The nuclear mutant AB1-4A/8/100, a respiratory-competent strain altered in the regulation of ATP synthesis, has been shown to be modified in the relative stoichiometry of the mtDNA-encoded proteolipids of the F0 sector of ATP synthase: the ratios mutant/wild type of the proteolipids were equal to 0.4/0.7/1 for Su8/Su6/Su9, respectively. This defect results from the simultaneous presence of two nuclear genes which promote a cryosensitive phenotype on a nonfermentable carbon source. Measurements of mitochondrial protein synthesis carried out "in vivo" and "in organello" evidenced a specific defect in the synthesis of subunits 6 and 8. Measurements of the steady state levels of mitochondrial mRNA showed that the defect in subunits 6 and 8 was correlated with a modification of the expression of a cotranscript ATP8-ATP6. This cotranscript is matured at a unique site to give two cotranscripts of 4600 and 5200 bases in length. In mutant mitochondria, the ratio between both cotranscripts, 5200/4600, was lowered. In parallel, expression of the whole mitochondrial transcription unit supporting the genes COXI, ATP8, ATP6, and RF3 was enhanced. However, despite this over expression, the amount of the long cotranscript ATP8-ATP6 remained lower than in wild type mitochondria.