THE CHLAMYDOMONAS-REINHARDTII LI818 GENE REPRESENTS A DISTANT RELATIVE OF THE CABI II GENES THAT IS REGULATED DURING THE CELL-CYCLE AND IN RESPONSE TO ILLUMINATION/

In the green unicellular alga Chlamydomonas reinhardtii, as in higher plants, the expression of the genes encoding the chlorophyll a/b-bindi ng (CAB) polypeptides associated with photosystem I (PSI) and photosys tem II (PSII) is regulated by endogenous (circadian clock) and exogeno us signals (light and temperature). The circadian clock ensures that t he oscillation in the levels of the different cab mRNAs is continuousl y kept in phase with light/dark (LD) cycles and is maximal by the midd le of the day. On the other hand, light controls the amplitude of the oscillations. We report here the cloning and characterization of the C . reinhardtii LI818 gene, which identifies a CAB-related polypeptide a nd whose expression is regulated quite differently from the cab I/II g enes. We show: (I) that in LD synchronized Chlamydomonas cells LI818 m RNA accumulation is subject to dual regulation that involves separable regulation by light and an endogenous oscillator; (2) that LI818 mRNA . is fully expressed several hours before the cab I/II mRNAs and that the latter accumulate concomitantly; (3) that blocking the electron ho w through PSII using DCMU prevents cells from accumulating cab I/II mR NAs but not LI818 mRNA and (4) that the accumulation of LI818 mRNA is abolished by blocking cytoplasmic protein synthesis, suggesting that t hese regulatory mechanisms are mediated by labile proteins.