Here we report the primary structure of potato cytochrome c1, a nuclear-encoded subunit of complex III. Using heterologous antibodies directed against cytochrome c1 from yeast two types of clones were isolated from an expression library, suggesting that at least two different genes are present and expressed in the genome. Northern blot analysis reveals that slightly varying levels of cytochrome c1 transcripts are present in all potato tissues analysed. A 1304 bp insert of one of the cDNA clones (pC13II) encodes the entire 320 amino acids of the precursor protein corresponding to a molecular weight of 35.2 kDa. As revealed by direct amino acid sequence determination of the cytochrome c1 protein another cDNA clone (pC18I) encodes the major form of cytochrome c1 present in potato tuber mitochondria. Western blots of subfractionated potato mitochondria show that the mature protein present in the membrane fraction is smaller than the pC13II encoded protein synthesized in Escherichia coli. The transient presequence of the protein is 77 amino acids long and has a bipartite polarity profile characteristic of presequences involved in targeting to the intermembrane space of fungal mitochondria. It consists of a positively charged NH2-terminal part which resembles "matrix targeting domains" and an adjacent hydrophobic region showing sequence similarities to "intramitochondrial sorting domains". The amino-terminal region of potato cytochrome c1 is the first presequence of a plant protein of the mitochondrial intermembrane space to be determined and may be useful in the study of intramitochondrial sorting in plants.