title: Isolation and characterization of carotenosomes from a bacteriochlorophyll c-less mutant of Chlorobium tepidum.
abstract: Chlorosomes are the light-harvesting organelles in photosynthetic green bacteria and typically contain large amounts of bacteriochlorophyll (BChl) c in addition to smaller amounts of BChl a, carotenoids, and several protein species. We have isolated vestigial chlorosomes, denoted carotenosomes, from a BChl c-less, bchK mutant of the green sulfur bacterium Chlorobium tepidum. The physical shape of the carotenosomes (86 +/- 17 nm x 66 +/- 13 nm x 4.3 +/- 0.8 nm on average) was reminiscent of a flattened chlorosome. The carotenosomes contained carotenoids, BChl a, and the proteins CsmA and CsmD in ratios to each other comparable to their ratios in wild-type chlorosomes, but all other chlorosome proteins normally found in wild-type chlorosomes were found only in trace amounts or were not detected. Similar to wild-type chlorosomes, the CsmA protein in the carotenosomes formed oligomers at least up to homo-octamers as shown by chemical cross-linking and immunoblotting. The absorption spectrum of BChl a in the carotenosomes was also indistinguishable from that in wild-type chlorosomes. Energy transfer from the bulk carotenoids to BChl a in carotenosomes was poor. The results indicate that the carotenosomes have an intact baseplate made of remarkably stable oligomeric CsmA-BChl a complexes but are flattened in structure due to the absence of BChl c. Carotenosomes thus provide a valuable material for studying the biogenesis, structure, and function of the photosynthetic antennae in green bacteria.
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA. nuf@mit.edu
authors.forename: Niels-Ulrik
authors.initials: NU
authors.lastname: Frigaard
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Peter
authors.initials: P
authors.lastname: Martinsson
authors.forename: Somes Kumar
authors.initials: SK
authors.lastname: Das
authors.forename: Harry A
authors.initials: HA
authors.lastname: Frank
authors.forename: Thijs J
authors.initials: TJ
authors.lastname: Aartsma
authors.forename: Donald A
authors.initials: DA
authors.lastname: Bryant
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001429:Bacteriochlorophylls / Q000172:deficiency*; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism*; D002458:Cell Fractionation; D041883:Chlorobium / Q000737:chemistry / Q000166:cytology* / Q000235:genetics* / Q000648:ultrastructure; D018625:Microscopy, Atomic Force; D008854:Microscopy, Electron; D009154:Mutation / Q000235:genetics; D015388:Organelles / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D013057:Spectrum Analysis
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; C404706:CsmA protein, Chlorobium tepidum; D002338:Carotenoids; C002084:bacteriochlorophyll c