The overexpression of the CDC25 gene of Saccharomyces cerevisiae causes a derepression of GAL system and an increase of GAL4 transcription

The CDC25 gene product is an exchange factor for Ras proteins and it activates the Ras/cAMP pathway in the yeast Saccharomyces cerevisiae. The overexpression of the CDC25 gene in S. cerevisiae cells causes a partial glucose-derepressed phenotype which is particularly evident for expression of invertase. To define domains of Cdc25 protein relevant for this derepression and to test another glucose repressed system, different to invertase, we have overexpressed different regions of the CDC25 gene under the control of a GAL-promoter. We found that a derepression of both GAL-regulated promoters and invertase was related to the overexpression of CDC25 regions that contain a functional guanine nucleotide exchange (GEF) domain. The effect on GAL-promoters was particular evident when the CDC25 gene was under the control of a UAS(gal) element and operates at transcriptional level, although a moderate derepression was found also for UAS(gal)/lacZ reporter gene. Finally, the overexpression of the GEF domain of CDC25 also caused an increase in the expression of the GAL4 regulatory gene, while a constitutive activation of the Ras/cAMP pathway did not produce any increase in GAL4 expression. These findings indicate that the overexpression of the catalytic domain of CDC25 gene is necessary and sufficient to give a glucose-derepression of GAL promoters and of invertase. They also suggest that the derepression of GAL promoters occurs through an increase of GAL4 expression in a Ras/cAMP independent way