The absence of ribothymidine in specific eukaryotic transfer RNAS: I. Glycine and threonine tRNAS of wheat embryo

Abstract

Ribothymidine, generally considered a universal nucleotide in tRNA, is completely absent in five specific wheat embryo tRNAs. These consist of two species of glycine tRNA and three species of threonine tRNA. These tRNAs, all extensively purified, are acceptable substrates for E. coli - ribothymidine forming-uracil methylase, which produces one mole of ribothymidine per mole of tRNA. These five tRNAs account for about 90% of the wheat embryo tRNAs which are substrates for this methylase. Nucleotide sequence analysis of one of these tRNAs, tRNA Gly I, confirmed both the complete absence of ribothymidine at position 23 from the 3′end, and the presence of uridine at that site instead. In addition, it is shown that methylation with E. coli uracil methylase quantitatively converts uridine at position 23 to ribothymidine, while no other uridine in the molecule is affected. Using E. coli uracil methylase as an assay we have detected this class of ribothymidine lacking tRNA, in each case consisting of a few specific species, in other higher organisms, such as wheat seedling, fetal calf liver and beef liver, in addition to wheat embryo. We could not detect this class of tRNA in E. coli or yeast tRNA.