A system for inducing mutagenesis in Saccharomyces cerevisiae by insertion of Ty elements. Ty elements fused to a GAL1 promoter and marked with the HIS3 gene are induced to transpose from a plasmid to the genome of the host cell by plating on galactose. Transposition confers the His phenotype on the his host strain after loss of the plasmid. Insertion mutations in mating-type genes in MATa cells are selected by resistance to growth arrest by alpha-mating-type pheromone. Genetic analysis and Southern hybridization have identified 9 TyHIS sterile mutants, including one in ste2 and 4 which activate the silent mating-type cassette, HMLalpha. Unlinked suppressors have appeared in 3 of the HMLalpha- activated mutants, and will be analyzed in other Ty insertion mutants and in SPT and spt strains. Further experiments will refine the induction protocol to reduce insertion of multiple Ty elements, and simplify the process of identifying new HIS marked mutants by selecting random haploid progeny and screening for the co-segregation of the His phenotype with the mutant phenotype. Mutagenesis by Ty insertion to activate cryptic genes in other systems in addition to the mating-type genes will be attempted. This system, if successfully developed, will provide a useful tool for scientists in research laboratories. It will also serve to introduce community college students to state-of- the-art molecular genetics.
