Characterization and postulated structure of the primary emitter in the bacterial luciferase reaction.

An intermediate identifiable as the emitter in bacterial bioluminescence has been demonstrated. The reaction was carried out at 1 degrees C by mixing purified luciferase-bound FMN 4a-hydroperoxide with long-chain aldehyde (decanal). Simultaneous kinetic measurements of bioluminescence and absorbance showed that the decay of light emission occurred more rapidly than the appearance of the stable product, oxidized FMN, indicating the formation of a transient intermediate species subsequent to light emission. The same species was found in reaction mixtures examined immediately after light emission was completed. It has a relatively short half-life (7 min at 9 degrees C); the chromophore is postulated to be the luciferase-bound flavin 4a-hydroxide and to decay to the stable product, FMN, by losing water. Both its absorption spectrum (lambda(max), 360 nm) and its fluorescence emission (lambda(max), 490 nm) are consistent with the hypothesis that this is the ground state of the primary emitter, the bioluminescent species produced in the reaction.