Effect of Copper on Expression of Functional Genes and Proteins Associated with Denitrification.

Nitrous oxide (NO) is a powerful greenhouse gas that contributes to climate change. Denitrification is one of the largest sources of NO in soils. The soybean endosymbiont is a model for rhizobial denitrification studies since, in addition to fixing N, it has the ability to grow anaerobically under free-living conditions by reducing nitrate from the medium through the complete denitrification pathway. This bacterium contains a periplasmic nitrate reductase (Nap), a copper (Cu)-containing nitrite reductase (NirK), a -type nitric oxide reductase (cNor), and a Cu-dependent nitrous oxide reductase (Nos) encoded by the and genes, respectively. In this work, an integrated study of the role of Cu in denitrification has been performed. A notable reduction in , and gene expression observed under Cu limitation was correlated with a significant decrease in NirK, NorC and NosZ protein levels and activities. Meanwhile, expression was not affected by Cu, but a remarkable depletion in Nap activity was found, presumably due to an inhibitory effect of nitrite accumulated under Cu-limiting conditions. Interestingly, a post-transcriptional regulation by increasing Nap and NirK activities, as well as NorC and NosZ protein levels, was observed in response to high Cu. Our results demonstrate, for the first time, the role of Cu in transcriptional and post-transcriptional control of denitrification. Thus, this study will contribute by proposing useful strategies for reducing NO emissions from agricultural soils.