Methylation process of mammalian ribosomal precursor RNA

The analysis of RNA from L5178 Y cells, by electrophoreis in 1.7% polyacrylamide gel, after short time labelling with [methyl-3H] methionine, shows that the three molecular species 47S, 46S and 45S RNA from the ribosomal RNA "45S fraction" are methylated. Comparative studies of the kinetics obtained after labelling for short periods with [methyl-3H] methionine and [3H] uridine, and brief labelling experiments with [methyl-3H] methionine in the presence of actinomycin D, show that the methylation of ribosomal precursor RNA occurs after the transcription process has been completed, and that methylation and transcription are not coordinated. These results lead us to propose the following scheme for ribosomal RNA biosynthesis : 1) transcription of ribosomal genes, 2) methylation of 47S RNA (and possibly 46S RNA), 3) maturation of these molecules which are converted to 45S RNA.