The crystallographic structure of the B800-820 LH3 light-harvesting complex from the purple bacteria Rhodopseudomonas acidophila strain 7050

The B800-820, or LH3, complex is a spectroscopic variant of the B800-850 LH 2 peripheral light-harvesting complex. LH3 is synthesized by some species a nd strains of purple bacteria when growing under what are generally classed as "stressed" conditions, such as low intensity illumination and/or low te mperature (<30 <degrees>C). The apoproteins in these complexes modify the a bsorption properties of the chromophores to ensure that the photosynthetic process is highly efficient. The crystal structure of the B800-820 light-ha rvesting complex, an integral membrane pigment-protein complex, from the pu rple bacteria Rhodopseudomonas (Rps.) acidophila strain 7050 has been deter mined to a resolution of 3.0 Angstrom by molecular replacement. The overall structure of the LH3 complex is analogous to that of the LH2 complex from Rps. acidophila strain 10050. LH3 has a nonameric quaternary structure wher e two concentric cylinders of alpha -helices enclose the pigment molecules bacteriochlorophyll a and carotenoid. The observed spectroscopic difference s between LH2 and LH3 can be attributed to differences in the primary struc ture of the apoproteins. There are changes in hydrogen bonding patterns bet ween the coupled Bchla molecules and the protein that have an effect on the conformation of the C3-acetyl groups of the B820 molecules. The structure of LH3 shows the important role that the protein plays in modulating the ch aracteristics of the light-harvesting system and indicates the mechanisms b y which the absorption properties of the complex are altered to produce a m ore efficient light-harvesting component.