Spectroscopic properties and chromophore conformations of the photomorphogenic receptor: Phytochrome

Abstract

Fluorescence lifetimes of ‘large’ (mol. wt. 120 000) and ‘small’ (mol. wt. 60 000) phytochromes isolated from oat and rye seedlings grown in the dark have been measured at 199 K and 298 K. Phytochrome model compounds have also been studied by phase modulation fluorometrically at 77 K for comparison with lifetime data for phytochrome. It was found that the fluorescence lifetime of ‘large’ phytochrome was significantly shorter than that of ‘small’ phytochrome and its chromophore models. The phytochrome chromophore of P r form has been analyzed by fluorescence polarization, CD, and molecular orbital methods. The fluorescence excitation polarization of ‘small’ phytochrome and the chromophore model in buffer/glycerol mixture (3 : 1, v/v) at 77 K is very high (0.4) at the main absorption band and is negative (−0.1) and close to 0 in the near ultraviolet band, respectively. Analyses of the spectroscopic data suggest that the chromophore conformation of P r and P fr forms of phytochrome are essentially identical. The induced ellipticity of ‘large’ rye phytochrome in the blue and near ultraviolet regions was found to be significantly higher than that of ‘small’ phytochrome, indicating that the binding interaction between the phytochrome chromophore and apoprotein is much tighter in the former than in the latter. In addition, the excitation energy transfer does occur from Trp residue(s) to the chromophore in ‘large’ phytochrome but not in ‘small’ P r. This illustrates one feature of the role played by the large molecular weight apoprotein in the binding site interactions and primary photoprocesses of P r. Finally, a plausible model for the primary photoprocesses and the mechanism of phytochrome interactions triggered by the P r → P fr phototransformation have been proposed on the basis of the above results.