BIOCHEMICAL AND SPECTROSCOPIC CHARACTERIZATION OF THE REACTION-CENTERLH1 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 assoc iated tetraheme cytochrome. This complex can be further fractionated t o 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 fir st that retains its carotenoid absorption properties. CD spectra in th e Q, region of bacteriochlorophyll a in both the RC-LH1 and the B820 s ubunit are bathochromically shifted as compared to other such complexe s. 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 bet a polypeptide. The CD spectra of these C. purpuratum pigment-protein c omplexes can be interpreted in terms of exciton interaction between ba cteriochlorophylls in the B820 subunit of the LH1 and in the B870, wit h additional spectral characteristics arising from interactions of the pigments with their protein environment.