The Association Kinetics Encode the Light Dependence of Arabidopsis Phytochrome B Interactions.

Plant phytochromes enable vital adaptations to red and far-red light. At the molecular level, these responses are mediated by light-regulated interactions between phytochromes and partner proteins, foremost the phytochrome-interacting factors (PIF). Although known for decades, quantitative analyses of these interactions have long been sparse. To address this deficit, we here studied by an integrated fluorescence-spectroscopic approach the equilibrium and kinetics of Arabidopsis thaliana phytochrome B binding to a tetramerized PIF6 variant. Several readouts consistently showed the stringently light-regulated interaction to be little affected by PIF tetramerization. Analysis of the binding kinetics allowed the determination of bimolecular association and unimolecular dissociation rate constants as a function of light. Unexpectedly, the stronger affinity of A. thaliana phytochrome B under red light relative to far-red light is entirely due to accelerated association rather than decelerated dissociation. The association reaction under red light is highly efficient and only 3-fold slower than the diffusion limit. The present findings pertain equally to the analysis of signal transduction in plants and to the biotechnological application of phytochromes.
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