used by many bacteria to detect the presence of other bacteria in their surroundings (reviewed in Taga and Bassler 2003). This method of monitoring the environment involves the production of a small molecule known as an autoinducer (AI) by an autoinducer synthase. Secreted into the environment, Als can be recognized in recipient cells either by a specific two-component sensor kinase, or more frequently in Gram-negative bacteria, by a DNA-binding protein in the LuxR family. In either case, the AI signal results in transcriptional control of target genes.

V. fischeri uses both the LuxR DNA binding protein and specific sensor kinases to detect at least three AI signals (Fig. 3). Both pathways contribute to the control of bioluminescence, a trait required for symbiosis. A mutant

o Autoinducer 'J] Sensor Kinase C> Regulator C3 Autoinducer synthase

Fig. 3. Regulatory circuits required for symbiosis. Dotted lines represent hypothesized regulatory events. Activities required for symbiosis -"luminescence," "motility," and "other" - are represented as genes on the V. fischeri chromosome. Regulation of these activities may be through activation of transcription, as is the case for FlrA, through activation of transcription of a repressor, as is predicted to be the case for LuxO, or through modulating the activity of the protein product. The V. fischeri proteins AinS,

AinR, and LitR are homologous to V. harveyi proteins LuxM, LuxN and LuxR, respectively o Autoinducer 'J] Sensor Kinase C> Regulator C3 Autoinducer synthase

Fig. 3. Regulatory circuits required for symbiosis. Dotted lines represent hypothesized regulatory events. Activities required for symbiosis -"luminescence," "motility," and "other" - are represented as genes on the V. fischeri chromosome. Regulation of these activities may be through activation of transcription, as is the case for FlrA, through activation of transcription of a repressor, as is predicted to be the case for LuxO, or through modulating the activity of the protein product. The V. fischeri proteins AinS,

AinR, and LitR are homologous to V. harveyi proteins LuxM, LuxN and LuxR, respectively defective for luxA, one of two genes that encode bacterial luciferase, exhibits a three- to four-fold reduction in colonization level within 48 h post-inoculation (Visick et al. 2000). Encoded upstream of luxA are LuxR and LuxI, an AI synthase that produces the AI detected by LuxR. Mutations in either luxR or luxI result in a colonization defect similar to that of the luxA mutant, suggesting that these regulators are required for symbiosis due to their role in transcriptional control of the lux operon.

A second AI synthase, AinS, produces a distinct AI that also is required for symbiosis. The pathway through which the AinS-synthesized AI is detected and transmitted is predicted based on studies in the related bacterium, V. harveyi (reviewed in Taga and Bassler 2003). In V. harveyi, AIs signal through two hybrid sensor kinases, LuxN and LuxQ, using a phosphotransferase protein, LuxU, to ultimately affect the activity of a response regulator, LuxO. In the absence of AIs, LuxO negatively regulates lux genes by indirectly controlling transcription of a transcriptional activator (LitR in V. fischeri [Fidopiastis et al. 2002]). In V. fischeri, the AinS-produced AI likely is recognized by AinR, a sensor kinase with significant homology to V. harveyi LuxN (Gilson et al. 1995; Lupp et al. 2003) (Fig. 3).

A mutant defective for ainS exhibits a colonization level indistinguishable from that of luxA, luxI and luxR mutants (Lupp et al. 2003). However, whereas the luxA, luxI, and luxR mutants produce no symbiotic bioluminescence (at least 1000-fold decreased [Visick et al. 2000]), the ainS mutant produces approximately 10-20% of the wild-type bioluminescence. Thus, it seems probable that the role of ainS in colonization may be independent of its role in bioluminescence regulation. These phenotypes are difficult to separate, however: mutations in luxO, the response regulator through which the AI signals are transmitted, restore to wild-type levels both the slightly decreased symbiotic bioluminescence and the colonization defect of the ainS mutant (Lupp et al. 2003). Thus, an important direction will be to determine whether this pathway controls genes, other than lux, necessary for colonization.

V. fischeri encodes a third AI synthase, LuxS (Lupp and Ruby 2004). In V. harveyi, LuxS produces an AI that is detected by sensor kinase LuxQ through its interaction with the periplasmic protein LuxP (Taga and Bassler 2003). Because V. fischeri contains homologs for all of these genes (Lupp and Ruby 2004), it seems likely that this AI system functions similarly in the symbiotic organism (Fig. 3). A strain of V. fischeri in which luxS is mutated colonizes the LO as well as the wild-type strain; however the luxS mutation appreciably decreases the colonization efficiency of an ainS mutant, but not its per cell luminescence (Lupp and Ruby 2004). These data provide further support for a role of the AinS system in symbiosis distinct from that of luminescence. Further investigation of the three quorum sensing pathways likely will provide insight both into genes necessary for symbiotic colonization and, because the signals are known, signal transduction during symbiotic colonization.

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