Structural Analysis of Nucleolar Precursors of Ribosomal Ribonucleic Acid

Abstract To estimate the chain lengths based upon the 5' terminals and to provide information on methylated oligonucleotides, nucleolar 28 S, 35 S, and 45 S RNAs were subjected to alkaline hydrolysis and the resultant oligonucleotides were separated and identified. The 5' terminal of each RNA class was found to be mainly (80%) the methylated dinucleoside triphosphate pCmpUp. In addition, smaller amounts of pCp (10%) and pAp (10%) were found. Based on the 5' terminal analysis, the ratio of the approximate chain lengths of the polynucleotide chains is 1:1.5:2.5 in nucleolar 28 S, 35 S, and 45 S RNA, respectively, and is in good agreement with ratios derived from hybridization studies and equilibrium sedimentation studies. An alkali-stable trinucleotide (UmpUmpCp) is present in all of the nucleolar RNA classes in amounts equimolar to the terminal pCmpUp. The 16 types of 2'-O-methylated alkali-stable dinucleotides present in the nucleolar 28 S, 35 S, and 45 S RNAs accounted for 1.2, 1.1, and 0.9% of the total residues in 28 S, 35 S, and 45 S nucleolar RNA, respectively. From these values, and the cleavage patterns derived from the hybridization data, a working model of nucleolar 45 S RNA is presented in which the 28 S nucleolar RNA is placed at the 5' end of the molecule.