The 39-kDa fragment of oat phytochrome phyA, obtained by tryptic digestion at the amino acids 65 and 425, was studied by resonance Raman spectroscopy. The parent state Pr reveals far-reaching similarities with that of the native phytochrome implying that the structures of the tetrapyrrole chromophore and its immediate protein environment are not affected by the proteolysis. However, the resonance Raman spectrum of the final product of the Pr phototransformation, denoted as Pbl, is more closely related to that of the Pfr precursor of the native phytochrome, i.e. meta-RC, rather than to that of Pfr itself. The resonance Raman spectra indicate a high conformational flexibility of the chromophore in Pbl so that, unlike in Pfr, the tetrapyrrole rings C and D adopt a largely coplanar conformation. The protein interactions with ring D of the chromophore, which in the native phytochrome stabilize the specific chromophore structure of Pfr, cannot be established in the 39-kDa fragment due to the lack of the major C-terminal part of the protein. These findings, furthermore, support the view that the meta-RC→Pfr transition is associated with a coupling of chromophore and protein structural changes that represent crucial events for the photoactivation of phytochrome.
