2 DIFFERENT REPRESSORS COLLABORATE TO RESTRICT EXPRESSION OF THE YEAST GLUCOSE-TRANSPORTER GENES HXT2 AND HXT4 TO LOW-LEVELS OF GLUCOSE

Transcription of the yeast HXT2 and HXT4 genes, which encode glucose t ransporters, is induced only by low levels of glucose, This low-glucos e-induced expression is mediated by tyro independent repression mechan isms: in the absence of glucose, transcription of both genes is preven ted by Rgt1p, a C-6 zinc cluster protein; at high levels of glucose, e xpression of HXT2 and HXT4 is repressed by Mig1p. Only at low glucose concentrations are both repressors inactive, leading to a 10- to 20-fo ld induction of gene expression, Mig1p and Rgt1p act directly on HXT2 and HXT4 by binding to their promoters, This transcriptional regulatio n is physiologically very important to the yeast cell because it cause s these glucose transporters to he expressed only in low-glucose media , in which they are required for growth.