Structural Analysis of Nucleolar Precursors of Ribosomal Ribonucleic Acids: II. POLYPURINE SEQUENCES AS CHEMICAL MARKERS OF PROCESSING OF NUCLEOLAR 45 S RIBONUCLEIC ACID

Abstract

Abstract To compare the distributions and structures of complete pancreatic RNase digestion products of highly labeled 45 S, 35 S, and 28 S nucleolar RNA and 18 S ribosomal RNA of the Novikoff hepatoma, digests were chromatographed on DEAE-Sephadex A-25 columns at pH 7.6 and 3.3. Each purified oligonucleotide peak was completely digested with T1 RNase. In addition, the frequencies of the mono- and dinucleotides were determined by electrophoresis at pH 3.5. The distributions of the oligonucleotides were very similar for 45 S, 35 S, and 28 S nucleolar RNA and differed markedly from those found for 18 S ribosomal RNA. All the nona- and decanucleotides isolated thus far from nucleolar 45 S RNA were also found in nucleolar 35 S and 28 S RNA when analyzed by chromatography on DEAE-Sephadex A-25 columns at pH 3.3 and complete digestion with T1 RNase. Rechromatography of the nonanucleotides of 18 S ribosomal RNA produced only three well defined peaks compared to six for nucleolar 28 S RNA. The decanucleotides identified in 45 S, 35 S, and 28 S nucleolar RNA were absent in 18 S ribosomal RNA. The mono- and dinucleotide frequencies of 18 S ribosomal RNA differed significantly from those determined for 45 S, 35 S, and 28 S RNA. In a comparison of the longest oligonucleotide sequences present in complete pancreatic RNase digests of nucleolar 45 S and 28 S RNA and 18 S ribosomal RNA, a tridecanucleotide with the partial sequence (C3UGp)(A-Gp)(C-Gp)(Gp)(Gp)(Gp)Cp was found to be present in ribosomal 18 S RNA and nucleolar 45 S RNA but absent from nucleolar 28 S RNA. This result provides a chemical evidence for the presence of 18 S ribosomal RNA in 45 S nucleolar RNA.