A hydrogen-sensing multiprotein complex controls aerobic hydrogen metabolism in Ralstonia eutropha.

H(2) is an attractive energy source for many microorganisms and is mostly consumed before it enters oxic habitats. Thus aerobic H(2)-oxidizing organisms receive H(2) only occasionally and in limited amounts. Metabolic adaptation requires a robust oxygen-tolerant hydrogenase enzyme system and special regulatory devices that enable the organism to respond rapidly to a changing supply of H(2). The proteobacterium Ralstonia eutropha strain H16 that harbours three [NiFe] hydrogenases perfectly meets these demands. The unusual biochemical and structural properties of the hydrogenases are described, including the strategies that confer O(2) tolerance to the NAD-reducing soluble hydrogenase and the H(2)-sensing regulatory hydrogenase. The regulatory hydrogenase that forms a complex with a histidine protein kinase recognizes H(2) in the environment and transmits the signal to a response regulator, which in turn controls transcription of the hydrogenase genes.