Figure 7.

Proposed model for the hierarchical control of fimbrial expression in S. typhimurium mediated by CsrA. The exceptionally high transcript levels of the fimAICDHF 5′-UTR can antagonize the regulatory effects of CsrA by binding and sequestering this RNA binding protein through a mechanism similar to that proposed for CsrB RNA and CsrC RNA. Via this mechanism, the fimAICDHF mRNA synergizes with CsrB RNA and CsrC RNA in antagonizing the activity of CsrA, a regulatory protein that activates expression of plasmid-encoded fimbriae by binding a GGA motif in the 5′-UTR of pefACDEF transcript. This mechanism is ultimately responsible for the hierarchical control of type I fimbriae and plasmid-encoded fimbriae in S. typhimurium.