The Topology of the Plastoquinone and Herbicide Binding Peptides of Photosystem II in the Thylakoid Membrane

The 32 kDa herbicide and Q_B binding peptide (D-1 protein) and its homologous 34 kDa peptide (D-2 protein) are integral membrane subunits of photosystem II. A model for their folding through the thylakoid membrane in five transmembrane a-helices is proposed from the compari­son of amino acid sequence and hydropathy index plot homologies with subunits of the bacterial system. Following recent data on the X-ray structure of a bacterial photosystem the binding niche for Q_B is interpreted on the basis of the amino acid changes found in the 32 kDa peptide in herbicide tolerant higher plants and algae.