Evidence for transport of 75S RNA from a discrete chromosome region via nuclear sap to cytoplasm in Chironomus tentans.

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.