Transferable Denitrification Capability of Thermus thermophilus

Abstract

Laboratory-adapted strains of Thermus spp. have been shown to require oxygen for growth, including the model strains T. thermophilus HB27 and HB8. In contrast, many isolates of this species that have not been intensively grown under laboratory conditions keep the capability to grow anaerobically with one or more electron acceptors. The use of nitrogen oxides, especially nitrate, as electron acceptors is one of the most widespread capabilities among these facultative strains. In this process, nitrate is reduced to nitrite by a reductase (Nar) that also functions as electron transporter toward nitrite and nitric oxide reductases when nitrate is scarce, effectively replacing respiratory complex III. In many T. thermophilus denitrificant strains, most electrons for Nar are provided by a new class of NADH dehydrogenase (Nrc). The ability to reduce nitrite to NO and subsequently to N 2 O by the corresponding Nir and Nor reductases is also strain specific. The genes encoding the capabilities for nitrate (nar) and nitrite (nir and nor) respiration are easily transferred between T. thermophilus strains by natural competence or by a conjugation-like process and may be easily lost upon continuous growth under aerobic conditions. The reason for this instability is apparently related to the fact that these metabolic capabilities are encoded in gene cluster islands, which are delimited by insertion sequences and integrated within highly variable regions of easily transferable extrachromosomal elements. Together with the chromosomal genes, these plasmid-associated genetic islands constitute the extended pangenome of T. thermophilus that provides this species with an enhanced capability to adapt to changing environments.

Parallel Pathways for Nitrite Reduction during Anaerobic Growth in Thermus thermophilus

“…The three genes are conserved in all the Thermus spp. sequenced so far that contain a denitrification cluster, with the exception of NirM, which is not present in the denitrification cluster of Thermus scotoductus SA-01 (11).…”

“…The NCE encodes a complete set of genes for nitrate respiration, including a nitrate reductase (Nar), a dedicated NADH dehydrogenase (Nrc) (14,15), nitrate and nitrite transporters (NarK and NarT), and signal transduction systems required for expression under appropriate conditions, which are unrelated to those found in proteobacteria or Gram-positive organisms (DnrS and DnrT) (reviewed in reference 11). A major difference between the thermophilic Nar and those of other bacteria is the presence of a periplasmic cytochrome c extra subunit required for the maturation of the enzyme (11,16). This extra subunit has been proposed to be involved in electron transfer to the nitrite and nitric oxide reductases in CD strains when nitrate is scarce in the medium (17).…”

“…The nic cluster can be laterally transferred only to PD strains, because the expression of its genes is dependent on the presence of the NCE in the recipient strain (13). Actually, in CD strains, the NCE and the nic clusters are located close to each other in a region of the pTT27 megaplasmid surrounded by insertion sequences, which putatively could facilitate its dissemination as a mobile denitrification genetic island among T. thermophilus strains (11,13).…”

“…Whereas nitrate respiration in T. thermophilus has been extensively studied (11,18,19), few data exist on nitrite respiration in this species. In this work, we analyze the role in nitrite respiration of three genes of the nic cluster (nirS, nirJ, and nirM) that encode homologues of proteins involved in nitrite respiration in other bacteria (13).…”

“…This ancient bacterial species (9) includes strict aerobic strains, such as T. thermophilus HB27, and facultative anaerobes that require nitrogen oxides as electron acceptors (10). Among the latter, there is also strain variability, and some of these strains can grow anaerobically with only nitrate with final production of nitrite (partially denitrificant [PD] strains), whereas other strains can also respire nitrite with production of nitrogen gas (completely denitrificant [CD] strains) (11). Nitrate respiration is encoded by the nitrate conjugative element (NCE), a genetic element that can be transferred from PD or CD strains to aerobic strain HB27 (12,13).…”