Analysis of the Methylation Sites in Yeast Ribosomal RNA

The methylation sites and the number of methyl groups of 26‐S and 17‐S ribosomal RNA (rRNA) of Saccharomyces carlsbergensis were investigated. 28‐S and 17‐S rRNA contain 43 and 24 methyl groups per molecule, respectively, of which in both molecules 6 are attached to the heterocyclic bases. In 26‐S rRNA the base‐methyl groups are present as two copies of m 3 U, m 5 C as well as m 1 A, whereas the methylated bases in 17‐S rRNA are one m 7 G, one methylated cytidine and two of m 2 6 A. In 17‐S rRNA there are 11 different alkali‐stable ( i.e. ribose‐methylated) dinucleotides, while 26‐S rRNA contains 15 different alkali‐stable dinucleotides, and in addition 4 alkali‐stable trinucleotides (Am‐Cm‐Gp, Am‐Gm‐Cp, Am‐Am‐Up and Um‐Gm‐Ψp). Most of the methylated oligonucleotides found in a ribonuclease T 1 digest of 26‐S or 17‐S rRNA occur in approximately one‐molar amounts. The combined data suggest that yeast ribosomal RNA is essentially homogeneous in its methylated sequences and in addition that the methylation is a highly specific process.