Isolation of nucleolar methylase producing only 5-methylcytidine in ribosomal RNA

Abstract

Methylation of rRNA mostly occurs in the nucleolus of mammalian cells. We have isolated nucleoli from Ehrlich ascites tumor cells of mice and purified RNA methylase taken from them. This highly purified nucleolar methylase produces only 5-methylcytidine in hypomethylated 18S and 28S rRNAs prepared from the mouse tumor cells after treatment with cycloleucine. This enzyme, however, did not transfer the methyl-group to normally methylated rRNA from the same mouse tumor cells. This high substrate specificity and enrichment of this enzyme in the nucleoli strongly suggest that we have isolated one of the enzymes which physiologically methylate rRNA precursor in the nucleoli.