Regulation of chloroplast and cytoplasmic rRNA accumulation by light energy and its relation to reproductive events during the cell cycle of the alga Scenedesmus quadricauda

Abstract

Synchronous cultures of the green alga Scenedesmus quadricauda were grown at different mean irradiances (ranging from 20 W/m 2 to 150 W/m 2). At these irradiances, the algae were exposed to illumination regimes which differed in the ratio between light and dark intervals (from 02.22 to 24.00 h). The patterns of accumulation of chloroplast and cytoplasmic ribosomal RNA (chl- and cyt-rRNA) and variations in their ratio were followed under the above mentioned growth conditions. The chl-rRNA accumulated at a high specific rate even in the dark while the synthesis of cyt-rRNA was depressed. Consequently, the ratio of chl-/cyt-rRNA increased during the dark period to the high value of 0.4. After the start of the light period, this ration decreased gradually to the low value of 0.2. In continuously illuminated cells, the chl-rRNA and cyt-rRNA were accumulated at the same specific rate of that their ratio remained constant (0.2) during entire cell cycle. However, the absolute amount of cyt-rRNA accumulated in light was about 10 times higher than that of chl-rRNA. In an experiment with 5-fluorodeoxyuridine treated cells, the evidence was provided that DNA replications, nuclear divisions and chloroplast nucleoid fissions interferred with the course of neither chl-rRNA nor cyt-rRNA during the cell cycle. Chloroplastkinesis and cytokinesis were the only reproductive events that prevented the accumulation of both chl- and cyt-rRNA.