Base composition of ribosomal RNA precursors in the HeLa cell nucleolus: Further evidence of non-conservative processing

Abstract

The base compositions of the ribosomal RNA precursors in the HeLa cell nucleolus have been measured. This was difficult previously because of contamination of nucleolar RNA by heterogeneously sedimenting RNA of different composition originating in the nucleoplasm. Purified nucleolar RNA was obtained by modifying the technique of nucleolar isolation and by ultraviolet inactivation of nucleoplasmic RNA. Acrylamide gel electrophoresis was also used to achieve much improved resolution of the nucleolar RNA components. The base composition of 45 s ribosomal precursor was found to be 70 mole % guanine plus cytosine compared to 64% predicted for a precursor containing one 28 s and one 18 s piece of ribosomal RNA. This result is in agreement with the previous finding that only one-half of the 45 s RNA eventually becomes mature ribosomal RNA. The gel electrophoresis technique also made possible the separation of nucleolar 32 s and 28 s RNA. Labeling of the nucleolar 32 s and 28 s RNA is difficult under conditions of phosphate starvation since processing the newly synthesized 45 s precursor rapidly ceases. However, processing resumes upon the addition of cold phosphate. The 32 s RNA has a base composition of 70 mole % G + C compared to 67% for 28 s RNA.