FUNCTIONAL COMPLEMENTATION OF AN OXA1(-) YEAST MUTATION IDENTIFIES ANARABIDOPSIS-THALIANA CDNA INVOLVED IN THE ASSEMBLY OF RESPIRATORY COMPLEXES

The nuclear gene OXA1 is essential for respiratory growth in yeast. It codes for a chaperon-like protein, and has pleiotropic effects on the assembly of cytochrome c oxidase and ATP synthase of the mitochondria l respiratory chain. To study respiratory complex formation in plants, we have cloned a homolog of the yeast oxa1(-) in Arabidopsis thaliana , OXA1At, by functional complementation of a yeast oxa1(-) mutant. OXA 1At is a single copy gene and appears to be constitutively expressed i n A. thaliana. Although OXA1At encodes a protein sharing only 30% amin o acid identity with the yeast Oral protein, hydrophobic domains likel y corresponding to trans-membrane domains are strictly conserved. Cyto chrome spectra and measurements of respiratory activities show that re placement of the yeast Oxa1 protein with the A. thaliana homolog leads to correct assembly and activity of cytochrome c oxidase, but to part ial restoration of ATPase activity. Our results suggest that the Oxa1A t protein is essential for the respiratory complex assembly in A. thal iana, and that genes involved in mitochondrial multiprotein complex fo rmation can be conserved between plants and other organisms.