Breeding of flocculent industrial alcohol yeast strains by self-cloning of the flocculation gene FLO1 and repeated-batch fermentation by transformants

A nonflocculent industrial polyploid yeast strain, Saccharomyces cerevisiae 396-9-6V, was converted to a flocculent one by introducing a functional FLO1 gene at the URA3 locus. The flocculent strain FSC27 obtained was a so-called self-cloned strain, having no bacterial DNA. FSC27 cells could be easily recovered for reuse from fermentation mash without any physical energy, The strain produced a concentration of alcohol as high as 396-9-6V, although the fermentation rate of FSC27 was slightly lower than that of 396-9-6V. When uracil was added to the medium or when URA3 was reintroduced into FSC27 (named FSCU-L18), the fermentation rate and the growth rate increased, and the ethanol concentration produced was higher than that produced by the parent strain, The stable flocculation and high ethanol productivity were observed by using FSCU-L18 during 10 cycles of repeated-batch fermentation test.