Fig. 1.

Regulation of the SPI-1 T3SS. In the “off” state, H-NS silences the hilD promoter region, and there is not enough HilD activity to overcome this repression. Rho, working through NusG, is blocking spurious transcripts initiated outside this region by RNA polymerase (RNAP). If these transcripts escape Rho action, H-NS is dislodged from the DNA by the transcription complex. This lowers the threshold of HilD activity required to autoactivate, leading to the “on” state.