Kinetic evidence that a discrete messenger-like RNA is formed by post-transcriptional size reduction of heterogeneous nuclear RNA

A discrete fraction of heterogeneous nuclear RNA, in addition to the previously described 75 S RNA, has been identified in salivary gland cells of Chironomus tentans. On the basis of its electrophoretic mobility in 1% agarose gel, it is tentatively designated as 35 S RNA. The 35 S RNA possesses distinct and characteristic labelling kinetics. After a lag of 20–45 min it emerges on the chromosomes and in the nuclear sap, but significant quantities of labelled 35 S RNA in the cytoplasm can be found only after 45–90 min of incorporation. The origin of 35 S RNA from genes manufacturing heterogeneous nuclear RNA has been established on the basis of its sensitivity against the nucleoside analogue, 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole, DRB. RNA extract derived from chromosome IV, carrying the Balbiani rings, lacks 35 S RNA. Furthermore, an essentially normal appearance of 35 S RNA was observed in glands having very small or no Balbiani rings and no detectable 75 S RNA synthesis. Both these findings are consonant with the interpretation that 35 S RNA is not generated by genes harboured in the Balbiani rings. When the continuous synthesis of chromosomal RNA is interrupted by actinomycin D, most of the 35 S RNA accumulated in the nuclear sap disappears, whereas a corresponding RNA fraction arrives in the cytoplasm. Kinetic studies as well as the chase experiment with actinomycin D suggest the existence of a kinetic relationship between nuclear sap 35 S RNA and its cytoplasmic counterpart. In addition, studies with the initiation inhibitor DRB strongly indicate that 35 S RNA derives from chromosomal RNA molecules of higher S values than 35 S, probably from RNA in the 75–100 S range, by post-transcriptional cleavages.