Positive Feedback Genetic Circuit Incorporating a Constitutively Active Mutant Gal3 into Yeast GAL Induction System

The GAL expression system is the most frequently used induction technique in the yeast Saccharomyces cerevisiae. Here we report a simple but powerful genetic circuit for use with the GAL induction system. Briefly, an artificial positive feedback circuit was incorporated into the GAL regulatory network. We selected green fluorescent protein (GFP) as a reporter of GAL1 induction, and designed a strain that expressed a constitutively active Gal3 mutant protein (Gal3^c) under control of the GAL10 promoter. In the resulting strain, GAL1 and GAL10 promoters regulate the expression of GFP and GAL3^c, respectively. Because Gal3^c sequesters the Gal80 repressor away from the Gal4 transcriptional activator in the same manner as the galactose-bound Gal3, the expressed Gal3^c protein provokes further expression of GFP and Gal3^c, yielding further enhancement of GAL induction. Thus, this GAL3^c-mediated positive feedback circuit permits substantially enriched induction of a target gene at extremely low concentrations, or even in the absence, of galactose, while maintaining the strict glucose-mediated repression of the target.