PCK1 encoding phosphoenolpyruvate carboxykinase is transcriptionally regulated by two upstream activating elements. By screening for mutants that failed to derepress a UAS2PCK1-CYC1-lacZ reporter gene we isolated the new recessive derepression mutation cat5. The CAT5 gene encodes a protein of 272 amino acids showing a 42% identity to the ZC395.2 gene product of Caenorhabditis elegans whose function is unknown. Deletion of CAT5 caused a complete loss of glucose derepression affecting gluconeogenic key enzymes. Respiration, but not mitochondrial cytochrome c oxidase activity, was also affected. CAT5 expression is 5- to 6-fold repressed by glucose, and CAT5 transcriptional activation was dependent on CAT1 (SNF1), CAT8 and CAT5 itself. The CAT5 gene is necessary for UAS1PCK1 and UAS2PCK1 protein binding since a carbon source-specific interaction was no longer detectable in cat5 mutants. Glucose derepression of gluconeogenesis depends on the active Cat1 (Snf1) protein kinase and the Cat8 zinc cluster activator. Mig1p-independent overexpression of CAT8 did not stimulate activation of gluconeogenic promoters in cat1 and in cat5 mutants. Since Cat8p multicopy expression suppresses the ethanol growth deficiency in cat1 (snf1) mutants, these results indicate that activation of Cat8p by the Cat1p (Snf1p) kinase and the Cat5p protein might be necessary for release from glucose repression.