Chlorobi: Green Sulfur bacteria
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.
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.
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.
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.
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.