Isolation and development of chlorosomes in the green bacterium Chloroflexus aurantiacus

Abstract

Freeze-fracture electron microscopy was used to study further the changes in chlorosome structure during the development of the photosynthetic apparatus in Chloroflexus aurantiacus J-10-fl. During development, in response to decreased light intensity or lower oxygen 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 (i) approximately 5-nm particles (in a lattice configuration) in the membrane attachment site, (ii) the crystalline baseplate material (with a periodicity of approximately 6 nm) adjacent to the membrane attachment site, and (iii) the chlorosome envelope layer preceded addition of longitudinally oriented, rodlike elements (diameter, congruent to 6 m) to the chlorosome core. It is estimated that each chlorosome can funnel energy into approximately 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-molecular-weight polypeptides (less than 15,000).