HIGHLY EFFICIENT ASSIMILATION OF LACTOSE BY A METABOLICALLY ENGINEERED STRAIN OF SACCHAROMYCES-CEREVISIAE

A diploid strain of Saccharomyces cerevisiae able to metabolize lactos e with high efficiency has been obtained. Haploid strains of Saccharom yces able to grow on lactose were constructed by cotransformation with two genes of Kluyveromyces lactis required for the utilization of the sugar, LAC4 and LACI2, encoding beta-galactosidase and lactose permea se respectively. Both genes were placed under the control of a galacto se-inducible promoter and targeted to the rDNA encoding region (RDNI l ocus) of the Saccharomyces genome. Lac(+) transformants were selected on medium with lactose as the only carbon source. These transformants were mitotically stable, they maintained the Lac(+) phenotype after gr owing in non-selective medium for more than 60 generations, but their growth was slow. We found that this lack of vigour was caused by their genetic background and not by a deficient expression of the heterolog ous genes. Therefore, their performance could be improved by crossing with a wild-type strain. Among the offspring of the crosses, two strai ns of opposite mating type were selected and mated to obtain a fast-gr owing Lac(+) diploid. This diploid strain showed the typical fermentat ive behaviour of S. cerevisiae when it was grown in aerated liquid med ium with glucose. Ln lactose medium, it exhibited a respiro-fermentati ve metabolism similar to that of K. lactis, with low ethanol productio n and high biomass yield. (C) 1998 John Wiley & Sons, Ltd.