J. Biochem, 1994, Vol. 115, No. 2 309-317
© 1994 Japanese Biochemical Society
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Aspartate Aminotransferase from a Thermophilic Formate-Utilizing Methanogen, Methanobacterium thermoformicicum Strain SF-4: Relation to Serine and Phosphoserine Aminotransferases, but Not to the Aspartate Aminotransferase Family
*Department of Biology, Faculty of Science, Osaka City University Suniyoshi-ku, Osaka 558
**Department of Biochemistry, Osaka Medical College Takatsuki, Osaka 569
1To whom correspondence should be addressed.
The primary structure of the aspartate aminotransferase (AspAT) of an archaebacterium, Methanobacterium thermoformicicum strain SF-4, has been determined by cloning and sequencing of the gene for the enzyme. The gene had a consensus promoter and a ribosome binding sequence of methanogens in the 5' untranslated region, followed by an open reading frame starting with ATG and terminating with TGA. The deduced amino acid sequence was identical with the partial amino acid sequences of the enzyme including the N-terminal sequence, and the deduced molecular weight of 41, 684 was virtually identical to that reported earlier for this enzyme [Tanaka, T., Yamamoto, S., Taniguchi, M., Hayashi, H., Kuramitsu, S., Kagamiyama, H., & Oi, S. (1992) J. Biochem. 112, 811–815]. The gene was expressed in Escherichia coli by inserting it into an expression vector just downstream of the lacZ promoter, and this verified that the cloned gene really encodes the Methanobacterium AspAT. The primary structure of the Methanobacterium AspAT showed extremely low homology, 5%, with AspATs of eubacteria, eukaryotes, and a thermoacidophilic archaebacterium, Sulfolobus solfataricus. On the other hand, the Methanobacterium AspAT showed remarkable amino acid sequence homology, 31.5%, with rat serine: pyruvate aminotransferase and, 13.5%, with E. coli phosphoserine aminotransferase. Thus, the Methanobacterium AspAT apparently belongs to subgroup IV of the aminotransferases [Mehta, P.K., Hale, T.I., & Christen, P. (1993) Eur. J. Biochem. 214, 549–561], but not to subgroup I, in which all the AspATs known so far are included.
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