Relationship Between Methylation and Maturation of Ribosomal RNA in Prokaryotic and Eukaryotic Cells

Abstract

All ribosomal RNAs (except the 5 S rRNA present in the large subunit of ribosomes) contain a relatively large number of methylated nucleosides (about 1 % of the total number) (Attardi and Amaldi, 1970 ; Hall, 1971), but the distribution of methyl groups in prokaryotic and eukaryotic ribosomal RNAs is strikingly different (see Table 1 of Maden, 1976 and Table 1 of Björk, 1983). E.coli 16 S and 23 S rRNAs contain a small number of 2′−0 methylated ribose residues (Xm) (Nichols and Lane, 1967, 1968) and most (> 80 %) of methyl groups occurs as methylated bases (Fellner and Sanger, 1968 ; Van Charldorp et al., 1981), whereas more than 90 % of the methyl groups found in eukaryotic 18 S and 28 S rRNAs are present in 2′−0 methylated ribose residues (Singh and Lane, 1964 ; Hashimoto et al., 1975 ; Maden and Salim, 1974 ; Cecchini and Miassod, 1979 ; Connaughton et al., 1984). The 5.8 S rRNA, hydrogen-bonded to the 28 S rRNA and present in the large subunit of eukaryotic (but not of prokaryotic) ribosomes, contains two 2′−0 methylated ribose residues (Nazar et al., 1975a, b ; MacKay and Doolittle, 1981 ; Schnare and Gray, 1982). Whereas phosphodiester bonds adjacent to methylated bases in RNA display normal sensitivity to ribonucleases and alkaline pH, those which bear a 2′−0 methylated ribose on the 3′ side are resistant to these agents since formation of a 2′−3′ cyclic phosphate intermediate is impossible.