Nuclear and Organelle Rna Synthesis in Ochromonas: the Effects of Light

In a companion study on the site of synthesis of chloroplast and mitochondrial RNA (7), it was shown that when dividing greening cells of Ochromonas danica were fed a radioactive RNA precursor, both the chloroplast and mitochondria as well as the nucleus were heavily labeled after a short labeling time, whereas a lag was observed in the labeling of the cytoplasmic matrix. These data were interpreted to mean that organelle RNA is synthesized within the organelles themselves, whereas the RNA of the cytoplasm is synthesized in the nucleus and subsequently transferred to the cytoplasm . However, since it is possible to interpret such radioautographic data in other ways, the present experiment was designed to answer more directly the question of where chloroplast RNA is synthesized. Preliminary studies had shown that when cells of Ochromonas are starved in a substrate-free medium, they are able to develop a small green chloroplast when placed in the light, even though they are incapable of further cell divisions. Under such conditions, it seemed likely that the only kind of RNA in the cell which would show a light-induced increase in its rate of synthesis would be chloroplast RNA. If this is so and if all chloroplast RNA is synthesized in situ, then one would expect that the only cell structure to show a light-induced increase in RNA synthesis would be the chloroplast. Consequently, in this study, dark-starved and light-induced nondividing cells of Ochromonas were labeled with tritiated orotic acid for a short interval and the amount of labeled RNA in each cell structure was determined by electron microscopic radioautography . For comparison purposes, similar observations were made on dark-grown and light-induced dividing cells . Contrary to expectations, it was observed that even in nondividing cells, light stimulates RNA synthesis in the nucleolus as well as in the chloroplast. However, other results obtained add further support to the hypothesis that chloroplast and mitochondrial RNA are synthesized in situ .