An evaluation of electron micrographs of bacteriochlorophyll a-protein crystals in terms of the structure determined by x-ray crystallography.

Previous electron microscopic studies of different crystalline aggregates of the water-soluble bacteriochlorophyll a -protein (Bchl-protein) from the green photosynthetic bacterium Chlorobium limicola are reevaluated in terms of the structure of the protein determined by X-ray crystallography. The overall molecular packing arrangement proposed by J. M. Olson, D. F. Koenig and M. C. Ledbetter [ Arch. Biochem. Biophys. 129 , 42 (1969)] and L. W. Labaw and R. A. Olson [ J. Ultrastruct. Res. 31 , 456 (1970)] for the hexagonal crystals of the Bchl-protein is confirmed by the X-ray analysis, although the molecules are shown to be trimers rather than tetramers as assumed previously. Consideration of the molecular contacts exhibited in the hexagonal and trigonal crystal forms of the protein leads to a revised model for the tubular paracrystalline aggregates. Implications of the molecular symmetry and the observed crystal packing for the arrangement of the Bchl-protein in vivo are discussed.