Chlorophyll organization in green photosynthetic bacteria.

Light-harvesting BChl c, d or e is thought to be located inside the rod elements which fill the chlorosome appressed to the inside of tbe cytoplasmic membrane of green photosynthetic bacteria. BChl a is known to be a part of BChl a-protein which forms a crystal-line baseplate between the rod elements in the chlorosome and the inside of the cytoplasmic membrane. Reaction-center complexes are most probably buried under the baseplate inside the membrane. Energy transfer is from BChl c, d or e in the rod elements to BChl a in the baseplate and then to BChl a in the reaction-center complexes. The rod elements in green sulfur bacteria are thought to be composed of approx. 15-kdalton protein subunits, each associated with 12-14 BChl c, d or e molecules. Six subunits would be required to form a 10-nm ring, and about 35 rings would be necessary to form a 100-nm rod element. The baseplate appears to be a two-dimensional crystal (trigonal space group P31) of BChl a-protein trimers with the 3(1) screw axis tilted 25 degrees out of the plane membrane. The reaction-center complex is thought to be made up of a 100-kdalton carotenoid reaction-center core and five 50-kdalton subunits, each containing seven BChl a molecules. Each reaction-center complex is apparently linked directly to two BChl a-protein trimers in the baseplate. The reaction centers in green sulfur bacteria may be of one type (containing P-840), or of two types (containing P-830 or P-842). In filamentous gliding bacteria the reaction centers appear to contain only P-865. The number of BChl a molecules in these reaction centers is not known, but is assumed to be at least two.