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Modified Nucleosides in tRNA

Susumu Nishimura


One of the characteristics of tRNA is the presence of a variety of modified nucleosides, more than 50 of which have been isolated and characterized so far. The structures of the modified nucleosides thus far found in tRNA, except the 2′-O-methylated nucleosides, are shown in Appendix II. Many of these modified nucleosides result from methylation at the base or at 2′-OH of ribose, but there are also a number of hypermodified nucleosides that reflect more complex modification.

I, ψ, s4U, and a number of methylated nucleosides were isolated and characterized over 10 years ago (Nishimura 1972; Dunn and Hall 1975). These modified nucleosides are present in most tRNA molecules. For this reason their detection and isolation in relatively large quantities are straightforward, with unfractionated tRNA being used as a source. By contrast, hypermodified nucleosides that are usually located in the anticodon region are present in only one or a few tRNA species. As a result, their detection in unfractionated tRNA is difficult. Their detection in pure tRNA species is easier, since they constitute at least 1% of the total nucleoside content. Therefore, in general, one must have a large quantity of pure tRNA species for isolation and characterization of a new modified nucleoside. In addition, knowledge of the exact location of the modified nucleoside in a specific tRNA molecule may be the only way to definitely establish that it is a legitimate component of tRNA—not an impurity carried through the isolation procedure. Using unfractionated tRNA as a source, this...

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