Sequence analysis of subunits of the membrane-bound nitrate reductase from a denitrifying bacterium: the integral membrane subunit provides a prototype for the dihaem electron-carrying arm of a redox loop.

Three genes, narH, narJ and narI, of the membrane-bound nitrate reductase operon of the denitrifying bacterium Thiosphaera pantotropha have been identified and sequenced. The derived gene products show high sequence similarity to the equivalent (beta, putative delta and gamma) subunits of the two membrane-bound nitrate reductases of the enteric bacterium Escherichia coli. All iron-sulphur cluster ligands proposed for the E. coli beta subunits are conserved in T. pantotropha NarH. Secondary structure analysis of NarJ suggests that this protein has a predominantly alpha-helical structure. Comparison of T. pantotropha NarI with the b-haem-binding integral membrane subunits of the E. coli enzymes allows assignment of His-53, His-63, His-186 and His-204 (T. pantotropha NarI numbering) as b-haem axial ligands and the construction of a three-dimensional model of this subunit. This model, in which the two b-haems are in different halves of the membrane bilayer, is consistent with a mechanism of energy conservation whereby electrons are moved from the periplasmic to the cytoplasmic side of the membrane via the haems. Similar movement of electrons is required in the membrane-bound uptake hydrogenases and membrane-bound formate dehydrogenases. We have identified two pairs of conserved histidine residues in the integral membrane subunits of these enzymes that are appropriately positioned to bind one haem towards each side of the membrane bilayer. One subunit of a hydrogenase complex involved in transfer of electrons across the cytoplasmic membrane of sulphate-reducing bacteria has structural resemblance to NarI.