nosX is essential for whole-cell N2O reduction in Paracoccus denitrificans but not for assembly of copper centres of nitrous oxide reductase

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas that is produced naturally as an intermediate during the process of denitrification carried out by some soil bacteria. It is consumed by nitrous oxide reductase (N2OR), the terminal enzyme of the denitrification pathway, which catalyses a reduction reaction to generate dinitrogen. N2OR contains two important copper cofactors (CuAand CuZcentres) that are essential for activity, and in copper-limited environments, N2OR fails to function, contributing to rising levels of atmospheric N2O and a major environmental challenge. Here we report studies ofnosX, one of eight genes in thenoscluster of the soil dwelling α-proteobateriumParaccocus denitrificans. AP. denitrificansΔnosXdeletion mutant failed to reduce N2O under both copper-sufficient and copper-limited conditions, demonstrating that NosX plays an essential role in N2OR activity. N2OR isolated fromnosX-deficient cells was found to be unaffected in terms of the assembly of its copper cofactors, and to be active inin vitroassays, indicating that NosX is not required for the maturation of the enzyme; in particular, it plays no part in the assembly of either of the CuAand CuZcentres. Furthermore, quantitative Reverse Transcription PCR (qRT-PCR) studies showed that NosX does not significantly affect the expression of the N2OR-encodingnosZgene. NosX is a homologue of the FAD-binding protein ApbE fromPseudomonas stutzeri, which functions in the flavinylation of another N2OR accessory protein, NosR. Thus, it is likely that NosX is a system-specific maturation factor of NosR, and so is indirectly involved in maintaining the reaction cycle of N2OR and cellular N2O reduction.