FERMENTATION OF LACTOSE BY YEAST-CELLS SECRETING RECOMBINANT FUNGAL LACTASE

Strains of Saccharomyces cerevisiae transformed with a yeast multicopy expression vector carrying the cDNA for Aspergillus niger secretory b eta-galactosidase under the control of ADH1 promoter and terminator we re studied for their fermentation properties on lactose (V. Kumar, S. Ramakrishnan, T. T. Teeri, J. K. C. Knowles, and B. S. Hartley, Biotec hnology 10:82-85, 1992). Lactose was hydrolyzed extracellularly into g lucose and galactose, and both sugars were utilized simultaneously. Di auxic growth patterns were not observed. However, a typical biphasic g rowth was observed on a mixture of glucose and galactose under aerobic and anaerobic conditions with transformants of a haploid S. cerevisia e strain, GRF167. Polyploid distiller's yeast (Mauri) transformants we re selected simply on the basis of the cloned gene expression on X-Gal bromo-4-chloro-3-indolyl-beta-D-galactopyranoside) plates. Rapid and complete lactose hydrolysis and higher ethanol (0.31 g/g of sugar) and biomass (0.24 g/g of sugar) production were observed with distiller's yeast grown under aerobic conditions. A constant proportion (10%) of the population retained the plasmid throughout the fermentation period (48 h). Nearly theoretical yields of ethanol were obtained under anae robic conditions on lactose, glucose, galactose, and whey permeate med ia. However, the rate and the amount of lactose hydrolysis were lower under anaerobic than aerobic conditions. All lactose-grown cells expre ssed partial galactokinase activity.