We report here on the formation of a complex between the two NrpR homologs present inMethanosarcina mazeiGo¨1 and their binding proper-ties to the nifHandglnK1 promoters. Reciprocal co-chromatography dem-onstrated that NrpRI forms stable complexes with NrpRII (at an NrpRI : NrpRII molar ratio of 1 : 3), which are not affected by 2-oxo-glutarate.