Molecular characterisation of the tyrosine tRNA genes of yeast

Abstract

MANY of the tRNA genes of yeast are observable genetically as nonsense suppressors¹. In particular, the set of unlinked suppressor loci SUP2 to SUP8 and SUP11 have been shown to insert tyrosine at nonsense codons in yeast iso-1-cytochrome c^2,3. Direct biochemical evidence for the presence of suppressor tRNA in yeast strains bearing these suppressor genes has been obtained using in vitro translation systems^4,5 and by the demonstration that a new minor tyrosine tRNA species with a suppressor anti-codon appears in a SUP5 strain⁶. Although these findings suggest that there are eight different loci in yeast which code for tRNA^tyr, only a single nucleotide sequence has been observed for tyrosine tRNA in wild-type yeast strains⁷. Thus it seems that all eight genes may share a common base sequence, at least in their structural regions. Because the yeast tRNA^tyr SUP genes occupy known positions on the genetic map⁸ and are complementary to a well characterised RNA molecule they are attractive candidates for molecular analysis. As a first step, we have labelled yeast tRNA^tyr with ¹²⁵I and have hybridised this probe to electrophoretically fractionated Eco RI fragments of yeast DNA; eight Eco RI fragments of different sizes hybridise to tRNA^{t yr}.