FLOW FORCE RELATIONSHIPS IN LETTUCE THYLAKOIDS .1. STRICT CONTROL OF ELECTRON FLOW BY INTERNAL PH

The regulation by the proton gradient of the electron flow from water to ferricyanide was investigated in thylakoids extracted from lettuce leaves. When the transmembrane proton current was varied by an uncoupl er or by the ATP synthase activity, a unique relationship was found be tween the rate of ferricyanide reduction and the proton gradient, rest ricted here to its DELTApH component. This behavior was conserved in C F1-depleted thylakoids where the passive proton flow was varied by the concentration of an F(o) inhibitor or by the concentration of an unco upler after 100% inhibition of F(o). This shows that under our experim ental conditions no direct proton transfer exists in steady state betw een the site of regulation of the redox chain and the ATPase. Studies at two different pH's indicate that the internal pH, and not the trans membrane pH difference, controls the electron transfer between PS2 and PS1. Modeling the data suggests that a single deprotonation step is k inetically limiting.