Figure 2.

The P. aeruginosa ferripyoverdine CSS is regulated by FpvR/FpvI/PvdS. A. In the absence of ferripyoverdine, the anti‐sigma factor FpvR sequesters the sigma factors FpvI and PvdS, preventing them from binding to the core RNAP and DNA. The expression of both ECF sigma factors is also repressed by Fur (in a similar way to that shown for FecI in Fig. 1 and not shown here). In these conditions, the target regulons for each sigma factor are not transcribed. B. When pyoverdine binds to Fe³⁺ to form ferrypioverdine, a signal is transmitted to FpvA and TonB, enabling the import of ferripyoverdine and the proteolysis of FpvR by the RseP/MucP protease, liberating FpvI and PvdS and allowing expression of the two regulons.