Although the 32 kOa-polypeptide (0-1) is a minor component of thyla­ koids in algae and higher plants. much attention has been focused on it. It was shown to be the binding site for herbicides (Pfister et al. 1981). the rapidly synthesized polypeptide of thylakoid membranes (Edelman and Reisfeld 1978; Owens et al. 1982) and to participate in Qa binding (Arntzen et al. 1984; Kyle et al. 1984; Ohad et al. 1984). Measurements of the stoichiometry of the various PS II polypeptides in C. reinhardii showed that thylakoids contain about one herbicide binding polypeptide per one PS II unit and the accelerated light sti­ mulated turnover of the 32 kOa-polypeptide was postulated to be the consequence of its function in quinone reduction. Based on comparison of amino acid sequence and hydropathy profile homologies of 0-1 and 0-2 with the subunits of the bacterial photosystem. recently a model was presented. in which the reaction center of photosystem II in algae and higher plants is formed by the polypeptides 0-1 and 0-2 (Trebst 1986). The structural gene of the 32 kOa-polypeptide (psbA) has been mapped on the chloroplast DNA and cloned from various plant species (for a review: Kyle 1985). The polypeptide is synthesized as a precursor on ribosomes bound to unstacked lamellae and there proc­ essed to its mature form (Edelman and Reisfeld 1978; Herrin and Michaels 1985). In C. reinhardii the 0-1 polypeptide is phosphorylat­ ed by a light-activated membrane bound kinase.