Inhibition of photophosphorylation by tentoxin, a cyclic tetrapeptide

Tentoxin, a fungal toxin which is known to cause seedling chlorosis, was found to inhibit cyclic photophosphorylation but not reversible proton accumulation by isolated chloroplasts. The toxin, at low concentrations, also inhibited coupled electron flow (in the presence of ADP and phosphate) but did not effect basal electron flow (in the absence of ADP and phosphate) or uncoupled electron transport. It is suggested that tentoxin is an energy transfer inhibitor which acts at the terminal steps of ATP synthesis.