We have cloned and sequenced a novel yeast nuclear gene ABC1 which suppresses, in multicopy, the cytochrome b mRNA translation defect due to the nuclear mutation cbs2-223. Analysis of the ABC1 gene shows that it is weakly expressed, it could code for a protein of 501 amino acids which has a typical presequence of a protein imported into mitochondria and which does not display a strong similarity to any known protein. Inactivation of the ABC1 gene is not lethal to the cell but leads to a respiratory defect: no oxygen uptake and no growth on non-fermentable media. A total absence of NADH-cytochrome c oxidoreductase and succinate-cytochrome c oxidoreductase activities concomitant with the presence of specific dehydrogenases, suggests a block in the bc 1 segment of the respiratory chain. However, all the cytochromes are spectrally detectable. Cytochrome b is quite efficiently reduced while cytochromes c1 and c are not. The function of ABC1 in the suppression of a defect in apocytochrome b mRNA translation and in the activity of the bc1 complex suggests that the ABC1 protein would be a novel chaperonin involved both in biogenesis and bioenergetics of mitochondria.