The Three-Dimensional Structure of the Herbicide Binding Niche on the Reaction Center Polypeptides of Photosystem II

Abstract

H erbicide Target, H erbicide R esistance, Q uinone Binding Protein, P hotosystem II, Thylakoid M embrane Protein The folding through the m embrane o f the plastoquinone and herbicide binding protein subunits of photosystem II and the topology o f the binding niche for plastoquinone and herbicides is described. The m odel is based on the hom ology in amino acid sequence and folding prediction from the hydropathy analysis o f the D -l and D-2 subunits o f photosystem II to the reaction center polypeptides L and M o f the bacterial reaction center. It incorporates the am ino acid changes in the D -l polypeptide in herbicide tolerant plants and those indicated by chem ical tagging to be involved in Q B binding. It proposes hom ologous amino acids in the D -l/D -2 polypeptides to those indicated by the X-ray structure o f the bacterial reaction center to be involved in Fe-, quinoneand reaction center chlorophyll-binding. The different chemical com pounds known to interfere with Q b function are grouped into two families depending on their orientation in the Q B binding niche.

Mutation in phenol‐type herbicide resistance maps within the psbA gene in Synechocystis 6714

“…A change in this amino acid to either alanine or glycine, depending on the organism, was correlated with the resistance to urea-triazine-type herbicides. Some of these mutants show increased sensitivity to ioxynil [24]. This is not the case for the Synechocystis mutant modified at this position [17] for the herbicide-binding environment in the DI protein.…”

New herbicide‐binding site in the photosynthetic electron‐transport chain

“…The effects of various herbicides were estimated in a similar way and listed in table 1. Phenanthrolines, which are known to bind to the QA and Qa sites of purple bacterial reaction centers or to the QB site of the PS II reaction center [5,18,19], showed Kd values of the order of 10-5-10 -4 M. These values are a little higher than those reported for PS II or purple bacteria [5,18,19]. Similar or lower Kd values were also estimated for HOQNO and myxothiazol, which are known to be potent inhibitors in the cytochrome b/cl complex [4] and in the QB site of purple bacterial reaction center [19].…”

“…In this type of reaction center, the prosthetic groups required for charge separation are associated with two, partially homologous, polypeptides of molecular mass about 30 kDa [1][2][3]. The Qa site, where QB is reduced to quinol, together with the quinol-binding sites in the cytochrome b/cl(f) complex, has long been a target of herbicide studies [4,5]. The other type contains the PS I reaction center from plants [3,6] and probably that of anaerobic green photosynthetic bacteria [7].…”

“…Many herbicides are known to bind to the QB site in a competitive manner with in situ ubi-or plastoquinone [5,18,19]. An X-ray crystallographic study of the Rps.…”

“…viridis reaction center [20] has clearly shown that different types of herbicides, terbutryne and o-phenanthroline bind to different positions in the QB site interacting with different amino acid residues, respectively. Mutational modifications of these residues are known to alter resistance to these herbicides [5,19].…”