Spectral properties and photoactivities of intermediates of photoconversion from the red-light-absorbing to far-red-light-absorbing form of pea phytochrome

The pathway for the phototransformation of the red-light-absorbing (Pr) to the far-red-light-absorbing (Pfr) form of native phytochrome A from pea was investigated by low temperature spectrophotometry. Four intermediates between Pr and Pfr were recognized. The first intermediate (lumi-R) was stable below -100 degrees C. The other intermediates and Pfr appeared during warming of lumi-R to -80 degrees C (metaPa), -40 degrees C (meta-Rb'), -20 degrees C (meta-Rc) and 0 degrees C (Pfr). Each intermediate (lumi-R, meta-Ra and meta-Rc) and Pr was photointerconvertible as a two-component system at the temperature at which the intermediate had initially appeared. Meta-Rb' is also photoconvertible to Pr. Prolonged red light irradiation of meta-Rb' at -40 degrees C resulted in appearance of another intermediate (meta-Rb).