The methylated nucleotide sequences in HeLa cell ribosomal RNA and its precursors

Abstract

The methylated nucleotide sequences in HeLa cell ribosomal RNA and its nucleolar precursors were examined by RNA fingerprinting and sequencing methods. 18 S RNA was found to contain approximately 46 methyl groups, 28 S RNA some 70 methyl groups and 5.8 S RNA one methyl group. Most methyl groups occur in different T 1 ribonuclease oligonucleotides, and most of these were recovered approximately once per molecule of 18 S or 28 S RNA. There are also, however, several multiply methylated oligonucleotides, a few short products that occur more than once and a few “fractional” products. The great majority of methylations occur at the level of 45 S RNA, but six further methylations occur late during the maturation of 18 S RNA, and one fractional one occurs during 28 S maturation. The transcribed spacer regions of the precursor molecules are unmethylated. Chemical analysis of the methylated components and sequences indicates that all except five “early” methylations are on ribose groups, the remaining five being on bases within the 28 S sequence. The late methylations are all on bases, four of those on 18 S RNA giving rise to the sequence, … Gpm 2 post6Apm 2 post6ApCp… The product, pCmpUp, previously reported by Choi & Busch (1970) as being the 5′ end-group of rat hepatoma 28 S, 32 S and 45 S RNA, is not present in HeLa cell 28 S RNA or its precursors. Implications of this work are discussed.