Steps in the Biosynthesis of Ribosomal RNA in Cultured Aedes aegypti Cells

Abstract

The continuous Aedes aegypti cell line developed by Grace in 1966 grows in a medium which is free of the non‐defined constituents commonly used in the continuous culture of insect cells. In the work presented, advantage has been taken of this fact to carry out labelling experiments which demonstrate the sequence of steps in the biosynthesis of rRNA in this continuous insect cell line under specified growth conditions. Preliminary experiments show that the large and small rRNAs of A. aegypti cells have molecular weights of 1.5 × 106 and 0.7 × 106, respectively, as determined by polyacrylamide gel electrophoresis; the corresponding sedimentation values are 26 S and 18 S. On incubating cells for 20 min at 30 °C with [3H]uridine four major RNA species are labelled. These have molecular weights of approximately 3.8 × 106 (41 S), 2.1 × 106 (30 S) 1.3 × 106 (25 S) and 0.7 × 106 (18 S). When prelabelled cells are incubated with actinomycin, 41‐S RNA is rapidly converted into the 30‐S and the 25‐S species. The 30‐S RNA thus formed is slowly degraded, probably by exonucleolytic breakdown, to give 26‐S RNA; the large rRNA is first labelled after 45‐min incubation with [3H]uridine. 25‐S RNA is the probable precursor of 18‐S RNA; the small rRNA is first labelled after 20 min incubation with [3H]uridine. The results obtained are discussed in relation to previous studies on the molecular weights and biosynthesis of rRNA in mammalian, insect, amphibian and plant tissues. Copyright © 1972, Wiley Blackwell. All rights reserved