Rosa-Nunez, E. | bioRxiv Science

bioRxiv2025

Azotobacter vinelandii glutaredoxin D delivers the core cluster to nitrogenase cofactor scaffold protein NifU

The scaffold protein NifU plays a central role in assembling the precursor [Fe4S4] clusters required for nitrogenase to function. The synthesis of these precursors depends on a catalytic [Fe2S2] group within NifU core ferredoxin domain. Here, we show that the monothiol glutaredoxin GrxD delivers this cluster to the NifU scaffold protein. Consistently, grxD mutants have reduced nitrogenase activity, the result of altered iron allocation to this enzyme. Biochemical assays show that GrxD unidirectionally transfers [Fe2S2] to NifU through protein-protein interaction. This allows GrxD to restore apo-NifU functionality, enabling proper [Fe4S4] synthesis, and NifH activation. These findings are crucial to understand how iron is allocated to nitrogenase for biological nitrogen fixation.

biochemistry↗

bioRxiv2019

Medicago truncatula Ferroportin2 mediates iron import into nodule symbiosomes

Iron is an essential cofactor for symbiotic nitrogen fixation. It is required by many of the enzymes facilitating the conversion of N2 into NH4+ by endosymbiotic bacteria living within root nodule cells, including signal transduction proteins, O2 homeostasis systems, and nitrogenase itself. Consequently, host plants have developed a transport network to deliver essential iron to nitrogen-fixing nodule cells. Model legume Medicago truncatula Ferroportin2 (MtFPN2) is a nodule-specific gene that encodes an iron-efflux protein. MtFPN2 is located in intracellular membranes in the nodule vasculature, and in the symbiosome membranes that contain the nitrogen-fixing bacteria in the differentiation and early-fixation zones of the nodules. Loss-of-function of MtFPN2 leads to altered iron distribution and speciation in nodules, which causes a reduction in nitrogenase activity and in biomass production. Using promoters with different tissular activity to drive MtFPN2 expression in MtFPN2 mutants, we determined that MtFPN2-facilitated iron delivery across symbiosomes is essential for symbiotic nitrogen fixation, while its presence in the vasculature does not seem to play a major role in in the conditions tested.

plant biology↗