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

Abstract

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 Pfr (the far‐red‐absorbing form of phytochrome) showed that two Pfr molecules bind to the membrane for each Pr (the form with an absorption maximum in the red) photoconverted to Pfr 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. Pr binding can be simulated if Pr binds only as a consequence of the binding of Pfr, i.e. when Pfr is part of a Pr: Pfr dimer. The enrichment of the membranes with Pfr as a result of the binding of Pfr 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.