Studies on Nucleolar 4 to 6 S Ribonucleic Acid of Novikoff Hepatoma Cells

Nucleolar 4 to 6 S RNA of Novikoff hepatoma ascites cells was fractionated by exclusion chromatography on Sephadex G-100. Three peaks of RNA were separated: I, a fraction that emerged in the void volume; II, a fraction emerging in the exclusion volume of 5 S RNA; and III, a 4 S RNA peak. Peak I, eluted in the void volume, contained approximately half the nucleolar 4 to 6 S RNA and had the highest incorporation of radioactivity among the three fractions 20 to 40 min after 32P-orthophosphate was injected into the ascites fluid. Each peak contained one major and one or more minor RNA bands of different mobility on polyacrylamide gel electrophoresis; the fastest moving bands were in the 4 S RNA fraction of Peak III, the slowest moving bands were in Peak I, and RNA bands of intermediate mobility were in Peak II. In addition, Peak I contained an RNA band which has not hitherto been described. Although the template activity of the RNA in each of the peaks was low, that of Peak I was the highest of the three fractions. The RNA in Peak II, which corresponds to ribosomal 5 S RNA in electrophoretic mobility and elution from Sephadex, had a higher content of uridylic acid (28.8%) and a lower content of cytidylic acid than ribosomal 5 S RNA. Virtually all of the amino acid acceptor activity was found in the 4 S RNA of Peak III. The presence of RNA other than amino acid acceptor RNA in Peak III is indicated by the base composition of the RNA, the low level of amino acid acceptor activity, and the low level of incorporation of 14C-methylmethionine. These studies indicate that nucleolar 4 to 6 S RNA contains components that are different from those of the cytoplasmic soluble 4 S RNA or the ribosomal 5 S RNA as indicated by base composition, template activity, and amino acid acceptor activity.