Non-selective transformation of Saccharomyces cerevisiae.

Wild or industrial yeast strains cannot be transformed by most selective vectors due to a lack of auxotrophic mutations. To enable identification of transformants of such yeast species, we have developed a 2-µm DNA vector with an indicator gene that can be used without any additional marker. The Escherichia coli gene for β-lactamase (bla) was placed under the control of the yeast promoter for the structural gene encoding ADHI. This increased the amount of β-lactamase produced in Saccharomyces cerevisiae 100-fold giving an enzyme activity in transformant colonies which is high enough to be detected directly on indicator plates. Non-selectively, the transformation frequency is even higher than under selective conditions indicating that selection does not assist the establishment of new plasmids. Transformants isolated non-selectively were found to retain the endogenous 2-µm DNA. Under control of appropriate promoters, the bacterial bla gene may also provide a convenient marker for other eukaryotic transformation systems.