STRUCTURAL ELEMENTS IN YEAST TRANSFER-RNAS REQUIRED FOR HOMOLOGOUS MODIFICATION OF GUANOSINE-26 INTO DIMETHYLGUANOSINE-26 BY THE YEAST TRM1TRANSFER-RNA-MODIFYING ENZYME

In eukaryotic tRNAs, guanosines in position 26 (G26), located at the j unction between the D-stem and the anticodon stem of tRNA, are usually modified to N-2,N-2-dimethylguanosine (m(2)(2)G). Although G26 is a p rerequisite for biosynthesis of m(2)(2)G26, it is not self-sufficient for the formation of the dimethylated G26, since in exceptional cases eukaryotic tRNAs have an unmodified G26. In the yeast Saccharomyces ce revisiae the only tRNA species with an unmodified G26 is tRNA(Asp). Us ing in vitro transcripts of this tRNA, as well as of yeast tRNA(Phe), a tRNA containing m(2)(2)G26 in vivo, we have investigated the require ments on tRNA sequences and structures for the formation of m(2)(2)G26 by the yeast enzyme, i.e. in a homologous in vitro system. We have no w demonstrated that G26 was efficiently dimethylated in vitro also aft er deletion of the entire anticodon stem and loop. We conclude that th e elements necessary for a productive interaction between G26 in nucle ar coded yeast tRNAs and the yeast G26 modifying enzyme are located wi thin the core of the tRNA. For modification of G26 to m(2)(2)G26 via m onomethylated G26, important primary and secondary structural elements in the tRNAs are a size of at least five nucleotides in the variable loop together with two G-C base pairs in the D-stem. This is the first case reported where the minimal requirements on nuclear coded tRNAs f or a yeast modifying enzyme has been elucidated.