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The nitrogen regulatory (NtrC) protein of enteric bacteria, which binds to sites that have the properties of transcriptional enhancers, is known to activate transcription by a form of RNA polymerase that contains the NtrA protein (sigma 54) as sigma factor (referred to as sigma 54-holoenzyme). In the presence of adenosine triphosphate, the NtrC protein catalyzes isomerization of closed recognition complexes between sigma 54-holoenzyme and the glnA promoter to open complexes in which DNA in the region of the transcription start site is locally denatured. NtrC is not required subsequently for maintenance of open complexes or initiation of transcription.
Popham et al. (Fri,) studied this question.