FIGURE 8.
Schematic diagram to summarize regulatory network of PhoB-PhoR and Fur on T3SS and T6SS of E. tarda PPD130/91 through EsrC. The TCS PhoB-PhoR senses low Pi concentration for autoregulation and activation of pstSCAB-phoU through direct binding of a Pho box upstream of the promoters of phoB and pstS. PhoB binds simultaneously with EsrC on the promoter upstream of evpA to regulate transcription of genes from T6SS. PhoB requires and also interacts with PhoU (double line) to activate esrC and suppress fur indirectly through unidentified regulators (dotted line; through regulator “X”) in T3SS and T6SS through EsrC. EsrC binds promoters upstream of esaM and orf29 of T3SS and evpP and evpA of T6SS. Fur senses high iron concentration and binds directly to a Fur box upstream of evpP to inhibit binding of EsrC to the same promoter. Additionally, Fur suppresses transcription of phoB, pstSCAB-phoU, and esrC indirectly through unidentified regulators (dotted line; through regulator “Y”). Fur shows negative cross-talk with the Pho regulon and physically interacts (double line) with PhoU and EsrC; however, whether this interaction is an inhibitory mechanism remains to be determined.