Sequence and domain relationships of ntrC and nifA from Klebsiella pneumoniae: homologies to other regulatory proteins.

We have determined the nucleotide sequences of two genes from Klebsiella pneumoniae, nifA, the nif-specific activator of transcription and ntrC, the bifunctional regulatory protein involved in 'nitrogen control'. These sequences differ significantly from those previously published. In particular, nifA extends 40 codons beyond the stop codon reported earlier. This extension encodes a putative DNA-binding domain strongly homologous to the Rhizobium meliloti nifA protein and to some extent to the ntrC protein. In all three proteins this domain is linked by a segment of variable length to a strongly conserved central domain of 240 residues. A short segment having the properties of an interdomain linker joins the central region to an N-terminal domain, which is weakly related in the case of the two nifA proteins. This homology is not shared by the N-terminal domain of ntrC, which is clearly but unexpectedly related to the N-terminal domains of a diverse set of procaryotic pleiotropic control proteins, including ompR, dye and nusA from Escherichia coli and spoOA and spoOF from Bacillus subtilis.