Artificial conversion of the mating-type of Saccharomyces cerevisiae without autopolyploidization

Crossbreeding is a classical yeast hybridization procedure, where the mating of haploid cells of opposite mating-type, MATa and MATα cells, produces a new heterozygous diploid. Here, we describe a method to generate haploid MATa and MATα cells using mating-type conversion caused by expression of the HO gene, which encodes an endonuclease. Importantly, our method prevents the autopolyploidization that typically arises during artificial mating-type conversion. This facilitates isolation of the desired mating-type of yeast cells with simple and easy procedure. In the current study, we designed a suitable genetic circuit for each haploid cell and converted MATα haploid cells into MATa haploid cells and vice versa, demonstrating the utility of constructed artificial regulation network to prevent autopolyploidization. Via forced expression of the a1 gene in MATα haploid cells or of α2 in MATa haploid cells, the undesirable mating ability of yeast cells was completely suppressed. We confirmed the success in prevention of autopolyploidization by ploidy analysis. This new approach provides a reliable and versatile tool for yeast crossbreeding, so that it will be useful for scientific research and industrial applications of yeast.