An autoradiographic study of RNA synthesis in isolated salivary glands of Drosophila hydei. I. Autoradiographic studies

Abstract RNA synthesis in isolated Drosophila salivary glands was studied by autoradiography. The dry mass of cellular structures was determined by interference microscopy; a correction which compensates for self‐absorption of β‐particles, and consequent lowered grain counts, is discussed. The correction factors are 12.0 for nucleolus, 5.0 for nucleoplasm and 8.8 for cytoplasm. After 15 seconds in uridine‐H3, label was localized over the nucleolus; after one minute, nucleoplasm RNA became labeled also. In situ synthesis of some nucleolar RNA is indicated. Comparisons of uridine‐H3 and cytidine‐H3 incorporation were made in glands exposed briefly to isotope, followed by transfer to unlabeled nucleoside solutions. The data suggest two RNA fractions in nucleoli. Rapid turnover and higher uridine incorporation characterized one fraction, which resembled RNA of nucleoplasm metabolically. The second nucleolar RNA incorporated less uridine, remained longer in the nucleolus, and resembled cytoplasmic RNA in precursor incorporation pattern. Preincubation in Actinomycin D before uridine‐H3 labeling resulted in 80% inhibition of RNA synthesis in both nucleolus and nucleoplasm. Longer isotope exposures produced some increase in labeling. Actinomycin treatment delayed appearance of label in cytoplasmic RNA. After HCl extraction of uridine‐H3‐labeled RNA, some activity, presumed to be incorporated in DNA, remained. This non‐extractable label appeared earliest over nucleoli, and subsequently over chromatin. Actinomycin treatment abolished incorporation of non‐extractable label into nucleoli.