Cava Felipe, Zafra Olga, da Costa Milton S, Berenguer José
Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid-C.S.I.C., 28049 Madrid, Spain.
Environ Microbiol. 2008 Feb;10(2):522-33. doi: 10.1111/j.1462-2920.2007.01472.x.
The nitrate conjugative element (NCE) encodes the ability to respire nitrate in the facultative Thermus thermophilus NAR1 strain. This process is carried out by two heterotetrameric enzymes that catalyse the oxidation of NADH (Nrc) and the reduction of nitrate (Nar), whose expression is activated by the NCE-encoded transcription factors DnrS and DnrT. We report the presence of NCE in other facultative strains of T. thermophilus and analyse its role in subsequent steps of the denitrification pathway. We encountered that nrc mutants of denitrifying strains show a decrease in anaerobic growth rates not only with nitrate, but also with nitrite, NO and N(2)O, which is concomitant to their lower NADH oxidation activities in vitro. We show that nitrate, nitrite and NO are activating signals for transcription of nrc in these strains. Finally, we demonstrate that DnrS and DnrT are required for anaerobic growth not only with nitrate, but also with nitrite, NO and N(2)O. These data allow us to conclude that: (i) Nrc constitutes the main electron donor for the four reductases of the denitrification pathway, and (ii) the NCE controls the expression of the whole denitrification pathway and the repression of the aerobic respiration through the transcription factors DnrS and DnrT.
硝酸盐接合元件（NCE）编码嗜热栖热菌NAR1兼性菌株中利用硝酸盐进行呼吸的能力。这个过程由两种异源四聚体酶完成，它们分别催化NADH的氧化（Nrc）和硝酸盐的还原（Nar），其表达由NCE编码的转录因子DnrS和DnrT激活。我们报道了NCE在其他嗜热栖热菌兼性菌株中的存在，并分析了其在反硝化途径后续步骤中的作用。我们发现，反硝化菌株的nrc突变体不仅在以硝酸盐为底物时厌氧生长速率降低，在以亚硝酸盐、NO和N₂O为底物时也降低，这与它们在体外较低的NADH氧化活性相一致。我们表明，硝酸盐、亚硝酸盐和NO是这些菌株中nrc转录的激活信号。最后，我们证明DnrS和DnrT不仅是硝酸盐，也是亚硝酸盐、NO和N₂O厌氧生长所必需的。这些数据使我们得出以下结论：（i）Nrc构成反硝化途径四种还原酶的主要电子供体，（ii）NCE通过转录因子DnrS和DnrT控制整个反硝化途径的表达以及有氧呼吸的抑制。
