Particle-bound phytochrome: a function of light dose and steady-state level of the far-red-absorbing form.

Abstract

The binding of phytochrome to a particulate fraction from maize coleoptiles has been examined as a function of the level ofP fr 1 offeredin vivo. Further evidence is provided that the degree of binding is a function of both the form of the phytochrome and the state of the binding sites; and thatP fr induces a change in the state of the binding sites such that the subsequent affinity forP r is enhanced. Increasing the steady-state level ofP fr offeredin vivo results in a subsequent binding curve forP r that is suggestive of cooperativity. However, increasing both time and irradiance parameters of the light dose while holding the steady-stateP fr level constant results in increased binding ofP r up to a saturation level. This indicates that the response is the product of both the steady-stateP fr concentration and the light dose and is determined therefore ultimately by the cycling rate. The system appears to respond therefore to the total number ofP fr molecules integrated over time rather than simply to the steady-state concentration ofP fr .