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.
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.