THE CIRCADIAN OSCILLATOR COORDINATES THE SYNTHESIS OF APOPROTEINS ANDTHEIR PIGMENTS DURING CHLOROPLAST DEVELOPMENT

Greening has been studied at circadian times of maximal and minimal le vels of mRNA for the light-harvesting chlorophyll a/b binding protein in photosystem II (Cab mRNA) after circadian synchronization of etiola ted barley plantlets (Hordeum vulgare cv Apex) by heat-shock treatment s. It was found that greening occurs faster and without a lag period w hen illumination was started at the time of maximal Cab mRNA accumulat ion. This holds true for the rate of accumulation of Cab and early lig ht-inducible protein mRNAs, the levels of their correspondent proteins , and the levels of chlorophyll a and b. When illumination was started at the time of Cab mRNA minimum, a lag in the appearance of all compo nents mentioned above was observed. Under these conditions, the lag in chlorophyll b accumulation was by far more pronounced than that found for chlorophyll a. The circadian oscillation in the capacity of chlor ophyll synthesis appears to be controlled via delta-aminolevulinic aci d (delta-ALA) synthesis. delta-ALA accumulation after levulinic acid t reatment is itself under circadian control; the maxima in stationary c oncentrations coincide with those of Cab mRNA levels. The amounts of p rotochlorophyllide and photoconvertible protochlorophyllide showed onl y minor differences between circadian minima and maxima, the levels be ing slightly lower during the time of minimum.