Cell survival and energy production requires a functional mitochondrial respiratory chain. Biogenesis of cytochrome c oxidase (COX), the last enzyme of the mitochondrial respiratory chain, is a very complicated process and requires the assistance of a large number of accessory factors. Defects in COX assembly alter cellular respiration and produce severe human encephalomyopathies. Mutations in SURF1, a COX assembly factor of exact unknown function, produce Leigh's syndrome (LS), the most frequent cause of COX deficiency in infants. In the yeast Saccharomyces cerevisiae, deletion of the SURF1 homologue SHY1 results in a similar COX deficiency. In order to identify genetic modifiers of the shy1 mutant phenotype, we have explored for genetic interactions involving SHY1. Here we report that overexpression of Hap4p, the catalytic subunit of the CCAAT binding transcriptional activator Hap2/3/ 4/5p complex, suppresses the respiratory defect of yeast shy1 mutants by increasing the expression of nuclear-encoded COX subunits that interact with the mitochondrially encoded Cox1p. Analogously, overexpression of the Hap complex human homologue NF-YA/B/C transcription complex in SURF1-deficient fibroblasts from an LS patient efficiently rescues their COX deficiency.
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