Yeast tRNA Precursors: Structure and Removal of Intervening Sequences by an Excision-Ligase Activity

Abstract

The tRNA genes provide an attractive system for studying gene structure and expression in eukaryotes. Using recombinant DNA techniques, we have isolated and determined the structure of the yeast sequences coding for tRNA^Tyr (Goodman et al. 1977) and tRNA^phe (Valenzuela et al. 1978). These studies resulted in the discovery that these DNAs contain additional sequences that are not used in encoding the final gene products. Immediately to the 3′ side of their anticodon triplets, the yeast genes coding for tRNA^Tyr and tRNA^phe contain an intervening DNA segment that is not present in the mature tRNAs.

There are eight unlinked genetic loci for tRNA^Tyr in yeast (Olson et al. 1977) and the nucleotide sequences of three have been determined (Goodman et al. 1977). All three genes contained a 14-bp intervening sequence, ATTTAYCACTACGA (Y is a pyrimidine). One of these was identified as coding for the suppressor tRNA associated with the genetic locus sup4. This demonstrated that the intervening sequence was present in an active gene, since the sup4 locus for tRNA^Tyr containing the intervening sequence confers a dominant phenotype on sup4-o strains.

There are at least ten unlinked tRNA^phe genes in a tetraploid yeast strain (Feldman 1977; Valenzuela et al. 1978). The nucleotide sequence of four alleles has been determined to contain an intervening 19- or 18-bp sequence, AAAAACTTCGGTCAAGTTA or AATACTTCGGTCAAGTTA (Valenzuela et al. 1978). Thus, the intervening sequences in the tRNA^phe and tRNA^Tyr genes have no obvious sequence homology or structure, yet both occur in a similar position in the...