Functional study of the Hap4-like genes suggests that the key regulators of carbon metabolism HAP4 and oxidative stress response YAP1 in yeast diverged from a common ancestor.

The transcriptional regulator HAP4, induced by respiratory substrates, is involved in the balance between fermentation and respiration in S. cerevisiae. We identified putative orthologues of the Hap4 protein in all ascomycetes, based only on a conserved sixteen amino acid-long motif. In addition to this motif, some of these proteins contain a DNA-binding motif of the bZIP type, while being nonetheless globally highly divergent. The genome of the yeast Hansenula polymorpha contains two HAP4-like genes encoding the protein HpHap4-A which, like ScHap4, is devoid of a bZIP motif, and HpHap4-B which contains it. This species has been chosen for a detailed examination of their respective properties. Based mostly on global gene expression studies performed in the S. cerevisiae HAP4 disruption mutant (ScΔhap4), we show here that HpHap4-A is functionally equivalent to ScHap4, whereas HpHap4-B is not. Moreover HpHAP4-B is able to complement the H2O2 hypersensitivity of the ScYap1 deletant, YAP1 being, in S. cerevisiae, the main regulator of oxidative stress. Finally, a transcriptomic analysis performed in the ScΔyap1 strain overexpressing HpHAP4-B shows that HpHap4-B acts both on oxidative stress response and carbohydrate metabolism in a manner different from both ScYap1 and ScHap4. Deletion of these two genes in their natural host, H. polymorpha, confirms that HpHAP4-A participates in the control of the fermentation/respiration balance, while HpHAP4-B is involved in oxidative stress since its deletion leads to hypersensitivity to H2O2. These data, placed in an evolutionary context, raise new questions concerning the evolution of the HAP4 transcriptional regulation function and suggest that Yap1 and Hap4 have diverged from a unique regulatory protein in the fungal ancestor.

Analyzing the suppression of respiratory defects in the yeast model of human mitochondrial tRNA diseases.

The respiratory defects associated with mutations in human mitochondrial tRNA genes can be mimicked in yeast, which is the only organism easily amenable to mitochondrial transformation. This approach has shown that overexpression of several nuclear genes coding for factors involved in mitochondrial protein synthesis can alleviate the respiratory defects both in yeast and in human cells. The present paper analyzes in detail the effects of overexpressed yeast and human mitochondrial translation elongation factors EF-Tu. We studied the suppressing activity versus the function in mt translation of mutated versions of this factor and we obtained indications on the mechanism of suppression. Moreover from a more extended search for suppressor genes we isolated factors which might be active in mitochondrial biogenesis. Results indicate that the multiplicity of mitochondrial factors as well as their high variability of expression levels can account for the variable severity of mitochondrial diseases and might suggest possible therapeutic approaches.