The reporter gene for luciferase was cloned from the firefly (Photinus pyralis), a bioluminescent organism (6). Firefly luciferase catalyzes the following reaction (7):

Luciferin + ATP + 02 oxyluciferin + PPi + C02 + light + cAMP

The expression of firefly luciferase is measured in the presence of ATP and luciferin by the emission of light, which allows very rapid detection of transgene expression. The emission of light is measured using a luminometer, which detects both the peak of the light flash and the integrated light output over a period of time. The assay is very sensitive and may be used in the analysis of the transcriptional activity of weak promoters. The luciferase assay also has the advantage of using a nonradioactive substrate, and there is no similar enzyme activity in mammalian cells.

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