Expression of a foreign eukaryotic gene in Saccharomyces cerevisiae: beta-galactosidase from Kluyveromyces lactis.

Three recombinant DNA vectors carrying the beta-galactosidase structural gene, LAC4, from the yeast Kluyveromyces lactis were constructed and transformed into Saccharomyces cerevisiae. All transformants expressed the beta-galactosidase activity of LAC4. However, the level of enzyme activity varied, being highest in cells transformed with vectors which are maintained as multicopy plasmids and lowest in cells transformed with a vector which integrates into chromosomes. Enzyme levels probably reflect gene dosage. LAC4 is very stable when integrated into a chromosome, but unstable when carried on a plasmid. Therefore, stability is a property of the recombinant vector rather than of LAC4, LAC4-coded beta-galactosidase synthesized in either S. cerevisiae or in K. lactis is the same as judged by two-dimensional polyacrylamide gel electrophoresis. However S. cerevisiae transformed with LAC4 cannot grow on lactose, probably because lactose does not enter the cell.