The nitrogenase system consists of two components, an iron (Fe-) protein and a molybdenum-iron (MoFe-) protein, which reduce dinitrogen in an ATP dependent reaction. The fundamental role of ATP and the Fe-protein in the mechanism of nitrogenase will be examined by X-ray diffraction studies of recently crystallized Fe-protein from Azotobacter vinelandii. The main areas to be investigated are: 1) the nature and environment of metal centers in Fe-protein; 2) the molecular symmetry of Fe-protein; 3) location of the ATP binding site, and the possible structural consequences of ATP binding; 4) distribution of conserved and nonconserved residues in the three-dimensional structure, and the implications for Fe-protein function. The Fe-protein structure will provide a framework for correlating an extensive body of spectroscopic, kinetic and chemical studies on the nitrogenase system.