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Journal of Bacteriology, September 2007, p. 6176-6184, Vol. 189, No. 17
0021-9193/07/$08.00+0     doi:10.1128/JB.00519-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Bacteriochlorophyllide c C-82 and C-121 Methyltransferases Are Essential for Adaptation to Low Light in Chlorobaculum tepidum{triangledown}

Aline Gomez Maqueo Chew,1 Niels-Ulrik Frigaard,2 and Donald A. Bryant1*

Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802,1 Copenhagen Biocenter, Department of Molecular Biology, University of Copenhagen, Copenhagen, Denmark2

Received 5 April 2007/ Accepted 15 June 2007

Bacteriochlorophyll (BChl) c is the major photosynthetic pigment in the green sulfur bacterium Chlorobaculum tepidum, in which it forms protein-independent aggregates that function in light harvesting. BChls c, d, and e are found only in chlorosome-producing bacteria and are unique among chlorophylls because of methylations that occur at the C-82 and C-121 carbons. Two genes required for these methylation reactions were identified and designated bchQ (CT1777) and bchR (CT1320). BchQ and BchR are members of the radical S-adenosylmethionine (SAM) protein superfamily; each has sequence motifs to ligate a [4Fe-4S] cluster, and we propose that they catalyze the methyl group transfers. bchQ, bchR, and bchQ bchR mutants of C. tepidum were constructed and characterized. The bchQ mutant produced BChl c that was not methylated at C-82, the bchR mutant produced BChl c that was not methylated at C-121, and the double mutant produced [8-ethyl, 12-methyl]-BChl c that lacked methylation at both the C-82 and C-121 positions. Compared to the wild type, the Qy absorption bands for BChl c in the mutant cells were narrower and blue shifted to various extents. All three mutants grew slower and had a lower cellular BChl c content than the wild type, an effect that was especially pronounced at low light intensities. These observations show that the C-82 and C-121 methylations of BChl c play important roles in the adaptation of C. tepidum to low light intensity. The data additionally suggest that these methylations also directly or indirectly affect the regulation of the BChl c biosynthetic pathway.

* Corresponding author. Mailing address: Department of Biochemistry and Molecular Biology, S-235 Frear Building, The Pennsylvania State University, University Park, PA 16802. Phone: (814) 865-1992. Fax: (814) 863-7024. E-mail: dab14{at}psu.edu

{triangledown} Published ahead of print on 22 June 2007.

Journal of Bacteriology, September 2007, p. 6176-6184, Vol. 189, No. 17
0021-9193/07/$08.00+0     doi:10.1128/JB.00519-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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