A third group of antibiotics, the aurachins, have been isolated from the myxobacterium Stigmatella aurantiaca. The aurachins chemically are quinolones. and four of them (aurachins A-D) have been tested for their inhibitory activity in photosystem II and cytochrome b/c-complexes. Aurachin C is the best inhibitor in photosystem II (pI50-value 7.2); its biochemical behaviour being the same like that of other photosystem II herbicides. Both aurachins C and D are also excellent inhibitors in the cytochrome b6/f-complex (pI50-values of 7.00 and 7.49. respectively). In its mechanism of action, aurachin C resembl|es antimycin, whereas aurachin D is different from either antimycin or myxothiazol.

The property of the D-l subunit of photosystem II in binding herbicides in its quinone-binding niche has provided important approaches to study its structure and function. In the D-l protein, amino acid residues Tyr-254 and Tyr-237 are labeled by an [Ylazido-urea derivative, as identified by protein sequencing of proteolytic fragments. Whereas Tyr-254 is in a parallel cc-helix already indicated to contribute to the herbicide-binding site, Tyr-237 is in a hydrophilic sequence that is partly accessible from the matrix space of the chloroplasts.This area has been implicated to contain a cleavage site for a protease involved in the rapid turnover of the D-l polypeptide.The photoaffinity labeling results show that some of the amino acids in this cleavage site are actually part of the quinone-binding niche. It allows a refined and extended prediction of the three-dimensional folding of the reaction center of photosystem II.

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