Synthesis and processing of ribosomal ribonucleic acid in salivary gland cells of Chironomus thummi

Experimental data on the RNA content and RNA base composition in different microisolated cell compartments, and on the synthesis and processing of ribosomal RNA in cells of the Chironomus thummi salivary gland are presented. The nucleolus of a Chironomus salivary gland cell contains about 250 pg RNA representing nearly 50% of the RNA of chromosomal material. The RNA content of chromosomes I–III from 82, 79 and 73 pg corresponds well to their relative lengths. The base composition of nucleolar RNA (46.5% G + C) is similar to that of cytoplasmic RNA (47% G + C) but different from Balbiani ring RNA (37% G + C). The synthesis and processing of precursor and ribosomal RNA were studied in vitro by incubation of glands in Cannon's insect medium supplemented with [3H]uridine and [3H]cytidine, and in vivo by injection of the isotopes into animals followed by gel electrophoresis of RNA extracted from microisolated nuclear components (nucleolus, chromosomes, nuclear sap) and cytoplasm. During 1 h of incubation, mainly one RNA fraction with a molecular weight of about 3 × 106 (38‐S RNA) is synthesized within the nucleolus. This becomes converted into smaller RNA molecules of 1.66 ×106 (30‐S) and 0.96 × 106 (23‐S) during long‐term labelling (or under chase conditions). In some of the nucleolar RNA preparations 0.72 × 106 (18‐S) RNA was also detected. Low‐molecular‐weight RNA in the 4–5‐S range is present after all incubation times, but without a distinct 4‐S peak while 5‐S RNA increases in amount with time of incubation. An additional low‐molecular‐weight RNA possessing a molecular weight of about 0.77 × 105 was detected in preparations of nucleolar RNA labelled for at least 3 h. RNA from chromosomes I–III contains, besides HRNA and low‐molecular‐weight RNA, RNAs with molecular weights 1.55 × 106 (“30‐S”), 0.96 × 106 (23‐S) and/or 0.72 × 106 (18‐S) representing (pre)‐rRNA species. “30‐S” and 18‐S RNA are also present in the nuclear sap while a distinct 23‐S peak is absent. Although in whole larvae mature rRNAs can only be found after 1‐h labelling, clear rRNA peaks in cytoplasmic RNA from salivary glands are not found earlier than after a 6‐h incubation. An RNA of molecular weight 0.65 × 105 first observed in nucleolar RNA preparations and accumulated in cytoplasm is suggested to be (pre)‐rRNA formed during processing by an endonucleolytic activity.