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Since the initial discovery of Aqueoria victoria ’s green fluorescence off the coast of Washington’s Puget Sound, biofluorescent marine organisms have been found across the globe. The variety of colors of biofluorescence as well as the variability in the organisms that exhibit this fluorescence is astounding. The mechanisms of biofluorescence in marine organisms are also variable. To fluoresce, some organisms use fluorescent proteins, while others use small molecules. In eels, green biofluorescence was first identified in Anguilla japonica . The green fluorescence in A. japonica was discovered to be caused by a fatty acid binding protein (UnaG) whose fluorescence is induced by the addition of bilirubin. Members of this class of proteins were later discovered in Kaupichthys eels (Chlopsid FP I and Chlopsid FP II). Here, we report the discovery and characterization of the first member of this class of green fluorescent fatty acid binding proteins from the moray eel Gymnothorax zonipectis . This protein, GymFP, is 15.6 kDa with a fluorescence excitation at 496 nm and an emission maximum at 532 nm upon addition of bilirubin. GymFP is 61% homologous to UnaG and 47% homologous to Chlopsid FP I. Here, we report de novo transcriptome assembly, protein expression, and fluorescence spectroscopic characterization of GymFP. These findings extend the fluorescent fatty acid binding proteins into a third family of true eels (Anguilliformes). 

Biofluorescence has been found to be an increasingly widespread phenomenon in the ocean. The reclusive Caribbean chlopsid eel, Kaupichthys hyoproroides displays bright green fluorescence in its native marine environment. We have previously shown the fluorescence to be attributed to a fluorescent fatty acid-binding protein, Chlopsid FP, part of a larger family of fluorescent fatty acid-binding proteins, including the homologous UnaG. All require the addition of exogenous bilirubin for fluorescence. Here, we report the generation of a series of point mutants, and deletions that result in the quenching of fluorescence in Chlopsid FP. In addition, we report the binding constants of bilirubin to Chlopsid FP and mutants, measured by fluorescence titration. This study provides key insights into the potential mechanism of fluorescence in this class of fluorescent fatty acid-binding proteins.