The Topology of the Reaction Center Polypeptides of Photosystem II
The function of the product of the psbA gene — the chloroplast coded rapidly turning over protein D-1 — was correlated in 1981 with the herbicide and QB binding protein of photosystem II (1). When the X-ray structure of the reaction center of Rh. viridis had become available (2) the homology of the L and D-1 protein allowed to propose a new folding of the D-1 protein — and of the D-2 protein as well (3). The folding model was based on a) amino acids conserved at essential positions for redox center binding in both the L/M and D-1/D-2 subunits and b) on the then known amino acid changes in the D-1 protein in herbicide tolerant plants and algae (quoted in 3). These changes were found in a common binding niche; furthermore, the mutant changes at ser 264 and phe 255 were amino acids equivalent to those involved in QB function according to the Rh. viridis structure. The model suggested immediately that the D-1 and D-2 protein constitute the reaction center of photosystem II (2,3) and that the QB protein not just binds QB, but also the reaction center chlorophyll of photosystem II and pheophytin. This was contrary to belief and to experimentation at that time. In 1986 Nanba and Satoh isolated a reaction center of photosystem II consisting of just the D-1 and D-2 protein and cytochrome b-559 (4) proving the prediction correct.
KeywordsRapid Turnover Purple Bacterium psbA Gene Photoaffinity Label Pest Sequence
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