However, some experiments in T. thermophilus HB8 suggest NarK1 might also function in nitrite extrusion [39]. T. oshimai JL-2 and T. scotoductus SA-01 also contain homologs of NarK2 (annotated as nep in T. scotoductus SA-01 [41]), which likely encodes a nitrate/nitrite antiporter [44,48]. No significant BLASTP selleck chemical U0126 hits for periplasmic nitrate reductase subunits NapB and NapC were found in T. oshimai JL-2 and T. thermophilus JL-18, consistent with the use of the Nar system in the Thermales. Figure 6 Map showing the organization of nar operon and neighboring genes involved in denitrification located on the megaplasmids of T. oshimai JL-2 (pTHEOS01) and T. thermophilusJL-18 (pTTJL1801) and the chromosome of T. scotoductus SA-01. Fe: heme protein-containing …

All three strains contain a dnrST operon adjacent to, but divergently transcribed from, the narGHJIK operon. dnrST encodes transcriptional activators responsible for upregulation of the nitrate respiration pathway in the absence of O2 and the presence of nitrogen oxides or oxyanions [42] (Figure 6). Both the species contain a putative nirK, which encodes the NO-forming, Cu-containing nitrite reductase. In addition, T. oshimai JL-2 and T. scotoductus SA-01 both harbor nirS [41], which encodes the isofunctional tetraheme cytochrome cd1-containing nitrite reductase. Previous studies have suggested that bacteria use either NirK or NirS, but not both, for the reduction of nitrite [49]. The unique presence of NirK and NirS in T. oshimai JL-2 and T. scotoductus SA-01 likely enhances their denitrification abilities since isoenzymes are typically kinetically distinct and/or regulated differently.

This idea is consistent with the distinct denitrification phenotypes of T. oshimai strains as compared to T. thermophilus strains reported previously, including strains T. oshimai JL-2 and T. thermophilus JL-18 [6]. In those studies, nitrite accumulated in the medium at concentrations of <150 ��M in T. thermophilus strains, whereas it was rapidly produced to concentrations >200 ��M but consumed rapidly to below method detection limits in T. oshimai strains. NirK functions as a homo-trimer [50] and contains type 1 (blue) and type 2 (non-blue) copper-binding residues [49]. Comparison of the NirK from T. oshimai JL-2 and T.

scotoductus SA-01 with previously studied NirK amino acid sequences revealed that six of the seven copper-binding residues are conserved, except for a single methionine (M) to glutamine (Q) substitution in both Thermus proteins (Figure Batimastat 7; indicated by an asterisk (*)). Glutamine, not methionine, is the copper-binding ligand in the case of stellacyanin, a blue (type 1) copper-containing protein [52,53]. A M121Q recombinant protein of Alcaligenes denitrificans azurin showed similar electron paramagnetic resonance (EPR), but exhibited a 100-fold lower redox activity when compared to wild-type azurin [54].