A thermophilic nitrate reductase is responsible for the strain specific anaerobic growth of Thermus thermophilus HB8.

T. thermophilus HB8 contains a nitrate reductase gene cluster which is absent from closely related strains. This cluster encodes 4 ORFs (a-d) similar in organization and protein sequence to those encoded by respiratory nitrate reductase operons (narGHJI) of Escherichia coli, Bacillus subtilis, Pseudomonas fluorescens, and Thiosphaera pantothropha. The highest similarity is shown between the proteins encoded by the ORFa, ORFb and ORFd, and the structural components of the mesophilic nitrate reductases NarG (alpha), NarH (beta), and NarI (gamma) proteins, whilst ORFc encodes a protein which showed lower similarity to NarJ, a protein of unknown function encoded between narH and narI genes in all the nar cluster so far sequenced. This T. thermophilus HB8 narGHJI cluster is strongly induced by the combined effect of nitrate and low oxygen concentration, giving rise to the synthesis of an enzyme whose optimal temperature and pH was determined to be 80 degrees C, and pH 10, respectively. We also demonstrate that insertional inactivation of the narG and narH genes of this cluster results in strictly aerobic mutants, showing its sole responsibility in the strain specific ability of T. thermophilus HB8 to grow anaerobically.