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Mol. Cell. Biol., 05 1997, 2502-2510, Vol 17, No. 5
F Randez-Gil, N Bojunga, M Proft and KD Entian
The Cat8p zinc cluster protein is essential for growth of Saccharomyces
cerevisiae with nonfermentable carbon sources. Expression of the CAT8 gene
is subject to glucose repression mainly caused by Mig1p. Unexpectedly, the
deletion of the Mig1p-binding motif within the CAT8 promoter did not
increase CAT8 transcription; moreover, it resulted in a loss of CAT8
promoter activation. Insertion experiments with a promoter test plasmid
confirmed that this regulatory 20-bp element influences glucose repression
and derepression as well. This finding suggests an upstream activating
function of this promoter region, which is Mig1p independent, as delta mig1
mutants are still able to derepress the CAT8 promoter. No other putative
binding sites such as a Hap2/3/4/5p site and an Abf1p consensus site were
functional with respect to glucose-regulated CAT8 expression. Fusions of
Cat8p with the Gal4p DNA-binding domain mediated transcriptional
activation. This activation capacity was still carbon source regulated and
depended on the Cat1p (Snf1p) protein kinase, which indicated that Cat8p
needs posttranslational modification to reveal its gene-activating
function. Indeed, Western blot analysis on sodium dodecyl sulfate-gels
revealed a single band (Cat8pI) with crude extracts from glucose-grown
cells, whereas three bands (Cat8pI, -II, and -III) were identified in
derepressed cells. Derepression-specific Cat8pII and -III resulted from
differential phosphorylation, as shown by phosphatase treatment. Only the
most extensively phosphorylated modification (Cat8pIII) depended on the
Cat1p (Snf1p) kinase, indicating that another protein kinase is responsible
for modification form Cat8pII. The occurrence of Cat8pIII was strongly
correlated with the derepression of gluconeogenic enzymes
(phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase) and
gluconeogenic PCK1 mRNA. Furthermore, glucose triggered the
dephosphorylation of Cat8pIII, but this did not depend on the Glc7p (Cid1p)
phosphatase previously described as being involved in invertase repression.
These results confirm our current model that glucose derepression of
gluconeogenic genes needs Cat8p phosphorylation and additionally show that
a still unknown transcriptional activator is also involved.
Copyright © 1997, American Society for Microbiology
Glucose derepression of gluconeogenic enzymes in Saccharomyces cerevisiae correlates with phosphorylation of the gene activator Cat8p
Institut fur Mikrobiologie, Johann Wolfgang Goethe-Universitat Frankfurt, Biozentrum, Niederursel, Frankfurt am Main, Germany.
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