Fig. 3.

Proposed model for the origins and evolution of NifD, K, E, and N sequences. The NifDK-first hypothesis postulates: (A) Originally there were two linked proto-protein genes that evolved to NifDK in one lineage and to Bch/ChlNB and BchYZ in another lineage. (B) The NifDK was expanded to Group IV sequences. (C) One of the NifDK sequences in Group IV was tandemly duplicated to become NifDKEN. In this scheme, NifE was derived from NifD and NifN was derived from NifK. (D) NifDKEN was expanded into Groups I, II, and III, whereas Group IV, which contains only NifDK sequences but no NifEN, continue to evolve.