FLOW FORCE RELATIONSHIPS IN LETTUCE THYLAKOIDS .2. EFFECT OF THE UNCOUPLER FCCP ON LOCAL PROTON RESISTANCES AT THE ATPASE LEVEL

The relationship between the steady-state proton gradient (DELTApH) an d the rate of phosphorylation was investigated in thylakoids under var ious conditions. Under partial uncoupling by carbonyl cyanide p-(trifl uoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was redu ced by less than expected from the decrease of DELTApH. This was obser ved in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O --> photosystem 2 --> cytochrome b6f --> photosystem 1 --> methyl viologen system. In state 4, a unique relation was found between DELTApH and the ''phosphate potential'', D ELTAG(p), regardless of whether the energy level was controlled by lig ht input or FCCP. The anomalous effect of FCCP on the rate of ATP synt hesis disappeared when the ATPase was partially blocked by the reversi ble inhibitor venturicidin, but not in the presence of tentoxin, an ir reversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the A TPase. This resistance would be collapsed by FCCP.