Methanococcus jannaschii
A Microbial Biorealm page on the genus Methanococcus jannaschii
Classification
Higher order taxa
- Superkingdom: Archaea
- Phylum: Euryarchaeota
- Class: Methanococci
- Order: Methanococcales
- Family: Methanocaldococcaceae
- Genus: Methanocaldococcus (syn: Methanococcus)
Species
Genus species - Methanocaldococcus jannaschii
- synonym: Methanococcus jannaschii
Description and significance
Methanococcus jannaschii is an autotropic hyperthermophillic organism that belongs to the kingdom of Archaea. They were found to live in extreme environments such as hypothermal vents at the bottom of the oceans in which water reaches boiling temperature or pressure is extremely high (Bult, C.J. et. al., 1996). The evolutionary of these organisms and the biological mechanisms that they use to not only survive but also thrive in such extreme environments are of great interest to current researches.
In 1982, M. jannaschii was first isolated in the East Pacific Rise, near the western coast of Mexico. The organism was found from a sample sediment taken from a 2600 m deep “white smoker” chimney. Its extreme habitat of temperature between 48oC-94oC, pressure at 200atm, a pH of 5.2-7, and 1.0-5.0% NaCl leads experts to a conclusion that these organisms must have modified and unique adaptations for optimal growth at high temperature, high pressure, and also moderate salinity level (Tsoka et. al, 2003). Moreover, cellular structure of M. jannaschii revealed that they are irregular cocci (have irregular spherical shape). Their motility is by utilizing polar bundles of flagella (Tsoka et. al., 2003). Characteristic to any organism that belongs to the kingdom Archaea, M. jannaschii lacks cell wall; however, they do possess cell envelope that is made up of cytoplasmic membrane and a protein surface layer (S-layer) (Sleytr et. al., 2007).
Methanococcus jannaschii belongs to a specific group called methanogens, or methane producers (Tumbula and Whitman, 1999). Methanogens are physiologically specialized to undergo fueling reactions to produce methane as the end product (Reeve, 1992). They are ultimately autotropic single-celled organisms. Being an autotropic organism, M. jannaschii is strictly anaerobic and uses only carbon dioxide as its sole carbon source. Its main pathway for energy production is through methanogenesis, a process during which hydrogen is used as an energy source to reduce carbon dioxide to methane (Zhu et. al., 2004). Methanogens are extremely important to anaerobic environments because they convert organic compounds into methane, which then rises into the aerobic environment. By doing so, these organisms provide a pathway for compounds that exist in anaerobic environment to escape into the atmosphere, where it acts as a natural gas resource (Reeve, 1992).
M. jannaschii became the first organism in the archaea to have its complete genome sequenced (Bult, C. J. et. al, 1996). Its genomic structure, along with the unique lifestyle, has gradually provided insights into understanding the organism’s adaptations to its extreme habitat. More importantly, the knowledge gained from studying the metabolic processes, the enzymes, and the proteins that are specifically involved in these processes will lead to deeper understanding of the evolution of Archaea. In studying the interaction between M. jannaschii and its environment, many evolution questions about our environments could potentially be answered.
Genome structure
Methanococcus jannaschii was the first organism of the kingdom Archaea to be completely sequenced in 1995 (Bult, C.J. et. al., 1996). Three characteristic elements that make up the genome of M. jannaschii are: a large circular chromosome, a large-circular extra chromosome, and a small circular extra-chromosome (Bult, C.J. et. al., 1996). Using whole-genome random sequencing, it was found that:
1. The large circular chromosome has a length of 1.66 mega base pair. Within the chromosome, there are a total of 1729 protein-coding regions, and the percentage of G+C content is 31.4%.
2. The large circular extra-chromosome has a length of 58 kilo base pair. The total protein-coding regions are 45, and the percentage of G+C content is reported as 28.2%.
3. The small circular extra-chromosome contains 16.5 kilo base pair. The predicted protein-coding regions are 12, and the G+C content percent is 28.9%.
The genome structure of Methanococcus jannaschii finally provided solid support for the hypothesis that Archaea is indeed a separate domain of life from Bacteria and Eukarya, and that Archaea is more closely related to Eukarya. This hypothesis was made several years before genome sequencing even existed (Tsoka et. al., 2003). Although the M. jannaschii genome is quite small compared to the genome of well-known species such as Eschericia coli (only 40% of E. coli genome), researchers believe that the genome is very specific to the autotropic lifestyle of the organism (Zhu et. al., 2004). Until now, only about 52% of the proteins in the genome have been identified and assigned to functions (Zhu et. al., 2004). However, their hyperthermophillic properties and their special mechanisms to their extreme environments leads to conclusion that these proteins are strictly involved in energy production, specifically methanogenesis, cell division, metabolism, and also maintaining the organism’s lifestyle (Zhu et. al., 2004).
Cell structure and metabolism
1. Features of cell structure of archaeal organisms.
Cellular structure and metabolism of M. jannaschii allow for a deeper understanding of the domain Archaea, and also the evolutionary relationship between this domain and the other two, Bacteria and Eukarya. M. jannaschii belongs to the domain Archaea, and therefore exhibit cellular structures that are characteristics of this domain. They are unique because they do not have a cell wall made of peptidoglycan (called murein) like most bacteria. Most archaeal species have walls made of proteins or glycoproteins, which form the outermost envelope of the cell called the surface layer (S-layer) (Sleytr. et. al., 2007). Because of the lack of a cell wall, this explains why Archaea are resistant to antibacterial antibiotics, which are often used as genetic markers (Tumbula and Whitman, 1999). More remarkably, they are different from all other organism due to the glycerol ethers that make up their cytoplasmic membrane, while other species’ membranes are composed of phospholipids (glycerol diesters) (Sleytr. et. al., 2007). Equally distinctive is the fact that no known Archaea have been identified as pathogens (Tumbula and Whitman, 1999).
2. Proteins that have been identified using shotgun proteomics method
Even though the genome of Methanococcus janaschii have been sequenced and the predicted protein functions have been assigned, the proteome, which provides the products of the actual expressed genome, will eventually give greater insights into studying the importance of these macromolecules in the organism’s lifestyle. Using a shotgun proteome method, which is called Multidimensional Protein Identification Technology (MudPIT), researchers were able to identify the actual amount of proteins, and also assigned them with specific functions (Zhu. et. al., 2004).
M. jannaschii protein mixtures were first digested in a more complicated protein mixtures by allowing M. jannaschii to grow on a complex medium enriched with H2. This mixture was further loaded on a “multidimensional chromatographic capillary column,” which is then separated and resolved. With this method, hundreds of proteins were identified at the same time (Zhu. et. al., 2004). This method of protein identification was extremely convenient for it provided a faster and more efficient way of searching for functional proteins without having to separate each protein ahead of time. Out of 1775 genes predicted from the sequenced genome, 963 (54.2%) were identified. These genes are identified to be involved in amino acid biosynthesis, biosynthesis of cofactors, prosthetic groups, and carriers, DNA metabolism, energy metabolism, synthesis of purines, pyridines, nucleosides, and nucleotides, fatty acid and phospholipids metabolism, transcription and other cellular processes. Due to the abundance of these proteins in the genome, the report concluded that these proteins are “housekeeping” proteins that are crucial in allowing cell maintenance and growth (Zhu. et. al., 2004).
Out of the 963 genes identified, 137 or 9% were proposed to be involved in energy metabolism. This group of proteins appears to have a high abundance within the proteome. These proteins include many enzymes used in the process of methanogenesis, and also many electron carrier proteins such as ferredoxin and subunits of proton transporting ATP synthase. The second large portion found in the genome includes genes that function in protein synthesis, specifically ribosomal proteins. 67% of the predicted genes are involved in the synthesis of cell envelope. The majority of proteins that are expressed by these genes are S-layer structural proteins. About 80% of the predicted genes are involved in fatty acids and phospholipids metabolism. Even though a large amount of genes have unknown functions or are hypothetical genes, it was reported that these genes are most likely crucial to cellular growth (Zhu et. al., 2004).
3. Methanogenesis pathway of Methanococcus jannaschii
As introduced earlier, methanogenesis is the main biochemical pathway from which Methanococcus jannaschii obtains energy. This pathway depends of a rich supply of H2 to reduce CO2 to CH4 (Teske et. al., 2003). There are seven steps involved in the pathway, and each step is catalyzed by a specific enzyme (Zhu et. al., 2004). Since these enzymes are synthesized by hyperthermophiles, they are called hyperthermophillic enzymes (Vieille and Zeikus, 2001). Most interestingly, these enzymes are optimally active at high temperatures, temperatures that would denature most enzymatic functions. In the first step of methanogenesis, carbon dioxide is introduced into the pathway, forming formyl-methanofuran (formyl-MF). This step is catalyzed by the enzyme formyl-methanofuran dehydrogenase (FMD), which is a membrane-bound protein complex. The second step of the pathway produces formyl-tetrahydromethanopterin by transferring the formyl group on the formyl-methanofuran. Formyl-MF:H4MPT formyl transferase (FTR) catalyzes this reaction. In the third step, formyl-H4MPT is reduced to methenyl-H4MPT by the removal of a water molecule. The enzyme involved in this step is H4MPT cyclohydrolase (MCH). The next step of the pathway involves two very functionally distinct enzymes that have been proven to be characteristic in the energy-producing metabolism of methanogens. They are coenzyme F420-dependent, N5, N10-methylene-H4MPT dehydrogenase (MTD), and the other is an H2-dependent, H2-forming N5-N10-methylene-H4MPT dehydrogenase (HMD). The product molecule in this pathway is methylene-H4MPT. N5, N10-methylene-H4MPT reductase (MER) is also a coenzyme F420-dependent, and in the fifth step, it catalyzes the production of methyl-H4MPT from methylene-H4MPT. Under rich hydrogen condition, this enzyme was shown to be highly expressed. In the sixth step, methyl S-coenzyme M is synthesized with the catalysis of the transmembrane enzyme complex, N5-methyl-H4MPT:Coenzyme M methyltransferase (MTR). This enzyme has proven to be extremely important in conserving energy for Methanococcus jannaschii. When MTR transfers a methyl group to conenzyme M, it also creates a proton gradient across the membrane by acting as a primary sodium pump. The Na+/H+ antiporter serves to aid the formation of methane in the next step. Methane is finally produced in the last step of the pathway by methyl conenzyme M reductase.
Ecology
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Application to Biotechnology
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Current Research
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References
Edited by Tina Nguyen Tran of Rachel Larsen and Kit Pogliano
