CHARACTERISTICS OF METHYLATION IN-VIVO OF RIBOSOMAL RNA IN YEAST

The characteristics of the methylation in vivo of ribosomal RNA (rRNA) in yeast has been studied by examining the pattern of incorporation of labeled methyl groups into rRNA and into ribosomal precursor RNA. 1. 1. Steady-state labeling with [Me-14C]methionine showed that the degree of methylation for 17-S and 26-S rRNA is very similar, being about 1 methyl group per 70 nucleotides for both rRNA's. The methylation of yeast rRNA was found to be mainly ribose methylation, amounting to 74 and 83% of the total methylation for 17-S and 26-S rRNA, respectively. Chromatographic analysis of the alkaline hydrolysates of the two separate rRNA's revealed that the distribution of the methyl groups along the polynucleotide chains is distinctly different for the two rRNA components. 2. 2. The kinetics of methylation was studied by simultaneous pulse-labeling with [Me-3H]methionine and [14C]uracil. The results showed that methylation starts already at the level of the first ribosomal RNA precursor, immediately after or at the time of its transcription. However, from the ratio of the incorporation of [3H]methyl and [14C]uracil, it could be inferred that additional methylation takes place at later stages of the maturation process of rRNA, presumably after the conversion of ribosomal precursor RNA to rRNA. 3. 3. By analysis of the distribution of the methyl label in rRNA after increasing periods of labeling with [Me-3H]methionine, it could be demonstrated that the additional methylation is base methylation, occurring in both 26-S and 17-S rRNA. © 1969.