Complex Intracellular Structures in Prokaryotes

Volume 2 of the series Microbiology Monographs pp 79-114


Chlorosomes: Antenna Organelles in Photosynthetic Green Bacteria

  • Niels-Ulrik FrigaardAffiliated withInstitute of Molecular Biology and Physiology, University of Copenhagen Email author 
  • , Donald A. BryantAffiliated withDepartment of Biochemistry and Molecular Biology, The Pennsylvania State University

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Chlorosomes are the light-harvesting antenna organelles found in two groups of bacteria, thegreen sulfur bacteria and the green filamentous bacteria, collectively known as photosynthetic greenbacteria. Chlorosomes consist mostly of aggregated bacteriochlorophyll (BChl) c,d, or e and are the largestantenna structures known. Unlike other light-harvesting antenna structures, the major antenna pigments(BChl c, d, or e)form aggregates that do not require a protein scaffold. This is possible because these BChlspossess structural modifications that do not occur in other naturally occurring chlorophyll derivatives.These properties allow the formation of very large and efficient antennae that permit phototrophicgrowth at remarkably low light intensities. The BChl aggregates are enveloped in a monolayerprotein–lipid membrane with a high content of glycolipids. Chlorosomes also contain smallamounts of BChl a bound to the CsmA protein, other protein speciesof mostly unknown function, carotenoids, and isoprenoid quinones. Chlorosomes from thermophilic greenbacteria also contain wax esters. Although very little is known about how chlorosomes are synthesizedby the cells, significant efforts have been devoted to understanding the structural organization andenergy transfer characteristics of the BChl aggregates in chlorosomes. This is in part because theseaggregates serve as models of self-assembling systems and their potential use as light-harvestingnanostructures in artificial photosystems. Recent progress in genetic manipulation of BChl c, carotenoids, and chlorosome envelope proteins in green sulfur bacteriahas allowed a better understanding of how these components function in chlorosomes.