Evidence for symmetry in the phytochrome subunit

Abstract

PHYTOCHROME is a chromoprotein that has been shown to initiate and control major developmental responses of plants to light¹. It has two spectrally different forms that can be reversibly interconverted by light, as indicated:The spectral properties of phytochrome are sensitive to changes in the microenvironment of the covalently bound chromophore(s). Relatively mild denaturing treatments, such as incubation at 25 °C for over an hour² and dehydration³, result in a significant loss of photoreversibility. In view of this susceptibility to spectral denaturation, it is remarkable that phytochrome can be proteolytically degraded to a molecular weight (MW) of 60,000, or half its native subunit size, without exhibiting any loss of photoreversibility or even any blue shift in its peak absorbance as Pr⁴. The additional fact that further proteolysis of phytochrome to 40,000 MW drastically reduces photoreversibility⁵ suggests that the 60,000 MW fragment has a sequence and configuration that are both sufficient and necessary to preserve photoreversibility. It is not clear whether this fragment is the product of one or more centrally located cleavages of the native subunit to yield two pieces of MW near 60,000 per native subunit or whether it is the product of extensive proteolysis which yields a single 60,000 MW fragment per native subunit. We have compared the amino acid composition and the peptide map of the proteolytically-derived 60,000 MW fragment with those of the native 120,000 MW subunit. Our results strongly support the conclusion that the native 120,000 MW subunit is composed of two halves with nearly identical sequences.