Stigmatellin Affects Both Hemes of Cytochrome b in Cytochrome b 6f/bc 1-Complexes

The antibiotic stigmatellin was purified from the gliding bacterium Stigmatella aurantiaca [1] and its inhibition of the mitochondrial cytochrome bcl-com plex was characterized in detail [2—4]. Stigmatellin was found to affect the mitochondrial cytochrome bc7-complex at the ubiquinol oxidation site (Q nsite), shifting the a-peak of cytochrome b to the red and raising the redox potential of the Rieske FeScenter [4, 5]. Stigmatellin also inhibits photosynthet­ ic electron flow in chloroplasts, where it affects photosystem II in addition to the cytochrome b6fcomplex [6]. The inhibitory mechanism on the cyto­ chrome b 6 f-complex, in contrast to an earlier report [7], has recently been shown to be similar to the one on the mitochondrial complex [8], i.e., stigmatellin dramatically raises the redox potential of the Rieske FeS-center, so that arriving electrons do not pass on to cytochrome /. Also a red-shift of the a-peak of cytochrome b6 has been reported [9]. In a more de­ tailed study we demonstrate here that this spectral effect is of complex nature in both, in the chloroplast — as well as in the mitochondrial system, and not only a simple absorption shift of the low-potential heme of cytochrome b/b6 at the Q0-site. Together with the spectral changes, changes of the redox potentials of the hemes, including the ones of cyto­ chrome c l a n d /, not only of the Rieske FeS-center, occur upon addition of stigmatellin in both systems.