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3. Genetic regulation of bacterial bioluminescence
There are several bacterial genes involved in bioluminescence and its regulation. The two subunits of luciferase, α and β, are encoded by genes luxA and luxB, respectively (Belas et al. 1982). In both V. fischeri and V. harveyi these genes are organized in an operon together with other genes involved in the bioluminescence reaction. In V. fischeri this operon consists of the luxI, luxC, luxD, luxA, luxB, luxE and luxG genes (the luxI is the most proximal gene to the promoter, luxG the most distal). In V. harveyi the lux operon is organized in a similar way to that of V. fischeri, but luxI is absent and luxG is followed by the luxH gene. LuxC, luxD and luxE code for proteins that form a complex of the fatty acid reductase. The products of genes luxG and luxH are responsible for the synthesis of the reduced flavine (Meighen 1994).
Expression of the lux operon in both V. fischeri and V. harveyi undergoes a specific regulation called quorum sensing (for detailed reviews see Swift et al. 1998, Winans & Bassler 2002). As a result of this mechanism, expression of the lux genes, and thus the efficiency of light emission, depends on the concentration of cells in the environment, that is, bacterial luminescence is effective when cells occur at a high density, whereas light emission is negligible in diluted cultures.
There are different mechanisms of the lux operon expression regulation by quorum sensing of V. fischeri and V. harveyi. In V. fischeri, the luxI gene codes for the enzyme responsible for the synthesis of N–(3–oxyhexanol)–L–homoserine lactone, which acts as an autoinducer interacting with the luxR gene product. The LuxR protein is a repressor of the promoter of the lux operon, and interaction of this protein with the autoinducer results in de-repression of this promoter (Meighen 1994, Bassler & Silverman 1995). Since the autoinducer is excreted from cells, it acts on other bacteria in the culture. Therefore, the denser the culture, the more efficient the expression of the lux operon, and the bacteria produce more light.
The lux regulatory system in V. harveyi seems to be more complicated than in V. fischeri, though recent studies have indicated that in the latter species additional regulatory mechanisms may also occur (Miyamoto et al. 2000). Apart from the luxCDABEGH operon, V. harveyi contains several additional genes involved in the regulation of bioluminescence. These are the regulatory genes luxR, luxO and luxU, genes coding for two autoinducer synthetases (luxL and luxM coding for the synthetase of an autoinducer called AI–1, luxS coding for the synthetase of the autoinducer AI–2), and genes coding for sensors of the autoinducers luxN (AI–1 sensor) and luxP and luxQ (AI–2 sensor) (Bassler et al. 1994, Freeman & Bassler 1999). The luxR gene product is an activator of the luxCDABEGH operon (note that this protein reveals no homology to LuxR of V. fischeri) (Chatterjee et al. 1996, Miyamoto et al. 1996). The negative regulator of this operon is the LuxO protein (Bassler et al. 1994). The sensory proteins LuxN and luxPQ are responsible for detecting autoinducers AI–1 and AI–2, respectively, and the subsequent signal transduction, mediated by the LuxU protein and based on phosphorylation and dephosphorylation reactions, leads to the inactivation of LuxO (Freeman & Bassler 1999, Freeman et al. 2000) and the subsequent stimulation of the expression of the luxCDABEGH operon. This leads to the efficient production of luciferase and other enzymes necessary for the luminescence reaction.
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