Figure 16.

(A) NifB domain architectures and conserved amino acid motifs. The simplest NifB proteins consist of a stand-alone SAM radical motif. More complex NifB proteins include a C-terminal NifX-like, or N-terminal NifN-like domain and C-terminal NifX-like domain. (B) Structure of the NifB homologue MoaA showing the [4Fe-4S]-SAM active site and its ligating Cys residues. Images created with NGL viewer³⁶ and RCSB PDB. No NifB crystal structure is yet available. (C) Structural model, generated with SWISS-MODEL,⁸⁷ of a 43 amino acid fragment of the A. vinelandii NifB sequence including the AdoMet motif Cx₃Cx₂C.