Biochemical and spectroscopic characterization of the reaction center-LH1 complex and the carotenoid-containing B820 subunit of Chromatium purpuratum.

Two complexes, the reaction center light-harvesting complex 1 (RC-LH1) and the B820 subunit of the LH1, have been isolated and characterized from the purple-sulfur photosynthetic bacterium Chromatium purpuratum. The RC-LH1 consists of the B870 antenna and a P-870 RC with an associated tetraheme cytochrome. This complex can be further fractionated to yield the B820 subunit of the LH1. The C. purpuratum B820 subunit is the first isolated from a purple-sulfur bacterium. It is also the first that retains its carotenoid absorption properties. CD spectra in the Qy region of bacteriochlorophyll a in both the RC-LH1 and the B820 subunit are bathochromically shifted as compared to other such complexes. Comparison of the sequence of the LH1 beta polypeptide to other LH1 beta s reveals the presence of additional aromatic amino acids in the vicinity of both of the conserved histidines in the C. purpuratum beta polypeptide. The CD spectra of these C. purpuratum pigment-protein complexes can be interpreted in terms of exciton interaction between bacteriochlorophylls in the B820 subunit of the LH1 and in the B870, with additional spectral characteristics arising from interactions of the pigments with their protein environment.