Precursors of chloroplast ribosomal RNA and their maturation in Chlorella

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Abstract
In Chlorella pyrenoidosa, tritiated uridine is incorporated specifically into the RNA of the chloroplast. The 16 S and 23 S ribosomal RNA become labeled after at least 15 min. Short pulse labeling of 5 min results in peaks of radioactivity in the 17 S region and at the heavy side of the 23 S peak, as shown by polyacrylamide electrophoresis. During chase treatment with unlabeled uridine after the pulse labeling, a shift of radioactivity from the 17 S to the 16 S region is observed. At the same time, the radioactivity over the 23 S speak becomes symmetrical. In the 17 S region, there are at least two peaks which appear and disappear during chase treatment. From data of specific radioactivity a precursor—end product relation can be deduced. After blocking of the chloroplast translation with spectinomycin, the RNA in the 17 S region is accumulated. This product is not stringently the same as that from pulse labeling experiments, because it migrates slightly faster than 17 S RNA. Removal of the antibiotic results in a shift of the radioactivity to the 16 S region. At the same time, the previously blocked chloroplast ribosome synthesis is reinitiated. Attempts have been made to localize the precursor molecules of 17 S and 23 S within the cell. By means of differential centrifugation it has been shown that the precursor RNA components are located in ribosomal particles. No free precursor molecules are found in the ribosome-free supernatant. This is the case in normal as well as in spectinomycin-treated cells. The results are discussed in view of the possible role of chloroplast ribosomal particles as processing agents for the maturation of chloroplast ribosomal RNA.