Quantitation of Turnover and Export to the Cytoplasm of HnRNA Transcribed in the Balbiani Rings

Some quantitative parameters of the intranuclear metabolism and export to the cytoplasm of Balbiani ring 1 and 2 RNA molecules in salivary gland cells of Chironomus tentans have been determined. Growing RNA chains in the Balbiani rings attain uniform labeling with RNA precursors after 20 min of incorporation. The specific activity of 75S RNA released from the Balbiani rings into the nuclear sap increases rapidly and reaches a maximum level between 90 and 180 min of labeling. After 20 min, labeled 75S RNA enters the cytoplasm and accumulates at a linear rate. However, only a small proportion of the RNA produced at the Balbiani ring loci can subsequently be recovered in the nuclear sap (14-17%) or cytoplasm (4-7%) as 75S RNA; presumably the remainder is degraded entirely. Experiments using inhibitors of elongation (actinomycin D) or initiation (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) revealed that no significant quanity of the 75S RNA transcribed can be chased into the cytoplasm. Both the kinetics of entry of labeled 75S RNA into the cytoplasm-that is, a constant rate of increase after a brief lag-and chase data are incompatible with a precursor-product relationship between the great majority of nuclear 75S RNA and cytoplasmic 75S RNA with messenger characteristics. The results are discussed in relation to the possibility that a post-transcriptional control mechanism is operating in these cells.