Fig. S1.

Sequence alignment of NifB^Av, NifB^Ma, and NifB^Mt. All three sequences contain a signature CxxxCxxC domain (highlighted gray) that houses the SAM-binding [Fe₄S₄] cluster. Other conserved Cys and His residues (highlighted yellow) in the three sequences could accommodate an FeS precursor to the nitrogenase cofactor. The “NifB domain” toward the N terminus of the NifB^Av sequence is also present in the sequences of NifB^Ma and NifB^Mt, whereas the “NifX domain” toward the C terminus of the NifB^Av is notably missing from the sequences of both NifB^Ma and NifB^Mt.