Dihydrogen oxidation is an important feature of bacterial energy conservation. In Escherichia coli hydrogen oxidation ('uptake') is catalysed by membrane-bound [NiFe] hydrogenase-1 and [NiFe] hydrogenase-2. The bulk of these uptake isoenzymes is exposed to the periplasm and biosynthesis of the proteins involves membrane transport via the twin-arginine translocation (Tat) pathway. Hydrogenase-2 is encoded by the hybOABCDEFG operon and the core enzyme is a heterodimer of HybO and HybC. HybO is synthesised with a twin-arginine signal peptide. HybOC is associated with two other proteins (HybA and HybB) that complete the respiratory complex. The HybOC dinner is bound to the cytoplasmic membrane and appears to be anchored via a hydrophobic transmembrane alpha-helix located at the C-terminus of HybO. Thus, hydrogenase-2 is an example of an integral membrane protein assembled in a Tat-dependent (Sec-independent) manner. Studies of the biosynthesis, targeting, and assembly of hydrogenase-2 would set a paradigm for all respiratory complexes of this type. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.