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Journal of Bacteriology, May 2000, p. 2716-2724, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

The H2 Sensor of Ralstonia eutropha Is a Member of the Subclass of Regulatory [NiFe] Hydrogenases

Laura Kleihues,dagger Oliver Lenz, Michael Bernhard, Thorsten Buhrke, and Bärbel Friedrich*

Institut für Biologie, Humboldt-Universität zu Berlin, 10115 Berlin, Germany

Received 25 October 1999/Accepted 21 February 2000

Two energy-generating hydrogenases enable the aerobic hydrogen bacterium Ralstonia eutropha (formerly Alcaligenes eutrophus) to use molecular hydrogen as the sole energy source. The complex synthesis of the nickel-iron-containing enzymes has to be efficiently regulated in response to H2, which is available in low amounts in aerobic environments. H2 sensing in R. eutropha is achieved by a hydrogenase-like protein which controls the hydrogenase gene expression in concert with a two-component regulatory system. In this study we show that the H2 sensor of R. eutropha is a cytoplasmic protein. Although capable of H2 oxidation with redox dyes as electron acceptors, the protein did not support lithoautotrophic growth in the absence of the energy-generating hydrogenases. A specifically designed overexpression system for R. eutropha provided the basis for identifying the H2 sensor as a nickel-containing regulatory protein. The data support previous results which showed that the sensor has an active site similar to that of prototypic [NiFe] hydrogenases (A. J. Pierik, M. Schmelz, O. Lenz, B. Friedrich, and S. P. J. Albracht, FEBS Lett. 438:231-235, 1998). It is demonstrated that in addition to the enzymatic activity the regulatory function of the H2 sensor is nickel dependent. The results suggest that H2 sensing requires an active [NiFe] hydrogenase, leaving the question open whether only H2 binding or subsequent H2 oxidation and electron transfer processes are necessary for signaling. The regulatory role of the H2-sensing hydrogenase of R. eutropha, which has also been investigated in other hydrogen-oxidizing bacteria, is intimately correlated with a set of typical structural features. Thus, the family of H2 sensors represents a novel subclass of [NiFe] hydrogenases denoted as the "regulatory hydrogenases."

* Corresponding author. Mailing address: Institut für Biologie, Humboldt-Universität zu Berlin, Chausseestr. 117, 10115 Berlin, Germany. Phone: 49-30-2093-8100. Fax: 49-30-2093-8102. E-mail: baerbel.friedrich{at}rz.hu-berlin.de.

dagger Present address: Max-Planck-Institut für molekulare Genetik, 14195 Berlin, Germany.

Journal of Bacteriology, May 2000, p. 2716-2724, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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