2-DIMENSIONAL STRUCTURE OF LIGHT-HARVESTING COMPLEX-II (LHII) FROM THE PURPLE BACTERIUM RHODOVULUM-SULFIDOPHILUM AND COMPARISON WITH LHII FROM RHODOPSEUDOMONAS-ACIDOPHILA

Background: Within the membranes of photosynthetic bacteria, up to thr ee types of light harvesting complexes (LHI, LHII, LHIII) are found. T hese complexes absorb photons and transfer the excitation energy to th e photosynthetic reaction centre. The LH complexes comprise units that contain alpha and beta polypeptides with associated pigment molecules . Results: The structure of LHII complex from Rhodovulum sulfidophilum has been examined to a resolution of 7 Angstrom using electron micros copy. The complex is a nonamer containing nine alpha beta subunits. Th ese are arranged in two radially symmetric concentric cylinders, with the nine a chains positioned in the inner cylinder and the nine beta c hains forming the outer cylinder. The 18 transmembrane helices are rea dily observed in the projection maps, along with 18 additional peaks a ttributed to the pigment molecules. Conclusions: The determination of more structures of LH complexes will uncover the full extent of the va riability of the oligomerization states in different bacteria and also in the native membrane. The analysis of two-dimensional crystals allo ws a rapid determination of key structural features and the oligomeric state of the complex, Comparison of our structure determined by elect ron microscopy with the recently solved X-ray structure indicates that the results of the two methods are complementary.