Phytochrome photoconversion

The spectral properties of native and modified phytochromes and the molecular events during phytochrome photoconversion, , are reviewed. Steady-state and time-resolved absorption spectra of native phytochrome A, as well as recombinant phytochromes (oat and potato phytochrome A and potato phytochrome B) reconstituted with phycocyanobilin and phytochromobilin as chromophores, are analysed. The vinyl double bond, present at position 18 in phytochromobilin and substituted by an ethyl group in phycocyanobilin, has a considerable influence on the photo-transformation kinetics of phytochromes A and B, evidently due to a strong interaction of this region of the chromophore with the protein surrounding. The kinetics of the phototransformation of potato phytochrome B differs from that of oat phytochrome A (wild-type and recombinant), indicating that the chromophore-protein interaction in phytochrome B is different from that in phytochrome A. It remains to be seen whether this difference is due to the di- versus monocotyledon origin of the phytochromes. Optoacoustic spectroscopy, applied to native oat phytochrome A, afforded thermo-dynamic, structural and kinetic parameters of the Pr→I₇₀₀ and the I₇₀₀→Pr phototransformations. Raman and infrared spectroscopic data for wild-type phytochrome A suggest that the protonated chromophore in Pr undergoes torsions around two single bonds in addition to the Z→E isomerization of the 15 ,16 double bond, and that all transients, possibly with the exception of I_bI, are protonated at the central pyrrole ring.