Mutations at the yeast SUP4 tRNATyr locus: DNA sequence changes in mutants lacking suppressor activity.

Yeast strains harboring indepjendent mutations within the SUP4 tyrosine tRNA gene have been selected by virtue of their inactivating effect upon the SUP4-o UAA suppressor. Three fourths of the mutations at SUP4 are point alterations; the rest resemble the deletions described by Rothstein (1979). A meiotic genetic fine structure map of the locus was made by crossing 69 of the mutants in all combinations and testing for the frequency of SUP4-o recombinants. The sequences of SUP4 genes cloned from 32 mutant strains were determined by the dideoxynucleotide terminator method, using as primer a synthetic oligodeoxynucleotide corresponding to a sequence adjoining the SUP4 3' terminus. The positions of the DNA sequence alterations showed good colinearity with the positions of the mutations on the genetic map. One of the 26 mutant sites found by DNA sequencing lies within the intervening sequence. At this site three repeat mutations were found, each changing AT leads to TA. Whereas mutations were generally rather uniformly distributed throughout the tRNATyr coding sequence, none occurred in the DNA sequences flanking the mature tRNATyr sequence or in a 12 nucleotide sequence including the 10 bp which constitute the 3' side of the intervening sequence.