Isolation and development of chlorosomes in the green bacterium Chloroflexus aurantiacus

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Abstract
Freeze-fracture EM was used to study further the changes in chlorosome structure during the development of the photosynthetic apparatus in C. aurantiacus J-10-fl. During development, in response to decreased light intensity or lower O2 tension, the number of chlorosomes per cell increased. The same conditions also led to a general thickening of chlorosomes but did not affect their length or width. The thickening of the chlorosomes paralleled increases in the bacteriochlorophyll c/bacteriochlorophyll a ratio. Semiaerobic induction of the photosynthetic apparatus did not produce a synchronous assembly of chlorosomes in all cells of a given culture. Even adjacent cells of a single filament showed great variations in the rate and extent of response. Parallel appearance of .apprx. 5-nm particles (in a lattice configuration) in the membrane attachment site, the crystalline baseplate material (with a periodicity of .apprx. 6 nm) adjacent to the membrane attachment site, and the chlorosome envelope layer preceded addition of longitudinally oriented, rodlike elements (diameter .simeq. 6 nm) to the chlorosome core. Each chlorosome can funnel energy into .apprx. 100 reaction centers. Chlorosomes could be isolated by a simple density gradient procedure only from cells grown at low light intensity. A bacteriochlorophyll a species absorbing at 790 nm was associated with isolated chlorosomes. Lithium dodecyl sulfate-polyacrylamide gel electrophoresis of chlorosomes showed only a few low MW polypeptides (< 15,000).