The structures of the phytochrome chromophore in both photoreversible forms.

Spectral measurements of phytochrome are performed after unfolding of the peptide chain. By comparison with bile pigments of known structure, structure 1a, containing a hydrogenated ring A, is deduced for the PR chromophore. Its spectral properties indicate that the chromophore of the physiologically active PFR form has lost the double bond of the bridge joining rings A and B.