Stimulation of ribonucleic acid synthesis by steroid hormones. II. High molecular weight components.

In the preceding paper of this series (l), our studies on stimulat.ion of the synthesis of transfer ribonucleic acid in liver and seminal vesicle by the appropriate steroid hormones were described. The 32P-RNA formed during a brief exposure to isotope and isolated by phenol extraction at low temperatures was found to consist of labeled transfer RNA and a poorly defined RNA that was tentatively identified as partially degraded chains of high molecular weight RNA. As shown originally by Sibatani, Dekloet, Allfrey, and Mirsky (a), most of the rapidly labeled RNA is, however, not extracted at low temperature, but remains bound to nuclear components that form an interphase between aqueous and phenol phases after centrifugation. This labeled RNA can be extracted into the aqueous phase by performing the phenol extraction at elevated temperatures (2, 3), and Georgiev, Samarina, Lerman, Smirnov, and Severtzov (4) have described a differential thermal fractionation whereby different types of rapidly labeled RNA can be separated. In this paper, we have employed a modification of the procedure of Georgiev et al. (4) extracting all of the labeled RNA into two or three fractions. The RNA of each fraction has been characterized by base composition analysis and sucrose density gradient centrifugation. In liver, the labeled RNA remaining in the interphase after low temperature phenol extraction consists of a high molecular weight precursor to ribosomal RNA and an RNA that resembles deoxyribonucleic acid in base composition. Synthesis of each of these is stimulated by steroid hormones.