Methylation and processing of ribosomal RNA in HeLa cells.

The methylation of RNA in HeLa cells has been studied. After incorporation of radioactive methyl-labeled methionine, the cells were fractionated into cytoplasmic, nuclear, and nucleolar fractions. Methylation of high molecular weight RNA is shown to be exclusively in the 45 s ribosomal RNA precursor and takes place in the nucleolus. The methylation of the 45 s RNA apparently occurs close to the growing point of newly synthesized RNA as evidenced by the existence of methylated, partially completed molecules after very short pulse labeling. The methyl-labeled RNA was chased with non-radioactive methionine and the progression of the ribosomal RNA precursors was followed through the cell fractions. The data support the previously published hypothesis that nucleolar 45 s RNA is the precursor to cytoplasmic 18 s RNA and 32 s nucleolar RNA. The 32 s RNA is converted to 28 s RNA, which is then found in the nucleoplasm and is the apparent precursor to cytoplasmic 28 s RNA.