We describe the construction of a lactose- utilizing Saccharomyces cerevisiae that expresses the cDNA for a secreted, thermostable β-galactosidase (lacA) from Aspergillus niger. Yeast cells expressing the lacA gene from the yeast ADH1 promoter on a multicopy plasmid secrete up to 40% of the total β-galactosidase activity into the growth medium. The secreted product is extensively N-glycosylated, and cells expressing the lacA gene grow on whey permeate (4% w/v lactose) with a doubling time of 1.6 hours. Such strains may offer a solution to the increasing problem of waste whey disposal.
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