The Aurachins, Naturally Occurring Inhibitors of Photosvnthetic Electron Flow through Photosystem II and the Cytochrome b6/f-Complex

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 (pI 50 -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 b 6 /f-complex (pI 50 -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.