The yeast nuclear gene OXAI is essential for cytochrome oxidase assembly, so that a null mutation in the OXAI gene leads to complete respiratory deficiency. We have cloned by genetic selection a human OXAI (OXA1Hs) cDNA that complements the respiratory defect of yeast oxal mutants. The deduced sequence of the human protein shares 33% identity with the yeast OXA1 protein. The OXAIHs cDNA corresponds to a single and relatively highly expressed gene.

This publication marks the beginning of the construction of a gene-protein index that relates proteins which are resolved on the two-dimensional protein map of Saccharomyces cerevisiae with their corresponding genes. We report the identification of 36 novel polypeptide spots on the yeast protein map. They correspond to the products of 26 genes. Together with the polypeptide spots previously identified, this raises to 41 the number of genes whose products have been identified on the protein map. The proteins identified here are concerned with four major areas of yeast cellular physiology: carbon metabolism, heat shock, amino acid biosynthesis and purine biosynthesis. Given the molecular weight and isoelectric point of the identified proteins, and the codon-usage bias of the corresponding genes, it can be estimated that 25 to 35% of all the soluble yeast proteins are detectable under the labelling and running gel conditions used in this study.

We have cloned three distinct nuclear genes, NAM1, NAM7, and NAM8, which alleviate mitochondrial intron mutations of the cytochrome b and COXI (subunit I of cytochrome oxidase) genes when present on multicopy plasmids. These nuclear genes show no sequence homology to each other and are localized on different chromosomes: NAM1 on chromosome IV, NAM7 on chromosome XIII and NAM8 on chromosome VIII. Sequence analysis of the NAM1 gene shows that it encodes a protein of 440 amino acids with a typical presequence that would target the protein to the mitochondrial matrix. Inactivation of the NAM1 gene by gene transplacement leads to a dramatic reduction of the overall synthesis of mitochondrial protein, and a complete absence of the COXI protein which is the result of a specific block in COXI pre-mRNA splicing. The possible mechanisms by which the NAM1 gene product may function are discussed.

We have previously isolated the yeast nuclear gene OXA1 and showed that Oxa1p is required for the formation of the cytochrome c oxidase and ATP synthase complexes. We have expressed Oxa1p in E. coli and shown that it is toxic and rapidly degraded. Nevertheless, a truncated protein was successfully expressed and antibodies have been raised against this truncated protein. These antibodies recognise a protein in mitochondrially enriched fractions. In vitro mitochondrial import experiments demonstrate that the import of Oxa1p is accompanied by the cleavage of a long pre-sequence. Osmotic swelling and alkaline carbonate extraction show that Oxa1p is an integral membrane protein located in the inner membrane of mitochondria. The relationships between the sub-mitochondrial location and the function of Oxa1p are discussed.