Mutations at theSaccharomyces cerevisiae SUP4tRNATyrLocus: Isolation, Genetic Fine-Structure Mapping, and Correlation with Physical Structure

The SUP4 tRNA Tyr locus in Saccharomyces cerevisiae has been studied by the isolation and characterization of mutations at the SUP4 gene which result in the loss of suppressor function. Most of the mutations act as single-site mutations, whereas about a third of the mutations are deletions of the entire gene. Two meiotic fine-structure maps of the gene were made. The first mapping technique placed 10 mutations plus the sup4 + anticodon on a map by a measurement of levels of recombination between pairs of mutations. The second map utilized a more qualitative estimate of recombination frequency, allowing 69 mutations and the sup4 + anticodon to be mapped. The maps were compared with the physical structure of the gene for the 34 mutations whose nucleotide alteration has been determined by DNA sequencing (Koski et al., Cell 22 :415-425, 1980; Kurjan et al., Cell 20 :701-709, 1980). Both maps show a good correlation with the physical structure of the gene, even though certain properties of genetic fine-structure maps, such as marker effects and “map expansion,” were seen.