Properties of ATP-driven reverse electron flow in chloroplasts

Abstract

1. The reverse reactions induced by coupled ATP hydrolysis were studied in spinach chloroplasts by measurements of the ATP-induced increase in chlorophyll fluorescence reflecting reverse electron flow, and of the ATP-induced decrease in 9-aminoacridine fluorescence, representing formation of the transthylakoidal proton gradient (deltapH). ATP-induced reverse electron flow was kinetically analysed into three phases, of which only the second and third one were paralleled by corresponding phases in deltapH formation. The rapid first phase and formation of a deltapH occur also in the absence of the electron transfer mediator phenazine methosulfate. 2. The rate and extent of the reverse reactions were measured at temperatures in the range from 0 to 30 degrees C. The rate of formation of delta pH and of reverse electron flow were faster at high temperatures, but the maximal extent of delta pH and chlorophyll fluorescence increase were observed at the lowest temperature. Considering rate and extent of the ATP-stimulated reactions, a temperature optimum around 15 degrees C was found. Light activation of the ATPase occurred throughout the range studied. At 0 degrees C and in the presence of inorganic phosphate the activated state for ATPase was maintained for more than 10 min. 3. The ATP-induced rise in chlorophyll fluorescence yield was found to be of similar magnitude as the rise induced by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU), when both were measured with an extremely weak measuring beam. It is concluded, that both effects, although derived via distinctly different pathways, are limited by the same electron donating or electron accepting pool.