Lateral Transfer of the Denitrification Pathway Genes among Thermus thermophilus Strains

Nitrate respiration is a common and strain-specific property in Thermus thermophilus encoded by the nitrate respiration conjugative element (NCE) that can be laterally transferred by conjugation. In contrast, nitrite respiration and further denitrification steps are restricted to a few isolates of this species. These later steps of the denitrification pathway are under the regulatory control of an NCE-encoded transcription factor, but nothing is known about their coding sequences or its putative genetic linkage to the NCE. In this study we examine the genetic linkage between nitrate and nitrite respiration through lateral gene transfer (LGT) assays and describe a cluster of genes encoding the nitrite-nitric oxide respiration in T. thermophilus PRQ25. We show that the whole denitrification pathway can be transferred from the denitrificant strain PRQ25 to an aerobic strain, HB27, and that the genes coding for nitrite and nitric oxide respiration are encoded near the NCE. Sequence data from the draft genome of PRQ25 confirmed these results and allowed us to describe the most compact nor-nir cluster known thus far and to demonstrate the expression and activities of the encoded enzymes in the HB27 denitrificant derivatives obtained by LGT. We conclude that this NCE nor-nir super-cluster constitutes a whole denitrification island that can be spread by lateral transfer among Thermus thermophilus strains.