We have previously described a eukaryotic het-erologous expression system, with the urf13TW gene in yeast, which mimics the disease susceptibility associated with the Texas cytoplasmic male sterility in maize. This yeast model was used to isolate yeast nuclear mutants conferring methomyl resistance. The genetic strategy we have developed focused on screening for nuclear dominant yeast mutations which restore methomyl resistance. MRGI-1, a yeast nuclear dominant allele, was identified as a methomyl-resistance restorer. We have shown that methomyl resistance co-segregated with a pleiotropic phenotype in the heterozygous MRGI-1/MRG1 diploids, detectable even in the absence of the maize-derived mito-chondrial protein and/or methomyl. We observed an increase in oxygen uptake, a significant decrease of the levels of cytochrome aa 3, and a decrease in the growth yield. This phenotype is influenced by the carbon source and the results suggest a defect in the adaptation to the respiratory pathway in MRGI-1 yeast cells.