Isotope content of oligonucleotides of nuclear and nucleolar RNA of rat liver

Abstract

Nuclear and nucleolar RNA of rat liver labeled with [6-14C]orotic acid or [32P]-orthophosphate were separated into sedimentation classes of 18-S, 28-S, 35-S, 45-S and 55-S nuclear RNA and 28-S, 35-S and 45-S nucleolar RNA. After purification of each class of RNA, the distribution of radioactivity was determined in the oligonucleotides obtained by hydrolysis with pancreatic ribonuclease after they were separated by a mapping technique.For nucleolar 28-S, 35-S and 45-S RNA, no marked differences were found for distribution of either isotope in the oligonucleotides. The distributions of isotope in oligonucleotides of nuclear 28-S and 35-S RNA were very similar; these oligonucleotide distributions differed from those of the 18-S and 55-S nuclear RNA which were also very similar. On the basis of the distribution of isotope in the oligonucleotides, liver nuclear 28-S, 35-S and 45-S RNA appear to be mixtures of the nucleolar, GC-rich RNA and the cosedimenting AU-rich RNA.The data are interpreted by the hypothesis that 28-S, 35-S and 45-S nucleolar ribosomal precursor RNA have very similar primary structures. Since 18-S nuclear RNA has a very similar distribution of isotope in oligonucleotides to that of 55-S nuclear RNA, the latter may be a precursor of nuclear 18-S RNA.