Diurnal and Circadian Rhythms in the Accumulation and Synthesis of mRNA for the Light-Harvesting Chlorophyll a/b-Binding Protein in Tobacco

The steady state level of mRNA for the light-harvesting chlorophyll a/b binding protein (LHCP) varies considerably during a day-night light cycle in expanding leaves of tobacco (Nicotiana tabacum, cv Wisconsin 38). The maximum (day) level is at least one hundred-fold higher than the minimum (night) level. The oscillation in the LHCP mRNA level continues for several days even under constant illumination. However, no such circadian rhythm can be detected in constant darkness. Plants that have been maintained in constant darkness for a prolonged period reinitiate the circadian rhythm upon illumination: the LHCP mRNA level resumes its rhythmic behavior with a period of roughly 24 hours. The same circadian fluctuation of LHCP mRNA is observed upon illuminating etiolated tobacco seedlings, demonstrating that the circadian rhythm is not dependent on the presence of mature chloroplasts. In vitro transcription experiments with nuclei isolated from green plants during various times of the day indicate that the diurnal as well as the circadian changes of the level of LHCP RNA are due, at least in part, to changes in the rate of LHCP mRNA synthesis.