Effects of Photosystem II Herbicides on the Photosynthetic Membranes of the Cyanobacterium Aphanocapsa 6308.

The effects of the photosystem II herbicides diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) on the photosynthetic membranes of a cyanobacterium, Aphanocapsa 6308, were compared to the effects on a higher plant, Spinacia oleracea. The inhibition of photosystem II electron transport by these herbicides was investigated by measuring the photoreduction of the dye 2,6-dichlorophenol-indophenol spectrophotometrically using isolated membranes. The concentration of herbicide that caused 50% inhibition of electron transport (I(50) value) in Aphanocapsa membranes for diuron was 6.8 x 10(-9) molar and the I(50) value for atrazine was 8.8 x 10(-8) molar. (14)C-labeled diuron and atrazine were used to investigate herbicide binding with calculated binding constants (K) being 8.2 x 10(-8) molar for atrazine and 1.7 x 10(-7) molar for diuron. Competitive binding studies carried out on Aphanocapsa membranes using radiolabeled [(14)C]atrazine and unlabeled diuron revealed that diuron competed with atrazine for the herbicide-binding site. Experiments involving the photoaffinity label [(14)C]azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-2-triazine) and autoradiography of polyacrylamide gels indicated that the herbicide atrazine binds to a 32-kilodalton protein in Aphanocapsa 6308 cell extracts.