Constitutive forms of the enhancer-binding protein NtrC: evidence that essential oligomerization determinants lie in the central activation domain

Nitrogen regulatory protein C(NtrC) is a bacterial enhancer-binding protein that activates transcription by the σ 54 -holoenzyme. To activate transcription, NtrC must hydrolyze ATP, a reaction that depends upon its being phosphorylated and forming an appropriate oligomer, In this paper we characterize "constitutive" mutant forms of the NtrC protein from Salmonella typhimurium ; unlike wild-type NtrC, these forms are able to hydrolyze ATP and activate transcription in vitro without being phosphorylated. The amino acids altered in NtrC constitutive proteins are located in both the N-terminal regulatory domain and the central domain, which is directly responsible for transcriptional activation. The residues that are altered are not conserved among activators of the σ 54 -holoenzyme, and are not identical even among NtrC proteins from members of different subgroups of the proteobacteria (purple bacteria). NtrC constitutive proteins are phosphorylated normally; phosphorylation increases their ability to hydrolyze ATP and activate transcription. Moreover, the oligomerization of these proteins that occurs when they bind to an enhancer also increases the ATPase activity of both unmodified and phosphorylated forms. Removal of the N-terminal regulatory domain from two NtrC constitutive proteins with amino acid substitutions in the central domain (NtrC S160F and NtrC V288I ) leaves them active, indicating that essential oligomerization determinants lie outside the regulatory domain. This conclusion is confirmed by the observation that the ATPase activity of ΔN-NtrC S160F is greatly stimulated when it binds to an enhancer, and by the ability of this protein to activate transcription synergistically with a form of NtrC incapable of DNA-binding. Together with previous results indicating that oligomerization determinants do not lie in the C-terminal DNA-binding domain of NtrC; these results provide evidence that they lie in the central domain. f2