Autoradiographic Evidence for the in Situ Synthesis of Chloroplast and Mitochondrial Rna

The rate of appearance of labelled RNA in the chloroplast and mitochondria as compared with the rate in the remaining cytoplasm was studied in the unicellular flagellate, Ochromonas danica, by electron-microscope autoradiography. Greening cells were labelled withuridine5,6['H] for a short (30 min) and a long (2 h) interval and the concentration of label, expressed as grains/unit area, determined for each cell component. The data demonstrate that there is the expected lag in the labelling of the cytoplasm proper, but no apparent lag in the labelling of the chloroplast and mitochondria. This observation, combined with the fact that after the short labelling time the chloroplast and mitochondria have a much heavier concentration of labelled RNA than the surrounding cytoplasm, indicates that most, if not all, chloroplast and mitochondrial RNA is synthesized m situ. The three kinds of ribosomes present in the cell are distinctly different in size. The mitochondrial ribosomes measure 150-170 A in diameter, the chloroplast ribosomes average 170-200 A in diameter, whereas the cytoplasmic ribosomes are 210-230 A in diameter in glutaraldehyde-osmium-fixed cells. During chloroplast development in the light, the number of chloroplast ribosomes increases approximately tenfold.