The function of circadian RNA-binding proteins and their cis-acting elements in microalgae

An endogenous clock regulates the temporal expression of genes/mRNAs that are involved in the circadian output pathway. In the bioluminescent dinoflagellate Gonyaulax polyedra circadian expression of the luciferin-binding protein (LBP) is controlled at the translational level. Thereby, a clock-controlled RNA-binding protein, called circadian controlled translational regulator (CCTR), interacts specifically with an UG-repeat, which is situated in the lbp 3^' UTR. Its binding activity correlates negatively with the amount of LBP during a circadian cycle. In the green alga Chlamydomonas reinhardtii, a clock-controlled RNA-binding protein (CHLAMY 1) was identified, which represents an analog of the CCTR from the phylogenetically diverse alga G. polyedra. CHLAMY 1 binds specifically to the 3^' UTRs of several mRNAs and recognizes them all via a common cis-acting element, composed of at least seven UG-repeats. The binding strength of CHLAMY 1 is strongest to mRNAs, whose products are key components of nitrogen metabolism resulting in arginine biosynthesis as well as of CO₂ metabolism. Since temporal activities of processes involved in nitrogen metabolism have an opposite phase than CHLAMY 1 binding activity, the protein might repress the translation of the cognate mRNAs.