Base ratios in RNA molecules of different sizes from a Balbiani ring

Abstract

The base ratios in newly synthesized RNA from a chromosome puff (Balbiani ring 2) in the salivary glands of Chironomus tentans were compared with the corresponding ratios in RNA produced in chromosome I–III and in the nucleoli. Salivary glands were incubated in larval haemolymph supplemented with tritiated cytidine and uridine. Balbiani ring 2, chromosome I–III and nucleoli were isolated from 50 to 70 cells by micromanipulation. The labelled RNA from the two chromosomal samples was fractionated by electrophoresis in agarose. RNA from different parts of the migration path was eluted. Fractionated, as well as unfractionated, RNA was hydrolysed in alkali and the labelled products in the hydrolysate, CMP and UMP, were separated by thin-layer chromatography and their radioactivities determined. The CMP/UMP activity quotients were calculated and the following results obtained: 1. (1) The newly synthesized RNA in Balbiani ring 2 is different from that in chromosome I–III and in the nucleoli. 2. (2) Rapidly labelled, high molecular weight RNA of a given size in Balbiani ring 2 is different from RNA of the same size in chromosome I–III. 3. (3) There is no difference between rapidly labelled, high molecular weight RNA molecules of different sizes within Balbiani ring 2 and within chromosome I–III. The results show a similarity between giant RNA molecules of different sizes produced in a single chromosome puff. This may indicate that this newly synthesized RNA contains repeating sequences; but alternative explanations are possible.