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Biology and Biochemistry
Hydrogenobacter thermophilus TK-6 is a thermophilic, straight rod (bacillus) bacterium. The size is about .3-.5 microns in width and 2-3 microns in length. It is a Gram negative, non-motile, obligate chemolithoautotroph. H. thermophilus undergoes aerobic respiration using molecular oxygen as an electron acceptor, while oxidizing hydrogen. The electron donor is the molecular form of hydrogen. This bacterium utilizes a special form of the reductive tricyclic acid cycle to fix CO2. The optimum growth conditions are: temperature between 70 and 75°C, freshwater, pH around 7.2. The habitat is soil that contains hot water (70-75°C) from springs of the Izu Peninsula, Japan.

Genomics
In 2010, the entire genome of Hydrogenobacter thermophilus TK-6 was sequenced by Hiroyuki Arai et al. The method of sequencing was the Sanger dideoxy whole genome shotgun approach. It was found to consist of 1,743,135 base pairs arranged in a circular chromosome with an estimated 1,864 protein coding genes and 22 pseudogenes. The genome was found to contain one 16S-23S-5S rRNA operon and 44 tRNA coding genes without the presence of any plasmids. The GC content of the genome is 44%. It should also be noted that H. thermophilus lacks the typical PSP (phosphoserine phosphatase) genes. In addition, it is an obligate chemolithoautotroph, and so genes commonly used in carbon fixation were present. Genes that encode proteins involved in the RTCA (reductive tricyclic acid cycle) and gluconeogenesis were observed. The sox gene cluster, sqr gene and sorAB genes were also noted, and are involved in the sulfur oxidation protein complex, sulfide:quinone oxidoreductase and sulfite:cytochrome c oxidoreductase respectively. (1). H. thermophilus also contains the necessary genes for nitrate reduction and nitrate assimilation.

History
Hydrogenobacter thermophilus TK-6 was originally discovered by Toshiyuki Kawasumi at the Department of Agricultural Chemistry, University of Tokyo. TK-6 was found with four other previously unknown hydrogen oxidizing bacteria. Kawasumi published the original discovery paper in January of 1984. The bacterium was isolated from hot water containing soils samples from mines of the Izu Peninsula, Japan. The colonies were cultivated in an identical medium to the isolation medium. Prior to the discovery of Hydrogenobacter thermophilus, only one extremely thermophilic, aerobic and hydrogen-oxidizing bacterium had been described (Bacillus schegelii). In addition, H. thermophilus has both morphological and physiological differences that vary from processes in B. schegelii, suggesting multiple means for being viable in a difficult environment. Until the discovery of H. thermophilus TK-6, it was thought that no obligate chemolithotrophic hydrogen oxidizing bacteria existed.

proteomics
Hydrogenobacter thermophilus has several unique proteins that allow it to be viable in its environment. Cytochrome b and cytochrome c are present in all strains. H. thermophilus strains also possess a very distinctive sulfur containing quinone, 2-Methylthio-1,4-naphthoquinone. This is the first case of non-Calvin-type pathway that is utilized to convert carbon dioxide into cellular components. In addition to the unique quinone, novel types of phosphoserine phosphatase (PSPs) was discovered and has been analyzed by preliminary crystallization and X-ray diffraction. Both iPSP1 and iPSP2 proteins found in H. thermophilus employ metal-ion-independent pathways while typical PSPs need Mg2+ for activity and are considered to be part of the haloacid dehalogenase-like hydrolase superfamily. iPSP1 is comprised of two PspA subunits, while iPSP2 is a heterodimer and has both PspA and PspB subunits. iPSP1 and iPSP2 were observed to share a strong binding affinity towards L-phosphoserine, which supports its activity as a PSP. Novel proteins such as citryl-CoA synthetase (CCS) and ciitryl-CoA (CLL)are utilized within the reductive TCA cycle. The cleavage of citryl-CoA to acetyl-CoA and oxaloacetate occurs in a two step process. First, citryl-coA synthetase catalyzes the formation of citryl-CoA, which is immediately cleaved by citryl-CoA lyase. It was also observed that there is significant level of protein sequence homology between the CCL protein and the C-terminal region of ATP citrate lyase (ACL), an enzyme commonly employed by the reductive TCA cycle.