Protein modifications in the D2 protein of photosystem II affect properties of the QB/herbicide-binding environment

The D2 protein contains an extended loop (the D-de loop) between helices D and de at the reducing side of photosystem II (PS II). Characterization of D2 mutants of the cyanobacterium Synechocystis sp. PCC 6803 has indicated that the length and amino acid composition of the D-de loop are not critical for basic PS II functions, although most of the residues in that region are conserved phylogenetically. Here we show using herbicide binding and electron-flow inhibition measurements that drastic modifications in the D-de loop of the D2 protein modify the interaction of some PS II-directed herbicides with their binding niche. The stability of (semi)-reduced QB in its binding pocket is altered in at least two of the mutants, as indicated by a shifted peak temperature of the thermoluminescence signal originating from charge recombination involving QB. These results suggest a close functional association between the D-de loop of the D2 protein and the QB/herbicide-binding environment, which is viewed as being coordinated mostly by residues of the D1 protein. This represents one of the first examples of modification of the QB/herbicide-binding domain by mutations in the D2 protein.