Sequence of nifL from Klebsiella pneumoniae: mode of action and relationship to two families of regulatory proteins.

We present the nucleotide sequence of K. pneumoniae nifL, which negatively regulates nif transcription in response to oxygen and fixed nitrogen. It shows partial sequence homology to the general nitrogen regulatory proteins NtrB of K. pneumoniae and Bradyrhizobium parasponiae. This homology is weaker than that shown between the NifA and NtrC activator components of the nif and general nitrogen control systems. The N-terminal section of the NifL protein includes a structural duplication sharing sequence homology with part of NtrB, and a region containing a cysteine pair which might be implicated in redox control Unlike NtrB, NifL appears to lack a DNA-binding motif, consistent with evidence that NifL represses by interacting directly with NifA. The C-terminal section of NifL shows clear homology to NtrB and to a family of proteins involved in transcriptional control or chemotaxis, each of which probably interacts with a member of the family of regulatory proteins showing homology to NtrC.