QUATERNARY STRUCTURE OF PEA PHYTOCHROME I DIMER STUDIED WITH SMALL‐ANGLE X‐RAY SCATTERING and ROTARY‐SHADOWING ELECTRON MICROSCOPY

Abstract— The quaternary structure of pea phytochrome type I (PI) dimer in the red‐light‐absorbing form was studied by small‐angle X‐ray scattering (SAXS) technique and rotary‐shadowing electron microscopy. Structural parameters for PI 114 kDa chromopeptide dimer and its tryptically digested N‐terminal 59 kDa chromopeptide monomer, such as average electron density, molecular volume and the second moment of electron density distribution, were determined in terms of SAXS using the contrast variation method. Furthermore, by means of model simulation for the scattering profiles of the chromopeptides, most plausible structural models for both peptides were constructed. The distance between the chromophoric domains was estimated to be about 70 A in the resultant model for 114 kDa chromopeptide dimer. Furthermore, the model was consistent with the electron‐micrographic images of both the intact PI dimer and the PI 114 kDa chromopeptide dimer, so that the N‐terminal 7 kDa fragment did not significantly contribute the low‐resolution images of the dimer.