Genes for components of the cytochrome b-f complex

As with all the other photosynthetic membrane complexes, the genes for the components of the cytochrome complex are distributed between the nuclear and chlo-roplast genomes of higher plants. The chloroplast genes for the Cyt /, Cyt 6-563 and 17 kDa polypeptides have been extensively characterized, but the nuclear gene(s) for the Rieske Fe-S protein have not yet been isolated. 

A chloroplast location of the gene for Cyt / was suggested by two plastome mutants of Oenothera which were deficient in spectrally detected Cyt/[102], and by the maternal mode of inheritance of Cyt /in interspecific Fj hybrids of Nicotiana [103]. The structural gene (petA) for the Cyt / polypeptide was initially localized and characterized from pea chloroplast DNA [104,105], and subsequently from wheat [106], spinach [107] and Oenothera [108]. Open reading frames of 320 codons have been detected in chloroplast DNA from each of these plants. Comparison with the determined N-terminal sequence of Cyt / indicates the presence of a 35 amino acid residue pre-sequence and a mature polypeptide of 285 amino acid residues. A putative haem-binding site, Cys-Ala-Asn-Cys-His, is located near the 

N-terminus of the mature polypeptide and a single hydrophobic sequence, which may be involved in anchoring Cyt/in the thylakoid membrane, is located near the C-terminus. A model of Cyt / with a single membrane-spanning a-helix and the bulk of the polypeptide in the intrathylakoid space has been proposed and tested by partial proteolytic cleavage [105], 

The gene(s) for the Rieske Fe-S protein has not been isolated, but its location in nuclear DNA is predicted from the synthesis of a precursor form from poly(A) RNA isolated from spinach [109]. cDNA clones containing the coding sequence of the Rieske Fe-S protein should be forthcoming shortly. 

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