The photosystem I (PS I) reaction-centre polypeptides, encoded by the psaA and psaB genes, are shown to contain several highly conserved leucine repeats, consisting of a leucine residue every seventh amino acid, similar to the leucine zipper motifs known to mediate DNA-binding polypeptide dimerisation. In each of the PSI reaction-centre subunits the leucine zipper motif precedes highly conserved cysteine residues which have been proposed to ligate the interpolypeptide [4Fe-4S] centre, Fx. We propose that PS I reaction-centre dimerisation and [4Fe-4S] centre formation are mediated through the leucine zipper.
