ABSTRACT
In the unicellular marine alga Gonyaulax polyedra, many processes such as bioluminescence, photosynthesis and cell aggregation are regulated by the circadian clock. For these and other reasons, Gonyaulax polyedra constitutes an excellent model system for studying the molecular mechanism by which the clock controls cellular processes. Bioluminescence, which can be easily measured in an automated apparatus, reaches its peak during the night phase. Light emission originates from numerous small (0.5µm) spherical organelles called scintillons, which contain the three components required for the bioluminesence reaction: the enzyme luciferase, its substrate luciferin and a luciferin binding protein (LBP). These proteins, which are unambiguous biochemical markers for the in vivo rhythm, are synthesized and destroyed on a daily basis. Their circadian expression is regulated at the translational level. Translational control is often mediated by protein factors that bind to the 5' or 3' untranslated region (UTR) of the relevant mRNA. In case of the luciferin binding protein (LBP) we did not find any proteins that bind specifically the the lbp 5' UTR. However, we did detect a protein, apparently a dimer, which binds specifically to the 3' UTR of lbp mRNA. Its binding site was localized within a 22nt-long region in the 3' UTR containing seven (UG) repeats. It will be of interest to find out if this novel RNA binding motif may be also present in any of the other translationally regulated proteins in Gonyaulax, that are under circadian control. The binding activity of this protein cycles on a daily basis, decreasing at the beginning of the night when synthesis of LBP starts, and increasing again at the end of the night when synthesis of LBP stops. These daily variations in its binding activity persist under constant conditions, suggesting that this protein has a function as a clock controlled repressor, preventing the translation of lbp mRNA during the day. Preliminary data indicate that the activity change of this protein may be regulated via phosphorylation.