Figure 5.

Mechanism of action of NtrC-dependent enhancer during multiple-round transcription of the glnAp2 promoter. (1) Before transcription of NtrC-dependent genes is induced, the Eσ⁵⁴ holoenzyme forms a closed complex (RP_c) at the promoter (localized at the –24 to –12 DNA region) but cannot initiate transcription. NtrC is bound to the enhancer (two 17 bp NtrC-binding sites indicated by open boxes) but cannot communicate with the promoter. After induction and phosphorylation by NtrB, NtrC forms homooligomers and interacts with the holoenzyme causing looping out of the intervening DNA (not shown) and ATP-dependent formation of the open complex (RP_o) at the promoter (2). After formation of RP_o, enhancer–promoter interaction is broken and the DNA loop is opened. As the RNA polymerase leaves the promoter (3), the σ⁵⁴ subunit dissociates into solution and the holoenzyme has to re-bind to the promoter and re-establish the interaction with the enhancer for the next round of transcription to occur.