Methylation of Ribosomal-Precursor RNA, Synthesized in vitro, by Isolated Rat-Liver Nucleoli

Nucleoli isolated from rat liver were incubated for synthesis of RNA in vitro in the presence or absence of S-adenosyl [3H] methionine. The results obtained indicate that neither the rate of RNA synthesis not the processing of pre-ribosomal RNA was changed if methylation was allowed to take place. The methylation process acts on the RNA most recently synthesized, rather than on the bulk of the RNA already present in the nucleoli. The reaction seems to occur faithfully both quantitatively and qualitatively. It is calculated that 104 mol methyl groups were incorporated per mol of newly synthesized 45-S RNA. Methylation of the ribose rather than the bases predominated. The pattern of alkali-stable oligonucleotides of RNA methylated in vitro was analyzed and found to correspond closely to that of ribosomal RNA labelled in vivo.