Topography of the phytochrome molecule as determined from chemical modification of SH-Groups

Abstract

124 kDa phytochrome has been isolated from etiolated oat seedlings. Specific chemical modification of SH-groups has been performed using several agents, especially mercurials.These procedures allowed covalent fluorescence labelling of phytochrome. Bleaching of the tetrapyrrole chromophore and reduced photoreversibility has been investigated and analyzed in terms of steric effects, i.e.size of the reagents and number of modified residues.The Pr form was found to exhibit two highly reactive SH-groups, the Pfr form three, however.Two highly reactive groups have been localized on the 55kDa C-terminal half of the protein.The 3rd group predominantly reacting in the Pfr form has been localized on a 39 kDa chromopeptide, i.e.on the N-terminal half. The covalently attached fluoresceine chromophores were used as reporter groups. The environment of the fluoresceine chromophore is more hydrophobic in the Pfr form than in the P form, indicating that conformational changes take place during photoconversion. © 1988, Walter de Gruyter. All rights reserved.