Firefly luciferase

By Jennifer McDowall

Bioluminescence has arisen independently at least 30 times during the course of evolution - no less than three times in insects alone, which have produced the most diverse group of bioluminescent organisms. Bioluminescence plays many different roles in the animal kingdom, including communication, illuminating areas to find food, signalling for a possible mate, self-defence and camouflage. Bioluminescence in insects is also thought to play an auxiliary role in oxygen detoxification, a secondary function that may have evolved later. Luciferases have diverged between species with regards to patterns of luminescence (continuous or flashing) and their colour. Beetle luciferases are the most diverse in terms of their biochemistry. They also produce the widest range of colours, which are determined by the primary structure of the enzyme; in particular, changes in structure around the active site. Insect luciferases range in colour from yellow-green light (l_max=565 nm) produced by fireflies, green-orange light (l_max=532-593 nm) produced by click beetles (Elateridae) to the red light (l_max=536-623 nm) produced by glowworms (Phengodidae). Most other organisms capable of bioluminescence alter the colour through accessory pigments.

Firefly Luciferase, Biological Lanterns

From the Rocky Mountains east to the coast, North American fireflies can light up a summer night sky. In winter, the skies are clear, as fireflies often burrow underground or under the bark of trees. These luminescent beetles are also found in many different temperate and tropical countries around the world, especially in marshy or wet woodland areas. There are over 2000 known species of fireflies, the luminescent ones normally being nocturnal, while many non-luminescent species are diurnal (day-flying). However, all fireflies glow as larvae, even if they are non-luminescent as adults.

Fireflies emit green-yellow flashes from ventral lanterns for sexual attraction, displaying variations in the duration, interval and frequency of light flashes. The unique courtship flash patterns allow males to attract mates of the same species, the females flashing back in response. Sometimes, females of one species will mimic the flashing of other fireflies in order to feed on the unsuspecting males of a different species. Tropical fireflies, such as those found in Malaysian jungles, often synchronize flashes within a large group to produce a coordinated display.

In larvae, bioluminescence serves a different function, where it is thought to be a defence mechanism, warning potential predators that they contain distasteful or toxic chemicals. Larvae also use luminescence to attract prey, feeding off other larvae, terrestrial snails and slugs.

Bioluminescence is generated in specialised photogenic organs, called photocytes, located on the lower abdomen. In fireflies, these photocytes are arranged in rosettes, each rosette consisting of thousands of photocytes. The photocytes obtain their oxygen through tracheoles, fine branching tubes of the insect trachea. Bioluminescence is neurally controlled by nerves penetrating the photogenic organs, possibly through a mechanism involving oxygen admission.

The luciferase enzyme acts on luciferin as its substrate, the activated luciferin being responsible for light emission. In photocytes, luciferase is found in peroxisomes, and is directed there by the C-terminal signal tripeptide Ser-Lys-Leu found in all beetle luciferases. Luciferin is found in granules in photocytes.

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