NifK

NifK is the gene encoding the beta subunit of the MoFe protein, the catalytic component of nitrogenase responsible for reducing atmospheric nitrogen to ammonia. It is typically located in the nifHDK gene cluster, together with nifD, which encodes the alpha subunit, and nifH, which encodes the Fe protein.

The MoFe protein is a heterotetramer composed of two NifD and two NifK subunits. The NifK subunit contributes to the structure and function of the catalytic core and participates in binding of essential cofactors, including the FeMo-cofactor and the P-cluster, which are required for electron transfer and substrate reduction. Along with NifD, NifK forms the binding surface for electrons delivered by NifH and supports the chemical steps that reduce N2 to NH3.

Expression of nifK is co-regulated with nifD and nifH within nitrogen fixation gene clusters. In many bacteria, the nif regulon is controlled by NifA, a transcriptional activator, and, in some species, by NifL, which senses oxygen and redox status. Nitrogenase genes are typically induced under nitrogen-limiting conditions and low oxygen, and repressed when fixed nitrogen is available or under aerobic conditions.

NifK is widespread among diazotrophic bacteria and some archaea, including genera such as Klebsiella, Azotobacter, and Rhizobium, as well as cyanobacteria. The NifD/NifK pair forms the core nitrogenase MoFe protein, and mutations or disruptions in nifK disrupt MoFe protein assembly and nitrogenase activity, underscoring its essential role in biological nitrogen fixation.