ISOLATION and CHARACTERIZATION OF A SUBUNIT FORM OF THE B875 LIGHT‐HARVESTING COMPLEX FROM Rhodobacter capsulatus

A structural subunit (called B816) has been isolated from the B875 light‐harvesting complex of Rhodobacter capsulatus using a detergent‐mediated dissociation of chromatophores. Rb. capsulatus MW442 (B800–850⁻ B875⁺ car⁺) chromatophores were extracted with benzene and titrated with octyl glucoside (OG) to shift the near‐infrared absorption maximum from 873 to 816 nm. Gel filtration chromatography was then used to separate B816 from reaction centers. B816 could be quantitatively shifted back to a B875‐like form (le_max= 875 nm) by decreasing the OG concentration. A similar B816 species could be isolated in low yield from wildtype (B800–850⁺ B875⁺ car⁺) cells but not from SB203E (B800–850⁻ B875 ⁺ car⁻). In the latter case, the B816 subunit seemed too unstable to be isolated under equivalent conditions. The α: β polypeptide ratio, the CD spectrum, and the ability to reversibly dissociate B816 to free bacteriochlorophyll and α‐ and β‐polypeptides were found to be similar to those of the B820 subunit of Rs. rubrum previously reported by our laboratory.