INHIBITION OF A PHOTOSYSTEM-II ELECTRON-TRANSFER REACTION BY THE NATURAL PRODUCT SORGOLEONE

Effects of the alleochemical sorgoleone on photosynthetic electron tra nsport by oxygen-evolving chloroplast thylakoids and Triton X-100-prep ared Photosystem II (PSII) membranes were analyzed. The Hill activity of the thylakoids proved to be at least as sensitive to inhibition by sorgoleone as it was to DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] , a potent herbicidal inhibitor of PSII. However, a Photosystem I (PSI ) partial reaction was not affected by a 10-fold greater concentration of sorgoleone than is required for complete inhibition of Hill activi ty. Measurements of flash-induced chlorophyll a variable fluorescence showed that sorgoleone neither dissipated excitation energy nor dimini shed the amplitude of chlorophyll a variable fluorescence. However, it inhibited the decay of variable fluorescence as effectively as DCMU, which blocks the oxidation of the PSII-reduced primary electron accept or, Q(A)(-), by the PSII secondary electron acceptor, Q(B), by displac ing Q(B) from the D-1 protein. Additionally, sorgoleone competitively inhibited the binding of [C-14]atrazine to the Q(B) locus. Increasing durations of trypsin proteolysis of the PSII membranes or thylakoids a nd of the Q(B)-binding niche itself caused parallel losses of inhibiti on of O-2 evolution from sorgoleone and DCMU, as well as from bromoxyn il, a phenol-type herbicide also binding to the Q(B) locus.