Galactose as a gratuitous inducer of GAL gene expression in yeasts growing on glucose.

The promoters of the highly expressed and stringently regulated GAL genes of Saccharomyces cerevisiae, are useful for expressing proteins in this organism. However, two problems complicate their use. First, because growth on glucose causes prolonged repression of GAL expression, cells are most rapidly induced after growth on nonfermentable carbon sources, conditions which usually support poor growth. Second, because the inducer of the GAL genes (galactose) also serves as a carbon source, the level of inducer is continually diminishing during growth of a Gal+ strain, which may lead to reduced GAL expression. To solve the first problem, we have employed strains that carry the reg1-501 mutation, which eliminates glucose repression of GAL expression. This gene has been shown to be located on the right arm of chromosome IV, distal but tightly linked to the TRP1 gene. We demonstrate that expression from GAL promoters is efficiently and rapidly induced in these reg1 strains by the addition of galactose to a culture growing in glucose medium. Levels of galactose as low as 0.02% can be used to obtain a 1500-fold induction of gene expression from GAL promoters in this strain. To surmount the second problem, we have used a gal1 mutant, deficient in the enzyme that catalyzes the first step of galactose utilization. We show that high levels of expression from GAL promoters are achieved rapidly in these mutants, for which galactose is a gratuitous inducer.(ABSTRACT TRUNCATED AT 250 WORDS)