Improved stress resistance and ethanol production by segmental haploidization of the diploid genome in Saccharomyces cerevisiae.

Saccharomyces cerevisiae strains from industrial and natural geographical environments are reported to show great variation in copy number of chromosomal regions. Such variation contributes to the mechanisms underlying adaptation to different environments. Here, we created and phenotypically analyzed segmentally haploidized strains, each harboring a deletion of one copy of approximately 100-300 kb of the left or right terminal region of 16 chromosomes in a diploid strain by using a PCR-mediated chromosomal deletion method. No haploidized strain of the 158-kb deleted right terminal region of chromosome III or the 172-kb deleted right terminal region of chromosome VI was produced; however, segmentally haploidized strains of the remaining 30 terminal regions were obtained. Among these 30 strains, two exhibited higher lactic acid resistance and two displayed higher thermo-tolerance at 41°C versus the host diploid strain. By contrast, four and two segmentally haploidized strains showed sensitivity to 6% lactic acid and low temperature at 13°C, respectively. The effect of the decreased copy number of the chromosomal terminal regions on ethanol production was analyzed. As compared with the host diploid strain, a 3.8% and 4.3% improvement in ethanol production in 10% glucose medium was observed for two strains in which one of two copies of the 197-kb left terminal region of chromosome V and one of two copies of the 195-kb left terminal region of chromosome X was deleted, respectively. These results indicate that artificial segmental haploidization might contribute to improvement of industrially important phenotypes and provide a new approach to breeding superior yeast strains.