Photoreversible association of phytochrome with membranes. II. Reciprocity tests and a model for the binding reaction

Abstract A series of fluence-response curves for the binding of phytochrome to membranes in the absence of divalent cations, as described by Watson & Smith (1982), were constructed to demonstrate that the response obeys the law of reciprocity. Analysis of the binding of P_fr (the far-red-absorbing form of phytochrome) showed that two P_fr molecules bind to the membrane for each P_r (the form with an absorption maximum in the red) photoconverted to P_fr in the intrinsic membrane-bound phytochrome pool. Using this stoichiometry we have been able to model the binding curve of Pr and match the binding data. P_r binding can be simulated if P_r binds only as a consequence of the binding of P_fr, i.e. when P_fr is part of a P_r: P_fr dimer. The enrichment of the membranes with P_fr as a result of the binding of P_fr was also accurately simulated. There is no binding cooperativity. Phytochrome binding is a low-fluence response and the possibility that it has physiological significance as a mediator of phytochrome action is discussed.