Overexpression of HAP4 in glucose-derepressed yeast cells reveals respiratory control of glucose-regulated genes.

A link between control of respiration and glucose repression in yeast is reported. The HAP4 gene was overexpressed in a Delta mig1 deletion background, generating a mutant in which respiratory function is stimulated and glucose repression is diminished. Although this combination does not result in derepression of genes encoding proteins involved in respiratory function, it nevertheless generates resistance against 2-deoxyglucose and hence contributes to more derepressed growth characteristics. Unexpectedly, overexpression of HAP4 in the Delta mig1 deletion strain causes strong repression of several target genes of the Mig1p repressor. Repression is not restricted to glucose growth conditions and does not require the glucose repressors Mig2p or Hxk2p. It was observed that expression of the SUC2 gene is transiently repressed after glucose is added to respiratory-growing Delta mig1 cells. Additional overexpression of HAP4 prevents release from this novel repressed state. The data presented show that respiratory function controls transcription of genes required for the metabolism of alternative sugars. This respiratory feedback control is suggested to regulate the feed into glycolysis in derepressed conditions.

Hap4p overexpression in glucose-grown Saccharomyces cerevisiae induces cells to enter a novel metabolic state.

BACKGROUND: Metabolic and regulatory gene networks generally tend to be stable. However, we have recently shown that overexpression of the transcriptional activator Hap4p in yeast causes cells to move to a state characterized by increased respiratory activity. To understand why overexpression of HAP4 is able to override the signals that normally result in glucose repression of mitochondrial function, we analyzed in detail the changes that occur in these cells. RESULTS: Whole-genome expression profiling and fingerprinting of the regulatory activity network show that HAP4 overexpression provokes changes that also occur during the diauxic shift. Overexpression of HAP4, however, primarily acts on mitochondrial function and biogenesis. In fact, a number of nuclear genes encoding mitochondrial proteins are induced to a greater extent than in cells that have passed through a normal diauxic shift: in addition to genes required for mitochondrial energy conservation they include genes encoding mitochondrial ribosomal proteins. CONCLUSIONS: We show that overproduction of a single nuclear transcription factor enables cells to move to a novel state that displays features typical of, but clearly not identical to, other derepressed states.

Dissection of the promoter of the HAP4 gene in S. cerevisiae unveils a complex regulatory framework of transcriptional regulation.

In S. cerevisiae, the heteromeric Hap2/3/4/5 complex is necessary for induced transcription of a large number of genes involved in oxidative metabolism on non-fermentable carbon sources. The Hap4p subunit is the activator subunit and at the same time also the regulatory part of the complex, since it is the only one whose level is regulated by carbon source itself. HAP4 promoter analysis shows a 265 bp activating region at position -1006/-741 bp upstream of the ATG start codon. Specific and differential protein-binding to a 30 nt CSRE-like sequence within this region was observed with extracts from repressing and inducing carbon sources. Carbon source-dependent activation mediated by the 265 bp fragment, as well as protein binding to the 30 nt CSRE-like region, is dependent on the presence of CAT8 function, unveiling a complex framework by which the expression of the HAP4 gene is coordinated.