Small-angle X-ray scattering studies on the photoreversible conformational change of phytochrome

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Phytochrome is a noble photoreceptor chromoprotein in green plants for a variety of morphogenic and developmental responses to light including requlation of the expression of light-sensitive genes. Although the primary structure of phytochrome was recently determined, its tertiary structure has been obscure. It is difficult to apply X-ray crystallography to studies of phytochrome, since high molecular mass of phytochrome (about 250 kDa) has been failed to be crystallized. Hence, the small-angle X-ray scattering (SAXS) technique appears a useful technique for study on the structure of phytochrome in solution. We have, therefore, begun a series of structural studies of phytochrome using SAXS. we determined the molecular mass the radius of gyration, the maximum diameter, the average electron density, and the fluctuation of the elecron density in the molecule. Using the obtained results, the dimeric molecular structure of pea phytochrome (monomer molecular mass, 114 kDa) in the red-ligth-absorbing form was studied with reference to the structures of its molecular domains in a monomer (a n-terminal 59-kDa chromophoric domain and a C-terminal 55-kDa nonchromophoric domain) and a "four-leaved shape" model is proposed.