Fig. 6.

Model for activation of hilD and prgH promoters by overlapping transcription. (A) A spurious transcription initiation event occurs at the edge of a patch of oligomerized H-NS (orange circles). Transcript elongation through bound H-NS is prevented by NusG-mediated recruitment of Rho factor (stop sign). (B) Occasionally, the transcript eludes Rho termination and progresses along the DNA dislodging H-NS in front of its path. This action opens a kinetic window during which RNA polymerase (green ovals) can bind to promoters that become exposed, including hilD secondary promoters (not shown) and the primary hilD promoter. (C) Activation of the hilD promoter leads to an increase in the levels of HilD protein (blue double ovals), which, upon binding to the hilD regulatory region, further stimulates hilD transcription and protein production. (D) This locks the system in a positive feedback loop: Accumulation of HilD leads to more hilD transcription and more HilD protein made. Through HilA (not shown) it also results in high-level transcription of the prgH gene. Divergent transcription further enhances the accessibility of additional spurious promoter sequences, further contributing to runaway transcription activation.