Abstract
Bioluminescence in the unicellular dinoflagellate Gonyaulax polyedra represents an excellent model for studying a circadian controlled process. Two proteins involved in the bioluminescence reaction are the enzyme luciferase and a luciferin-binding protein (LBP), which sequesters the substrate luciferin at pH 7.5 and thus prevents it from reacting with the enzyme. In vivo bioluminescence and the amounts of both proteins peak during the night phase. LBP has been shown to be synthesized and degraded on a circadian basis. However, its mRNA is constant throughout the day-night cycle, indicating that LBP expression is controlled at the translational level. This control appears to be mediated by a protein that binds to the 3' untranslated region of the lbp mRNA. The binding activity of this protein decreases at the beginning of the night, when synthesis of LBP starts, and it increases at the end of the night, when synthesis of LBP stops. This indicates that the lbp 3' UTR binding protein functions as a clock-controlled repressor, preventing the translation of lbp mRNA during the day. The binding site of this protein embraces a 22 nt long region, which contains 7 UG repeats and is part of a hairpin-loop structure. This 3' UTR binding protein seems to occur as a dimer, or even multimer.