Structure of the B880 holochrome of Rhodospirillum rubrum as studied by the radiation inactivation method.

Chromatophores from photoreaction centerless strain F24 of Rhodospirillum rubrum were subjected to different doses of gamma radiation. Target theory was applied to the induced decay of the B880 holochrome pigments as analyzed by absorption spectroscopy of the membranes and of organic solvent extracts. Destruction of bacteriochlorophyll is associated with a target size of 7 kDa. This indicates that each one of the two different 6-kDa holochrome polypeptides binds one molecule of this pigment. The target size of spirilloxanthin, 12 kDa, suggests that both polypeptides contribute to the binding site of this carotenoid. The 880 nm absorption band and the oxidation-induced 1225 nm band have a target size of 14 kDA. Therefore, these bands are due to interaction between two bacteriochlorophyll molecules, each one of which resides on a different polypeptide. This 14-kDa complex decays into a bacteriochlorophyll monomer associated with a target size of 7 kDa. The absolute absorption spectra of the protein-bound bacteriochlorophyll pair and monomer are presented.