Genetics of Bioluminescence
The bacterial luciferin-luciferase system is encoded by a set of genes labelled the Lux operon. In A. fischeri, five such genes (LuxCDABE) have been identified as active in the emission of visible light, and two genes (LuxR and LuxI) are involved in regulating the operon. Several external and intrinsic factors appear to induce and inhibit the transcription of this gene set and produce or suppress light emission. More research is being done to improve our understanding of these processes.
The bioluminescent, Gram-negative bacterium Aliivibrio fischeri is one of many species of bacteria that commonly form symbiotic relationships with marine organisms (Distal, 1993). Marine organisms contain bacteria that use bioluminescence so they can find mates, ward off predators, attract prey, or communicate with other organisms (Widder, 2010). In return, the organism the bacteria are living within provides the bacteria with a nutrient-rich environment (Winfrey et al., 1997). The lux operon is a 9-kilobase fragment of the A. fischeri genome that controls bioluminescence through the catalyzation of the enzyme luciferase (Meighen, 1991). The lux operon has a known gene sequence of luxCDAB(F)E, where lux A and lux B code for the components of luciferase, and the lux CDE codes for a fatty acid reductase complex that makes the fatty acids necessary for the luciferase mechanism (Meighen, 1991). Lux C codes for the enzyme acyl-reductase, lux D codes for acyl-transferase, and lux E makes the proteins needed for the enzyme acyl-protein synthetase. Luciferase produces blue/green light through the oxidation of reduced flavin mononucleotide and a long-chain aldehyde by diatomic oxygen. The reaction is summarized below (Silverman et al., 1984): FMNH2+O2+R-CHO → FMN + R-COOH + H2O + Light To generate the aldehyde needed in the reaction above, three additional enzymes are needed. The fatty acids needed for the reaction are pulled out from the fatty acid biosynthesis pathway by the enzyme acyl-transferase. Acyl-transferase reacts with acyl-ACP to release R-COOH, a free fatty acid. R-COOH is reduced by a two-enzyme system to an adehyde. The reaction is shown below: R-COOH+ATP+NADPH→ R-CHO+AMP+PP+NADP+ (Winfrey et al., 1997). Although the lux operon encodes the enzymes necessary for the bacteria to glow, bioluminescence is regulated by autoinduction. An autoinducer is a transcriptional promoter of the enzymes necessary for bioluminescence. Before the glow can be luminized, a certain concentration of an autoinducer must be present. So, for bioluminescence to occur, high colony concentrations of A. fischeri should be present in the organism (Winfrey et al., 1997).
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