UNORGANIZED DATA

The green photosynthetic bacteria are characterized by the presence of chlorosomes appressed to the cytoplasmic side of the cytoplasmic membrane. The chlorosomes are filled with bacteriochlorophyll (BChl) c, d or e molecules in a highly aggregated state. The truly "green" bacteria contain mainly BChl c or d; while the others look orange or brown because of a high content of carotenoid. From a phylogenetic point of view the green bacteria are really two separate "phyla" based on 16S rRNA, reaction center (RC) type and physiology. It is truly remarkable that such different types of bacteria (green filamentous bacteria and green sulfur bacteria) contain such similar light-harvesting entities as chlorosomes. Green filamentous bacteria (Chloroflexaceae) contain a quinone-type RC similar to those found in purple bacteria (Proteobacteria), whereas the green sulfur bacteria (Chlorobiaceae) contain an iron-sulfur-type RC similar to those found in heliobacteria and in photosystem I of cyanobacteria and chloroplasts. The filamentous bacteria live either as facultative photoautotrophs that grow in relatively bright light or as respiring chemoheterotrophs. They are found predominantly in hot springs, often in mixed population with cyanobacteria that provide organic carbon compounds for them. Most of our knowledge about the filamentous bacteria at the molecular level comes from one species, Chloroflexus aurantiacus. A second species, Chloroflexus aggregans, has recently been isolated and characterized by Hanada et al. The sulfur bacteria are obligate photoautotrophs and strict anaerobes that grow in dim light in sulfide-rich environments. These conditions are found in effluents of sulfur springs and in the chemocline of stratified lakes and in marine habitats. In one extreme case green sulfur bacteria can be found living at a depth of 80 m in the Black Sea. Our knowledge about the sulfur bacteria comes from several species including Chlorobium limicola, Chlorobium phaeovibrioides, Chlorobium tepidum, Chlorobium vibrioforme, Pelodictyon luteolum and Prosthecochloris aestuarii.

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The crystal structure of the light-harvesting antenna complex (LH2) from Rhodopseudomonas acidophila strain 10050 shows that the active assembly consists of two concentric cylinders of helical protein subunits which enclose the pigment molecules. Eighteen bacteriochlorophyll a molecules sandwiched between the helices form a continuous overlapping ring, and a further nine are positioned between the outer helices with the bacteriochlorin rings perpendicular to the transmembrane helix axis. There is an elegant intertwining of the bacteriochlorophyll phytol chains with carotenoid, which spans the complex.