Nature, Published online: 08 January 2025; doi:10.1038/s41586-024-08311-1Cryo-electron microscopy of Azotobacter vinelandii FeSIInitrogenase reveals a core complex of molybdenumiron proteins (MoFePs), iron proteins (FePs) and FeSII, in which FeSII mediates interactions with MoFeP and FeP to position their FeS clusters in catalytically inactive but O2-protected states.