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The Kinetics of Phytochrome Conversion

From the ¹ From the Department of Chemistry, Brandeis University, Waltham, Massachusetts 02154
² From the United States Department of Agriculture, Agricultural Research Service, Beltsville, Maryland

It is shown, by flash technique, that the light-initiated interconversions between the red (P_r) and far red (P_fr) forms of phytochrome occur in a complex series of steps. For the transformation P_r P_fr 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 mµ and no far red absorption. In the reverse reaction, P_fr P_r, 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 P_r the photochemical act leads, through an intervening dark step, to three different intermediates, which then react independently to form P_fr. Two such parallel intermediates may appear in the formation of P_r from P_fr. Comparisons are made between the two photoreversible systems, phytochrome and rhodopsin.

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