Conversion of rat liver nucleolar 29.5-S RNA to 28-S RNA in vitro

Abstract

By employing nucleolar RNA, especially 28-S and 29.5-S RNAs, isolated from thioacetamide-treated rat liver nucleoli which were labelled with [ 14C]- or [ 3H]orotic acid, the conversion of 29.5-S RNA to 28-S RNA was investigated and the following results have been obtained. 1. 1. Nucleolar 29.5-S RNA disappears with incubation for 13 min, in the sodium acetate buffer (pH 7.0) above 42 °C, in sodium dodecyl sulphate-phenol above 37 °C and in 6 M urea above 25 °C. The slow cooling after heat treatment of 29.5-S RNA does not result in the reappearance of the RNA, suggesting the disappearance to be an irreversible process. The disappearance caused by heat treatment is inhibited by divalent metal ions (Mg 2+, Ca 2+, Ba 2+, Co 2+ and Mn 2+). 2. 2. With incubation in 10 mM sodium acetate buffer (pH 7.0) at 45 °C for 13 min, nucleolar 28-S RNA decreases to 60–75 %. The decrease is accompanied by the appearance of two low molecular weight RNAs (5.3-S and 6.7-S RNAs) in addition to the heterogeneous RNAs distributed from the 28-S to 12-S regions on acrylamide gel electrophoresis. 3. 3. Under the same conditions as above, the conversion of 29.5-S to 28-S RNAs occurs, which accompanies the release of a specific x component with a molecular weight of 180 000 corresponding to the difference in the molecular weight between 29.5-S and 28-S RNAs, although the heterogeneous RNAs distributed from the 28-S to 12-S regions are also observed. The x component has a similar specific activity to that of 29.5-S RNA after labelling periods of 2 and 6 h. The possible mechanism of the conversion of 29.5-S RNA to 28-S RNA is discussed.