RNA polysomes and RNA synthesis in the silk glands of the silkworm Bombyx mori;.

Total RNA polymerase activity, as well as the proportion of alpha-amantin-sensitive and resistant during activity, have been measured in the posterior silk glands of the silkworm as a function of growth the fifth larval instar. During the first 5 days, termed the growth phase, the total enzyme activity and particularly the portion that is alpha-amantin-resistant increases to reach a peak value and thereafter declines during the secretory phase, Much of the enzyme remains firmly bound and insoluble. Heparin only only does not inhibit this insoluble and probably chromatin-bound activity which would indicate lack of initiation, but it enhances the activity. A large proportion of newly transcribed RNA is released from the transcription complex. The synthesis of RNA has been studied both qualitatively and quantitatively during the same period. RNA synthesis becomes important on the second day of the fifth instar, as does the RNA polymerase, and stays at a high level for several more days. The results from these studied as well as those with incorporation of 32P indicate interference of varying precursor pools in quantitatively measured RNA synthesis. However, RNA content as well as RNA synthesis in vitro show a close correlation with RNA polymerase activity. The labeled RNAs extracted at different days of the fifth instar have been fractioned on sucrose gradients; this demonstrated that the predominant product of RNA synthesis, as followed by [3H]uridine incorporation at short time intervals, is 45-S preribosomal RNA and 4-5 S RNA. The 45-S RNA is transformed to 19-S and 30-S ribosomal RNA as time progresses or after a chase with unlabeled and/or actinomycin D. There also exists a component heavier than 45S which is fairly rapidly labeled to a small extent.