Evidence for Transport of 75S RNA from a Discrete Chromosome Region via Nuclear Sap to Cytoplasm in Chironomus tentans

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Abstract
Salivary glands of the dipteran Chironomus tentans were exposed to tritiated pyrimidines for different time periods either in vitro or in vivo . Nonribosomal, high molecular weight RNA molecules of very different sizes (15-100 S) were labeled on chromosomes I-III, while only one main species, 75S RNA, was recorded in Balbiani ring 2, a giant chromosome puff on chromosome IV. 75S RNA was present as a prominent fraction in nuclear sap, but not in cytoplasm, after 90 min of incubation in vitro . During the ensuing 90 min, 75S RNA also appeared in cytoplasm. After 1 week of labeling in vivo , radioactive 75S RNA accumulated heavily in cytoplasm. Absorbance measurements showed that 75S RNA constituted as much as 1.5% of the total salivary-gland RNA. The half-life of 75S RNA was estimated to exceed 35 hr. Available data on the intracellular distribution and metabolic stability of nonribosomal, high molecular weight RNA from Balbiani ring 2 and chromosomes I-III indicate that most of the cytoplasmic 75S RNA is transcribed in the Balbiani ring 2 region of chromosome IV. 75S RNA molecules are, therefore, likely to be transferred from Balbiani ring 2, via nuclear sap, to cytoplasm without being measurably reduced in molecular size.