CAT8, A NEW ZINC CLUSTER-ENCODING GENE NECESSARY FOR DEREPRESSION OF GLUCONEOGENIC ENZYMES IN THE YEAST SACCHAROMYCES-CEREVISIAE

The expression of gluconeogenic fructose-1,6-bisphosphatase (encoded b y the FBP1 gene) depends on the carbon source. Analysis of the FBP1 pr omoter revealed two upstream activating elements, UASI(FBP1) AND UASZ( FBP1), which confer carbon source-dependent regulation on a heterologo us reporter gene. On glucose media neither element was activated, wher eas after transfer to ethanol a 100-fold derepression was observed. Th is gene activation depended on the previously identified derepression genes C;ATI (SNF1) (encoding a protein kinase) and CAT3 (SNF4) (probab ly encoding a subunit of Catlp [Snf1p]). Screening for mutations speci fically involved in UBS1(FBP1) derepression revealed the new recessive derepression mutation cat8. The cat8 mutants also failed to derepress UAs2(FBP1) and these mutants were unable to grow on nonfermentable ca rbon sources. The CAT8 gene encodes a zinc cluster protein related to Sacchnromgces cerevisiae Ga14p. Deletion of CAT8 caused a defect in gl ucose derepression which affected all keg gluconeogenic enzymes. Derep ression of glucose-repressible invertase and maltase was still normall y regulated. A CAT8-lacZ promoter fusion revealed that the CAT8 gene i tself is repressed by Cat4p (Mig1p). These results suggest that glucon eogenic genes are derepressed upon binding of Cat8p, whose synthesis d epends on the release of Cat lp (Mig1p) from the CAT8 promoter. Howeve r, gluconeogenic promoters are still glucose repressed in cat4 mutants , which indicates that in addition to its transcription, the Cat8p pro tein needs further activation. The observation that multicopy expressi on of CAT8 reverses the inability of cat1 and cat3 mutants to grow on ethanol indicates that Cat8p might be the substrate of the Cat1p/Ca3p protein kinase.