SummaryA model for the domain structure of σ54‐dependent transcriptional activators, based on sequence data, has been tested by examining the function of truncated and chimaeric proteins. Removal of theN‐terminal domain of NtrC abolishes transcriptional activation, indicating that this domain is positively required for activator function. Over‐expression of this domain as a separate peptide appears to titrate out the phosphorylating activity of NtrB. Removal of theN‐terminal domain of NifA reduces activation 3–4‐fold. The residual activity is particularly sensitive to inhibition by NifL, suggesting that the role of theN‐terminal domain is to block the action of NifL in derepressing conditions. TheC‐terminal domain of NtrC showed repressor activity when expressed as a separate peptide. This domain is necessary for activator function even when NtrC binding sites are deleted from promoters. A point mutation in the ATP‐binding motif of the NtrC central domain, Ser 169 to Ala, also abolished activator function. Exchanging theN‐terminal domains ofKlebsiella pneumoniaeNtrC, NifA andEscherichia co/rOmpR, did not produce any hybrid activity, suggesting thatN‐terminal domains in the native proteins specifically recognize th