High-copy suppression of glucose transport defects by HXT4 and regulatory elements in the promoters of the HXT genes in Saccharomyces cerevisiae.

HXT4, a new member of the hexose transporter (HXT) family in Saccharomyces cerevisiae was identified by its ability to suppress the snf3 mutation in multicopy. Multicopy HXT4 increases both high and low affinity glucose transport in snf3 strains and increases low and high transport in wild-type strains. Characterization of HXT4 led to the discovery of a new class of multicopy suppressors of glucose transport defects: regulatory elements in the promoters of the HXT genes. We have designated these sequences DDSEs (DNA sequence dependent suppressing element). Multicopy HXT4 and DDSEs in the HXT1, HXT2, HXT3 and HXT4 promoters were found to restore growth to snf3 and grr1 strains on low glucose media. The DDSE in the HXT4 promoter was refined to a 340-bp sequence 450 bp upstream of the HXT4 translational start. This region was found to contain an 183-amino acid open reading frame. Extensive analysis indicates that the DNA sequence itself and not the encoded protein is responsible for suppression. The promoters of SNF3 and of other glycolytic genes examined did not suppress snf3 in multicopy. Suppression of snf3 by DDSE is dependent on the presence of either HXT2 or HXT3.