Armas, A.;Escudero, V.;Quintana, J.;Rodriguez-Simon, M.;Abreu, I.;Collantes-Garcia, J.;Gupta, B.;Ansorena, E.;Raimunda, D.;Rubio, L.;Gonzalez-Guerrero, M.

Summary Engineering nitrogen fixing crops requires not only transferring the nitrogenase structural genes, but also the accessory genes to synthesize its iron-sulphur cofactors. Scaffold protein NifU is a critical element in this system as the starting point of nitrogenase cofactor assembly. NifU has been successfully produced in plants, however, its optimal production required high levels of iron in the medium. This is likely due to a faulty connection with the endogenous iron trafficking network To identify specific elements targeting iron to NifU, pull-down assays were performed to identify showing bacterioferritin A (BfrA) as a likely candidate. Co-immunopurification, mutant characterization, iron transfer assays, and co-expression in Nicotiana benthamiana assays were carried out. BfrA transfers iron to NifU through protein-protein interactions. When these two proteins were co-expressed in N. benthamiana leaves, there was an increase in NifU production. In turn, it led to doubling NifH synthesis, a nitrogenase structural protein that is also required for the synthesis of the more complex nitrogenase cofactors. Our results provide a new element towards engineering nitrogen-fixing crops. They also underscore the importance of transferring the metal delivery systems when expressing metalloproteins in heterologous systems.