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Applied and Environmental Microbiology, February 2004, p. 722-728, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.722-728.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Cyanobacterial-Type, Heteropentameric, NAD⁺-Reducing NiFe Hydrogenase in the Purple Sulfur Photosynthetic Bacterium Thiocapsa roseopersicina

Institute of Biophysics, Biological Research Center, Hungarian Academy of Sciences, and Department of Biotechnology, University of Szeged, H-6726 Szeged, Hungary

Received 7 August 2003/ Accepted 4 November 2003

Structural genes coding for two membrane-associated NiFe hydrogenases in the phototrophic purple sulfur bacterium Thiocapsa roseopersicina (hupSL and hynSL) have recently been isolated and characterized. Deletion of both hydrogenase structural genes did not eliminate hydrogenase activity in the cells, and considerable hydrogenase activity was detected in the soluble fraction. The enzyme responsible for this activity was partially purified, and the gene cluster coding for a cytoplasmic, NAD⁺-reducing NiFe hydrogenase was identified and sequenced. The deduced gene products exhibited the highest similarity to the corresponding subunits of the cyanobacterial bidirectional soluble hydrogenases (HoxEFUYH). The five genes were localized on a single transcript according to reverse transcription-PCR experiments. A ⁵⁴-type promoter preceded the gene cluster, suggesting that there was inducible expression of the operon. The Hox hydrogenase was proven to function as a truly bidirectional hydrogenase; it produced H₂ under nitrogenase-repressed conditions, and it recycled the hydrogen produced by the nitrogenase in cells fixing N₂. In-frame deletion of the hoxE gene eliminated hydrogen evolution derived from the Hox enzyme in vivo, although it had no effect on the hydrogenase activity in vitro. This suggests that HoxE has a hydrogenase-related role; it likely participates in the electron transfer processes. This is the first example of the presence of a cyanobacterial-type, NAD⁺-reducing hydrogenase in a phototrophic bacterium that is not a cyanobacterium. The potential physiological implications are discussed.

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