Identification of HXT7 as a Suppressor of the snf3 Growth Defect in Wine and Wild-type Strains of Saccharomyces cerevisiae

Abstract

In the YPH500 laboratory strain background of Saccharomyces cerevisiae, disruption of the SNF3 open reading frame results in the inability to grow at low glucose concentrations. Conversely, in haploid derivatives of wine strains and several other wild laboratory strains, deletion of SNF3 does not lead to a discernable phenotype. Complementation and subsequent tetrad analysis demonstrated that the snf3 suppressor found in wine strains is a dominant gene encoded by a single locus, not allelic to the known dominant suppressor, RGT2-1. HXT7 was identified independently from two low-copy number genomic libraries of strains carrying the novel dominant suppressor as capable of suppressing the snf3Δ phenotype in YPH500. Cloned HXT7 plus 1 kb of promoter was sufficient for suppression. Sequence comparison of HXT7 from the YPH500 background to that of the archetype lab strain, S288C, demonstrated that YPH500-derived strains contain a chimera of the HXT6 promoter and the HXT7 gene (HXT6^P/7). Deletion of HXT7 in S288C results in expression of the snf3 phenotype in this genetic background, while deletion of HXT6 only slightly diminishes growth in the snf3 HXT7 strain. In contrast to cloned HXT7, expression of HXT6^P/7 cloned in a centromere (CEN)-based plasmid is not sufficient for snf3 suppression. A construct placing HXT6 under the control of the HXT7 promoter also suppresses the snf3 growth defect. Suppression of snf3 in the wine and wild strains of Saccharomyces evaluated is therefore due to the pattern of expression of the Hxt7 protein rather than to differences in the HXT7 and HXT6 coding sequences.