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Unique properties of hyperthermophilic archaea
This article discusses the Unique properties of hyperthermophilic archea. Hyperthermophiles are organisms that can live at temperatures ranging between 70 and 125 °C.〔Vieille, C. & Zeikus, G. J. "Hyperthermophilic Enzymes: Sources, Uses, and Molecular Mechanisms for Thermostability". ''Microbiol. Mol. Biol. Rev.'' 65, 1–43 (2001).〕 They have been the subject of intense study since their discovery in 1977 in the Galapagos Rift. It was thought impossible for life to exist at temperatures as great as 100 °C until ''Pyrolobus fumarii'' was discovered in 1997. ''P. fumarii'' is an unicellular organism from the domain Archaea living in the hydrothermal vents in black smokers along the Mid-Atlantic Ridge. These organisms can live at 106 °C at a pH of 5.5. In order to get energy from their environment these organisms are facultatively aerobic obligate chemolithoautotrophs, meaning these organisms build biomolecules by harvesting carbon dioxide (CO2) from their environment by using hydrogen (H2) as the primary electron donor and nitrate (NO3−) as the primary electron acceptor. These organisms can even survive the autoclave, which is a machine designed to kill organisms through high temperature and pressure. Because hyperthermophiles live in such hot environments, they need to have DNA, membrane and enzyme modifications in order to withstand the intense thermal energy. Such modifications are currently being studied to better understand what allows an organism or protein to survive such harsh conditions. By learning what allows these organisms to survive such harsh conditions, researchers will be better able to synthesize molecules that are harder to denature that can be used in industry.
==DNA structures of ''P. fumarii''==
Two DNA strands are held together by base pairing that allows the nucleotide bases adenosine (A) to bind with thymine (T), and guanine (G) to bind with cytosine (C). It has been proposed that thermophilic archaea would be expected to have higher GC content within their DNA, because GC pairings have three hydrogen bonds, while AT pairings have only two. Increasing the number of hydrogen bonds would increase the stability of the DNA, thereby increasing the energy required to separate the two strands of DNA. This would help the DNA to remain double stranded while at such high temperatures that would normally provide enough thermal energy to separate the DNA strands.
''P. fumarii'' was first sequenced in 2001 by the Diversa Corporation and the sequence was released to the public in 2014. The data from this analysis showed a GC content of 54.90%. This supports the hypothesis that thermophiles experience selective pressure to increase their GC content in order to stabilize their DNA. However, research has not conclusively supported this hypothesis. A study done by Hurst and Merchant (2001) showed no correlation between higher GC content in prokaryotes and increased optimal growing temperatures. However, their analysis did show that there was higher GC content for the third nucleic acid within the codon. This demonstrates that within the wobble position there is likely a selective pressure for more hydrogen bonds to increase stability within the DNA, but less selective pressure for GC pairings within the DNA as a whole. This supports what is seen in ''P. fumarii''. The majority of the DNA is composed of G and C nucleotides, but the DNA still contains many A and T nucleotides. These results likely indicate that along with increasing GC pairing in the wobble position, thermophilic archaea have other mechanisms for stabilizing their DNA at such high temperatures.
One possible mechanism for stabilizing DNA at such high temperatures are proteins such as a type I topoisomerase that supertwists the DNA making spontaneously untwisting of the DNA more difficult. The presence of this protein in multiple evolutionarily distant organisms supports the hypothesis that this protein plays a role in DNA stabilization.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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