title: Chlorobium tepidum mutant lacking bacteriochlorophyll c made by inactivation of the bchK gene, encoding bacteriochlorophyll c synthase.
abstract: The gene encoding bacteriochlorophyll (BChl) c synthase was identified by insertional inactivation in the photosynthetic green sulfur bacterium Chlorobium tepidum and was named bchK. The bchK mutant of C. tepidum was rusty-orange in color and completely lacked BChl c. Because of the absence of the BChl c antenna, the mutant grew about seven times slower than the wild type at light intensities that were limiting to the wild type (< 90 micromol m(-2) s(-1)). Various pheophorbides, which probably represent precursors of BChl c which had lost magnesium, accumulated in the mutant cells. A small fraction of these pheophorbides were apparently esterified by the remaining chlorophyll (Chl) a and BChl a synthases in cells. The amounts of BChl a, Chl a, isoprenoid quinones, carotenoids, Fenna-Matthews-Olson protein, and chlorosome envelope protein CsmA were not significantly altered on a cellular basis in the mutant compared to in the wild type. This suggests that the BChl a antennae, photosynthetic reaction centers, and remaining chlorosome components were essentially unaffected in the mutant. Electron microscopy of thin sections revealed that the mutant lacked normal chlorosomes. However, a fraction containing vestigial chlorosomes, denoted "carotenosomes," was partly purified by density centrifugation; these structures contained carotenoids, isoprenoid quinones, and a 798-nm-absorbing BChl a species that is probably protein associated. Because of the absence of the strong BChl c absorption found in the wild type, the bchK mutant should prove valuable for future analyses of the photosynthetic reaction center and of the roles of BChl a in photosynthesis in green bacteria. An evolutionary implication of our findings is that the photosynthetic ancestor of green sulfur bacteria could have evolved without chlorosomes and BChl c and instead used only BChl a-containing proteins as the major light-harvesting antennae.
authors.affiliation: Department of Biochemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. nxf10@psu.edu
authors.forename: Niels-Ulrik
authors.initials: NU
authors.lastname: Frigaard
authors.forename: Ginny D
authors.initials: GD
authors.lastname: Voigt
authors.forename: Donald A
authors.initials: DA
authors.lastname: Bryant
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001429:Bacteriochlorophylls; D019729:Carbon-Oxygen Ligases / Q000235:genetics*; D019414:Chlorobi / Q000235:genetics* / Q000254:growth & development / Q000648:ultrastructure; D045342:Light-Harvesting Protein Complexes; D008854:Microscopy, Electron; D009154:Mutation; D015388:Organelles / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D010860:Pigments, Biological / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; C086320:FMO bacteriochlorophyll protein, Bacteria; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D010860:Pigments, Biological; C002084:bacteriochlorophyll c; D019729:Carbon-Oxygen Ligases