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Journal of Bacteriology, June 2002, p. 3368-3376, Vol. 184, No. 12
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.12.3368-3376.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Chlorobium tepidum Mutant Lacking Bacteriochlorophyll c Made by Inactivation of the bchK Gene, Encoding Bacteriochlorophyll c Synthase

Niels-Ulrik Frigaard,* Ginny D. Voigt, and Donald A. Bryant

Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

Received 30 January 2002/ Accepted 25 March 2002

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 µmol 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.

* Corresponding author. Mailing address: 232 South Frear Building, Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802. Phone: (814) 863-7405. Fax: (814) 863-7024. E-mail: nxf10{at}psu.edu.

Journal of Bacteriology, June 2002, p. 3368-3376, Vol. 184, No. 12
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.12.3368-3376.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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