A Novel Ferredoxin-Dependent Glutamate Synthase from the Hydrogen-Oxidizing Chemoautotrophic Bacterium, Hydrogenobacter thermophilus TK-6

Department of Biotechnology, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

^* To whom correspondence should be addressed. Email: amishii{at}mail.ecc.u-tokyo.ac.jp.

	Abstract

Glutamate synthase is classified according to its specificity for the electron donor. Ferredoxin-dependent glutamate synthase had been found only in plants and cyanobacteria, whereas many bacteria have NADPH-dependent glutamate synthase. In this study, Hydrogenobacter thermophilus, a hydrogen-oxidizing chemoautotrophic bacterium, was shown to possess ferredoxin-dependent glutamate synthase like phototrophs. This is the first observation of a ferredoxin-dependent glutamate synthase in a non-photosynthetic organism to our knowledge. The purified enzyme from H. thermophilus was shown to be a monomer of a 168 kDa polypeptide homologous to ferredoxin-dependent glutamate synthases from phototrophs. In contrast to known ferredoxin-dependent glutamate synthases, the H. thermophilus glutamate synthase exhibited glutaminase activity. Furthermore, this glutamate synthase did not react with a plant-type ferredoxin (Fd3 from this bacterium) containing a [2Fe-2S] cluster but with bacterial-type ferredoxins (Fd1 and Fd2 from this bacterium) containing [4Fe-4S] clusters. Interestingly, the H. thermophilus glutamate synthase was activated by some of the organic acids in the reductive tricarboxylic acid cycle, the central carbon metabolic pathway of this organism. This type of activation has not been reported for any other glutamate synthases, and this property may enable the control of nitrogen assimilation by carbon metabolism.