Chlorobi: Green Sulfur bacteria

Background

The bacterial phylum Chlorobi, or the green sulfur bacteria, is one of only six bacterial phyla with chlorophyll-synthesizing, photosynthetic members. Green sulfur bacteria are typically found in anoxic, sulfur-rich aquatic or terrestrial environments, and characteristically the light intensities are extremely low where they usually live. Green sulfur bacteria are obligately anaerobic photoautotrophs, which oxidize sulfur compounds (for more information), hydrogen gas or ferrous iron; fix carbon by the reverse TCA cycle; synthesize BChls c, d, or e as well as smaller amounts of BChl a and Chl a; and have a photosynthetic apparatus composed of a type I reaction center, the Fenna-Matthews-Olson BChl a-binding protein, and chlorosomes that each contain ~250,000 BChl c, d, or e molecules.

Fig.1 The hot spring where Chlorobium tepidum was isolated.	Fig. 2 Light microscopic image of Chlorobium tepidum.
	Fig. 3 Transmission electron micrograph of Chlorobium tepidum.

Genome Sequencing

The genome of Chlorobium tepidum (2.15 Mbp) was completely sequenced (Eisen et al., PNAS 2002). Studies on important physiological characteristics such as bacteriochlorophyll and carotenoid biosynthesis, chlorosome structure and biogenesis, and oxygen tolerance have been greatly facilitated by the genome sequence. Our group collaborated with the Joint Genome Institute of the Department of Energy (JGI-DOE) to sequence another 11 green sulfur bacteria strains. By sequencing the genomes of many green sulfur bacteria from different environments and with different phenotypes, we hope to discover many novel and exciting findings about the physiology, metabolism, and evolutionary relationships among these and other bacteria.

Fig. 4 Phylogenetic tree of concatenated protein sequences of 813 core set genes shared among 12 sequenced green sulfur bacteria.

To download genome sequence (completed and draft), click here.

Chlorosome structure, function and assembly

Chlorosomes are large cellular organelles that contain a large number of light-harvesting molecules. A typical chlorosome from Chl. tepidum is about 100–200 nm long, 50 nm wide, and 20–30 nm high and contains ~250,000 BChl c molecules, 2,500 BChl a molecules, 20,000 carotenoid molecules, 15,000 chlorobiumquinone molecules, 3,000 menaquinone-7 molecules, and about 5,000 protein molecules. The functions of the ten chlorosome proteins have been studied, although some of them are still not clear. Interactions among these proteins and the structure of chlorosome envelope have been studied using chemical cross-linking and a detailed model has been proposed. Essentially nothing is known about chlorosome biogenesis. We have identified several proteins by comparative genomics and analysis of vestigial chlorosomes that may function in transporting components into the chlorosomes.

Fig. 5 The ten chlorosome proteins in Chl. tepidum. The shapes of the proteins correspond to the four structural motif families. CsmA is the major protein (>50% of the total chlorosomal protein mass) and binds BChl a and carotenoid(s).

Fig. 6 Proposed model of protein organization on the chlorosome envelope. BChl c aggregates are shown in the rod model (left side) and the more recently proposed lamellar model (right side). Chlorosome proteins are denoted with single letters (e.g., B represents CsmB). The thick red arrows indicate the paths for excitation energy transfer, and the blue arrows indicate possible electron transfer reactions. The quencher is likely to be chlorobiumquinone.

Bacteriochlorophyll biosynthesis

Green sulfur bacteria synthesize Bchl c, d, or e as their major chlorophyll species. They also synthesize small amounts of Bchl a and Chl a. The biosynthetic pathways for these molecules in Chlorobium tepidum have been elucidated by comparative genomic analysis and gene inactivation. Most genes involved in these pathways have been identified. The transformation from chlorophyllide a to 3-vinyl-bacteriochlorophyllide d and from bacteriochlorophyllide c to bacteriochlorophyllide e still remain to be discovered.

Fig. 7 Proposed biosynthetic pathways for BChl c, BChl a, and Chl a in Chl. tepidum. The structural features that differ between the three end products are indicated in red. Genes that have been inactivated are boxed and indicated in blue.

Fig. 8 Biosynthetic pathway of carotenoids in Chl. tepidum. All genes identified in this pathway have been insertionally inactivated, and the resulting mutant strains have been biochemically and physiologically characterized

Carotenoid biosynthesis

By inactivating candidate genes selected from the genome annotation, all of the genes involved in the biosynthesis of carotenoids in Chlorobium tepidum have been identified. Studies of various carotenoid mutants have shown that carotenoid play important roles in photoprotection, have structural roles in pigment protein complexes and BChl c aggregates in chlorosomes, and also are important for oxygen tolerance in Chlorobium tepidum.

References

Eisen JA, Nelson KE, Paulsen IT, Heidelberg JF, Wu M, Dodson RJ, Deboy R, Gwinn ML, Nelson WC, Haft DH, Hickey EK, Peterson JD, Durkin AS, Kolonay, JL, Yang F, Holt I, Umayam LA, Mason T, Brenner M, Shea TP, Parksey D, Venter JC, Volfovsky N, Gruber TM, Ketchum KA, Tettelin H, Bryant DA, and Fraser CM (2002) The complete genome sequence of the green sulfur bacterium Chlorobium tepidum. Proc. Natl. Acad. Sci. USA 99: 9509-9514.
Bryant DA and Frigaard N-U (2006) Prokaryotic photosynthesis and phototrophy illuminated. Trends Microbiol. 14: 488-496.
Frigaard N-U and Bryant DA (2004) Seeing green bacteria in a new light: genomics-enabled studies of the photosynthetic apparatus in green sulfur bacteria and filamentous anoxygenic phototrophic bacteria. Arch. Microbiol. 182: 265-276.
Li H, Frigaard N-U, and Bryant DA (2006) Molecular contacts for chlorosome envelope proteins revealed by cross-linking studies with chlorosomes from Chlorobium tepidum. Biochemistry 45: 9095-9103.
Gomez Maqueo Chew A and Bryant, DA (2007) Chlorophyll biosynthesis in bacteria: the origins of structural and functional diversity. Annu. Rev. Microbiol. 61: 113-129.
Gomez Maqueo Chew A, Frigaard N-U, and Bryant DA (2007) Bacteriochlorophyllide c C-82 and C-121 methyltransferases are essential for adaptation to low light in Chlorobaculum tepidum. J. Bacteriol. 189: 6176-6184.
Maresca JA and Bryant DA (2006) Two genes encoding new carotenoid-modifying enzymes in the green sulfur bacterium Chlorobium tepidum. J. Bacteriol. 188: 6217-6223.
Maresca JA, Graham JE, Wu M, Eisen JA, and Bryant DA (2007) Identification of a fourth family of lycopene cyclases in photosynthetic bacteria. Proc. Natl. Acad. Sci. USA 104: 11784-11789.