MoBio Activation of glnA Transcription by NTRC Chapter 4

Unlike the lac operon, the enhancer of the glnA gene is a little far from the promoter so that the bound activator does not immediately contact the polymerase. Nitrogen Regulatory Protein C (NTRC) can induce DNA loopingto bring the activator in contact with the polymerase.

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Figure 4-D-5. The mechanism of transcriptional activation by NTRC.
(a) The glnA gene is transcribed by the Sigma-54-containing polymerase which alone cannot initiate transcription. The unphosphorylated NTRC dimers can bind only one site at the enhancer, still insufficient to stimulate transcription.
(b) The phosphorylated NTRC dimers can bind both sites of the enhancer.
(c) Their binding induces DNA looping. Contact between the activator and the polymerase stabilizes the interaction between the polymerase and DNA, thereby initiating transcription.