PARENT SESSION
Posters P5C Biosynthesis and assembly: Pigments. Abstracts (643-659)

The role of the C-8² and C-12¹ methylations of bacteriochlorophyll c in the green sulfur bacterium Chlorobium tepidum. Aline Gomez Maqueo Chew^*,1, Niels-Ulrik Frigaard¹, Donald Bryant¹, ¹ Department of Biochemistry and Molecular Biology, University Park, PA, USA

ABSTRACT- Bacteriochlorophyll c (BChl c) accounts for more than 90% of the pigment in the green sulfur bacterium Chlorobium tepidum. Chlorosomes, the light harvesting antennae of green bacteria, can contain more than 200,000 BChl c, d, or e molecules that form self-aggregates. The pigments are methylated at the C-8² and C-12¹ carbons, but the roles of these methylations have not been previously described. The bchQ and bchR genes, which encode the C-8² and C-12¹ methyltransferases of C. tepidum, respectively, were identified by targeted inactivation. Strains harboring segregated mutations in both genes and a double mutant strain were obtained; complete segregation of alleles was confirmed by PCR. The proteins encoded by the bchQ and bchR genes belong to the radical-SAM superfamily of proteins and are thus related to the oxygen-independent isocyclic ring cyclase BchE. The pigmentation of the mutants was analyzed by reverse-phase HPLC, HPLC-mass spectrophotometry, and absorption spectroscopy. The HPLC elution profiles and mass spectroscopic analysis of the chlorophylls in the mutants confirmed the anticipated changes in methylation patterns. The in vivo absorption spectra show that the mutants have a narrower Q_Y peak and are red-shifted to varying degrees. The mutants show slower growth rates than wild type especially at low light intensities. Interestingly, the mutants have a lower BChl c content in relation to protein and other pigments. Our results demonstrate that these methylations have important roles in the adaptation of C. tepidum to low light.

KEY WORDS: bacteriochlorophyll biosynthesis, Bacteriochlorophyll c, Chlorobium tepidum, methylation