The Kinetics of Phytochrome Conversion

Abstract

It is shown, by flash technique, that the light-initiated interconversions between the red (Pr) and far red (Pf,) forms of phytochrome occur in a complex series of steps.For the transformation Pr -Pfr at 0 °, the initial photoreaction is followed by four dark processes in the interval from 0.1 msec to 5 sec after the flash.The initial photoproduct has a peak at 695 mAu and no far red absorption.In the reverse reaction, Pr -Pr, two dark processes are observed in the interval from 0.1 msec to 10 msec, and the intermediates show no far red absorption.The transformations are first order in phytochrome.Rate constants and spectra are given for all six distinguishable stages of the interconversion.The effects are attributed to a combination of sequential and parallel processes, involving conformational changes of the chromophore and possibly also of the protein.The kinetic and spectroscopic evidence indicates that in the photoconversion of Pr the photochemical act leads, through an intervening dark step, to three different intermediates, which then react independently to form PfP. Two such parallel intermediates may appear in the formation of Pr from Pf,. Comparisons are made between the two photoreversible systems, phytochrome and rhodopsin.The pigment phytochrome plays a key role in the photomrorphogenic and photoperiodic responses of higher plants (1-3).Studies on the physical chemistry of phytochrome are now in progress, as a step towards the detailed understanding of its biological function.This function depends on the ability of the phytochrome system to exist in two forms, Pr and Pf,, which are photochemically interconvertible.Light absorbed by Pr (peak at 664 m) transforms it to Pf, (peak at 724 mM) and vice versa.hv (664)