Iron-molybdenum cofactor biosynthesis in Azotobacter vinelandii requires the iron protein of nitrogenase

Nitrogenase is composed of two separately purified proteins called the Fe protein and the MoFe protein. In Azotobacter vinelandii the genes encoding these structural components are clustered and ordered: nifH (Fe protein)-nifD (MoFe protein alpha subunit)-nifK (MoFe protein beta subunit). The MoFe protein contains an ironmolybdenum cofactor (FeMo cofactor) whose biosynthesis involves the participation of at least five gene products, nifQ, nifB, nifN, nifE, and nifV. In this study an A. vinelandii mutant strain, which contains a defined deletion within the nifH (Fe protein) gene, was isolated and studied. This mutant is still able to accumulate significant amounts of MoFe protein subunits. However, extracts of this nifH deletion strain have only very low levels of MoFe protein acetylene reduction activity. Fully active MoFe protein can be reconstituted by simply adding isolated FeMo cofactor to the extracts. Fe protein is not necessary to stabilize or insert this preformed FeMo cofactor into the FeMo cofactor-deficient MoFe protein synthesized by the nifH deletion strain. Extracts of the nifH deletion strain can carry out molybdate and ATP-dependent in vitro FeMo cofactor biosynthesis provided Fe protein is added, demonstrating that they contain the products encoded by the FeMo cofactor biosynthetic genes. These data demonstrate that the Fe protein is physically required for the biosynthesis of FeMo cofactor in A. vinelandii.