Understanding the growth phenotype of the yeast gcr1 mutant in terms of global genomic expression patterns

The phenotype of an organism is the manifestation of its expressed genome. The gcr1 mutant of yeast grows at near wild-type rates on nonfermentable carbon sources but exhibits a severe growth defect when grown in the presence of glucose, even when nonfermentable carbon sources are available. Using DNA microarrays, the genomic expression patterns of wild-type and gcr1 mutant yeast growing on various media, with and without glucose, were compared. A total of 53 open reading frames (ORFs) were identified as GCR1 dependent based on the criterion that their expression was reduced twofold or greater in mutant versus wild-type cultures grown in permissive medium consisting of YP supplemented with glycerol and lactate. The GCR1-dependent genes, so defined, fell into three classes: (i) glycolytic enzyme genes, (ii) ORFs carried by Ty elements, and (iii) genes not previously known to be GCR1 dependent. In wild-type cultures, GCR1-dependent genes accounted for 27% of the total hybridization signal, whereas in mutant cultures, they accounted for 6% of the total. Glucose addition to the growth medium resulted in a reprogramming of gene expression in both wild-type and mutant yeasts. In both strains, glycolytic enzyme gene expression was induced by the addition of glucose, although the expression of these genes was still impaired in the mutant compared to the wild type. By contrast, glucose resulted in a strong induction of Ty-borne genes in the mutant background but did not greatly affect their already high expression in the wild-type background. Both strains responded to glucose by repressing the expression of genes involved in respiration and the metabolism of alternative carbon sources. Thus, the severe growth inhibition observed in gcr1 mutants in the presence of glucose is the result of normal signal transduction pathways and glucose repression mechanisms operating without sufficient glycolytic enzyme gene expression to support growth via glycolysis alone.