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

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.