COMPARISON OF TWO STRUCTURES OF BACTERIAL LIGHT HARVESTING COMPLEXES DETERMINED BY EM AND X-RAY CRYSTALLOGRAPHY. Hugh Savage, Guillermo Montoya, Irmgard Sinning. EMBL Postfach 102209, D-69012 Heidelberg, Germany

Within the membranes of photosynthetic bacteria, up to three types of light harvesting complexes (LHI, LHII and LHIII) are found. These complexes absorb photons and transfer the excitation energy to the photosynthetic reaction centre. The LH complexes comprise pairwise-units of [[alpha]] and [[beta]] polypeptides with associated pigment molecules. The polypeptides each contain one transmembrane alpha-helix with bacteriochlorophyll and carotenoid molecules bound between them.

The structure of the LHII complex from Rhodovulum sulfidophilum (RS) has been examined using cryo-electron microscopy to a resolution of 7Å. The complex is a nonamer containing nine [[alpha]][[beta]] subunits. These are arranged in two radially symmetric concentric cylinders, with the nine [[alpha]] chains positioned in the inner cylinder and the nine [[beta]] chains forming the outer cylinder. The positions of the eighteen transmembrane helices are readily observed in the EM projection maps, along with eighteen additional peaks, attributed to the pigment molecules.

The X-ray structure of the LHII complex from Rhodopseudomonas acidophila, strain 10050 (RA) has been determined recently (McDermott et al, 1995) and also contains nine [[alpha]][[beta]] subunits. Comparison of the RS and RA peak positions indicate small but significant differences The similarity of the two nonameric structures at 7Å in projection indicates that results obtained by the two methods of electron and X-ray crystallography, are directly comparable. EM analysis of 2D crystals allows a rapid determination of key structural features and the oligomeric state of the complex.The determination of further structures of LH complexes will uncover the full extent of the variability of the oligomerization states in different bacteria and also in the native membrane.

McDermott et al. (1995) Nature 374, 517-525.

Savage et al. (1996) Structure 4, in press.