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Bioluminescence is widespread in living organisms, occurring in such diverse groups as jellyfish, earthworms, insects, squid, fish, algae, and bacteria (1). Bioluminescence is especially common in the unlit ocean depths. A variety of biochemical mechanisms have evolved to emit light at many different wavelengths. The reasons for the emission of light are not always clear, but it often appears to be a recognition device, a way of attracting mates or prey, confusing predators, or promoting schooling in the dark depths of the ocean.

While many forms of life produce their own light, others have formed elaborate ectosymbiotic relationships with luminous bacteria that are sequestered within special light organs (Fig. 1 to 4). It is assumed that the hosts provide optimal conditions in these organs for the growth and luminance of the bacteria. These light organs contain a transparent lens and often have a layer of reflective tissue at the rear to maximize directional light emission (Fig. 1 and 2). Light output is often regulated by the host through systems that control the visibility of the light.

Fish in the genera Photoblepharon and Anomalops harbor luminous bacteria in special pouches under the eyes (Fig. 1 to 4). The Anomalops control the visibility of light by rolling the light organ downward against a pocket of black tissue, while the Photoblepharon, or flashlight fish, have a fold of black tissue they can draw over the light organ when required (Fig. 3). The marine bacteria Vibrio fischeri has been isolated from the flashlight fish (Fig. 4 and 5).

The majority of luminescent bacteria inhabit the ocean. Two genera of marine bacteria, Vibrio and Photobacterium, are among the most abundant luminous bacteria. They can be found in seawater and in the intestinal tract and on the body surfaces of marine animals. These bacteria are easily isolated by incubating a raw marine fish in a cold room for several days, after which the luminous patches that develop can be streaked for isolation on seawater-based agar medium. Their natural light emission is at a maximum near 490 nm, but mutants have been isolated or genetically produced which emit a variety of colors (Fig. 5b). Light emission by these bacteria, as well as many other luminescent organisms, is mediated by the enzyme luciferase. In the presence of oxygen, FMNH₂, and a fatty acid aldehyde (R-CHO), luciferase catalyzes the oxidation of the FMNH₂ to water, R-COOH, and excited FMN* which decays to ground state by emitting light. The genes for the luciferase system have been cloned from a number of luminescent species into bacteria (4, 5) (Fig. 5a).

The only terrestrial luminescent bacterial genus known is Photorhabdus. Members of the Photorhabdus are mostly insect pathogens that exist in a complex symbiotic relationship with a family of entomopathogenic nematodes (2, 3). Photorhabdus bacteria, carried by nematodes that invade insect larvae (Fig. 6), are released into the insect hemolymph, where they rapidly grow and kill the insect host. The dead insect subsequently serves as a source of nutrients for nematode reproduction. The bacteria produce pigments that turn the insect carcass a red-orange color (Fig. 6a and b), antibiotics that inhibit the growth of other microbes, and light that causes the carcasses to become luminous (Fig. 6c).

Luminescent systems are proving extremely valuable in a variety of molecular biological research areas as visible indicators (reporters) of gene regulation and as a way of following reactions that occur within a living cell (6, 7, 8). These images can be used to illustrate to students the concept of luminescent systems.

References

1. Moris, J. G., et al. 1975. Light for all reasons: versatility in the behavioral repertoire of the flashlight fish. Science 190:74-75.

2. Nealson, K. H., T. M. Schmidt, and B. Bleakley. 1990. Biochemistry and physiology of Xenorhabdus, p. 271-282. In R. Gaugler and H. K. Kaya (ed.), Entomopathogenic nematodes in biological control. CRC Press, Boca Raton, Fla.

3. Frackman, S., and K. H. Nealson. 1990. Molecular genetics of Xenorhabdus, p. 285-299. In R. Gaugler and H. K. Kaya (ed.), Entomopathogenic nematodes in biological control. CRC Press, Boca Raton, Fla.

4. Engebrecht, J., K. Nealson, and M. Silverman. 1983. Bacterial bioluminescence: isolation and genetic analysis of functions from Vibrio fischeri. Cell 32:773-781.

5. Zenno, S., and K. Saigo. 1994. Identification of the genes encoding NAD(P)H-flavin oxidoreductases that are similar in sequence to Escherichia coli Fre in four species of luminous bacteria: Photorhabdus luminescens, Vibrio fischeri, Vibrio harveyi, and Vibrio orientalis. J. Bacteriol. 176:3544-3551.

6. Stearns, T. 1995. Green fluorescent protein. The green revolution. Curr. Biol. 5:262-264.

7. Heim, R., and R. Y. Tsien. 1996. Engineering green fluorescent protein for improved brightness, longer wavelengths and fluorescence resonance energy transfer. Curr. Biol. 6:178-182.

8. Korpela, M. T., J. S. Kurittu, J. T. Karvinen, and M. T. Karp. 1998. A recombinant Escherichia coli sensor strain for the detection of tetracyclines. Anal. Chem. 70:4457-4462.