Abstract
The mitochondrial NADH-ubiquinone reductase (complex I) is an assembly of approximately 26 different polypeptides. In vertebrates and invertebrates, seven of its subunits are the products of genes in the mitochondrial DNA, and homologues of these genes have been found previously in the chloroplast genomes of Marchantia polymorpha and Nicotiana tabacum, although their function in the chloroplast is unknown. The remainder of the subunits of the mitochondrial complex are nuclear gene products that are imported into the organelle, amongst them the 49 kd subunit, a component of the iron--sulphur subcomplex of the enzyme. In the present work, the N-terminal sequence of this protein has been determined, and this has been used to design two mixtures of synthetic oligonucleotides, each containing 32 different sequences 17 bases long. These mixtures have been used as hybridization probes to isolate cDNA clones from a bovine library. The DNA sequences of these clones have been determined and they encode the mature 49 kd protein, with the exception of amino acids 1 and 2. The protein sequence of 430 amino acids is closely related to those of proteins that are encoded in open reading frames (ORFs) present in the chloroplast genomes of M.polymorpha and N.tabacum. Only one cysteine is conserved and the sequences provide no indication that the 49 kd protein contains iron--sulphur centres. These ORFs are found in the single copy regions of chloroplast DNA in close proximity to four of the homologues of the mammalian mitochondrial genes that encode subunits of complex I.(ABSTRACT TRUNCATED AT 250 WORDS)