The Rhizobium meliloti nifA product (NifA) shares extensive homology in its central region and at its C-terminal end with Rhizobium leguminosarum DctD and with NtrC from several species. All three proteins are transcriptional activators of NtrA (RpoN)-RNA polymerase-dependent promoters. Several large deletions of R. meliloti NifA were constructed to investigate the role of the conserved and divergent domains of NifA in transcriptional activity and posttranscriptional regulation by oxygen. The ability of NifA expressed from the Escherichia coli lacZ promoter to activate the R. meliloti nifH promoter in E. coli and R. meliloti was tested under a range of defined atmospheric oxygen partial pressures. Deletion of the divergent N-terminal domain of NifA had little effect on NifA activity and no effect on oxygen sensitivity. Deletion of the conserved C-terminal helix-turn-helix motif of NifA did not eliminate NifA-dependent activation of the nifH promoter, although it did decrease NifA activity about twofold in E. coli and 10-fold in R. meliloti. A NifA carrying both the N-terminal and C-terminal deletions and consisting of only the central highly conserved domain and 50 divergent amino acids retained the ability to activate transcription from the nifH promoter. The transcriptional activity of the conserved central domain is consistent with the prediction that the core domain is the part of NifA which interacts with the transcriptional machinery to stimulate transcription.