ON THE ORIENTATION OF PHOTOSYSTEM-II INHIBITORS IN THE Q(B)-BINDING NICHE - ACRIDONES, XANTHONES AND QUINONES

The orientation of acridones, xanthones, 1,4-benzo- and naphthoquinone s within the photosystem II Q(B) herbicide-binding niche was studied b y means of mild trypsination and by estimation of pI50-values in Chlam ydomonas reinhardtii D1 mutants (Val219 > Ile, Ala251 > Val, Phe255 > Tyr, Ser264 > Ala, Asn266 > Thr, and Leu275 > Phe). As judged from the R/S-values (ratios of I50-values resistant versus susceptible type) c lose to 1 in all mutants, the acridones and xanthones do not have stro ng interactions with the parent amino acids. Contrary, the quinones ex hibit extreme low R/S-values down to 0.003 (for 2,5-dibromo-3-methyl-6 -isopropyl-1,4-benzoquinone; DBMIB) in the Ser264 mutant. This extreme negative cross resistance or supersensitivity indicates that the quin ones do not form a hydrogen bond to the serine hydroxyl group.