ABC1, A NOVEL YEAST NUCLEAR GENE HAS A DUAL FUNCTION IN MITOCHONDRIA - IT SUPPRESSES A CYTOCHROME-B MESSENGER-RNA TRANSLATION DEFECT AND IS ESSENTIAL FOR THE ELECTRON-TRANSFER IN THE BC1 COMPLEX

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 ABC] 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 ABC] 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 bc1 segment of tie respiratory chain. However, all the cytochromes are spectrally, detectable. Cytochrome b is quite efficiently reduced while cytochromes cl and c are not. The function of ABC] in the suppression of a defect in apocytochrome b MRNA translation and in the activity of the bc I complex suggests that the ABC I protein would be a novel chaperonin involved both in biogenesis and bioenergetics of mitochondria.