Chapter 8 The Modified Nucleotides in Ribosomal RNA of Man and Other Eukaryotes

Abstract

Publisher Summary Ribosomes of man and other vertebrates contain more than 200 modified nucleotides. In particular, ribosomes from human cells contain about 110 2'-0-methylated nucleotides, a small number of base-modified nucleotides and about 95 pseudouridine residues. Approximately 80 of the modified nucleotides occur in 18S rRNA in the small ribosomal subunit; four occur in 5.8S rRNA and the rest (about 130) are in 28S rRNA in the large ribosomal subunit. All of the 2'–0–methyl groups are added to ribosomal precursor RNA in the nucleolus soon after transcription. Many or all of the pseudouridine modifications also take place on ribosomal precursor RNA. Some of the base modifications occur late during ribosome maturation. The exact locations of all the methyl groups in the primary structure of 18S rRNA from Xenopus laevis and man have been determined. The methyl groups are widely but non-uniformly distributed, with a major cluster of 2'-0-methyl groups in a section of the 5 region of the molecule which shows high phylogenetic sequence conservation. Some of these methyl groups are in sequence tracts which are relatively inaccessible to S1 nuclease, suggesting that they are buried within complex tertiary structure. Partial data on the locations of the methyl groups in 28S rRNA also indicate clustering in phylogenetically conserved sequences, with a major cluster in the 3' region of the molecule. Only a few pseudouridines in rRNA have so far been located.