Fig. 8: Schematic of the proposed role of CsrA focus in virulence regulation.

A CsrA switches from supporting to repressing T3SS expression. During the early exponential growth phase (left half of the illustrated bacterium), most bacteria either lack CsrA foci or have small foci. In these bacteria, the cytoplasmic CsrA levels are relatively low (represented by the light green shade), allowing Ler expression, which triggers a positive feedback loop leading to the transcription of the LEE operons (blue arrow). The transcripts of these operons are shown collectively in a blue transparent rectangle. In these bacteria, the CsrA levels are sufficient to support the translation of key T3SS proteins in the LEE4 transcript (Pink arrow), leading to T3SS assembly and virulence ON state. When reaching the late exponential growth phase (right half of the illustrated bacterium), the LEE1 and LEE7 mRNAs are localized in the expanding focus. In this compartment, the higher CsrA concentration acts to repress Ler production (pink blunt arrow). This breaks the positive feedback loop, thus leading to a decay in T3SS expression and diminishing infectivity. Created using BioRender. Rosenshine, I. (2025) https://BioRender.com/k60n247. B Model of CsrA focus composition. CsrA dimers bridged by CsrB form a network constituting the focus backbone. The CsrB function is complex, as in addition to its structural function and inhibition of CsrA, it surprisingly also collaborates with CsrA to achieve repression of ler, and possibly other genes. Additional mRNAs and regulatory sRNAs are distributed between the focus and cytoplasmic compartments, and localized to the different compartments based on their affinity to CsrA, number of CsrA binding sites, and abundance. Other RNAs might be recruited to the foci via base pairing with foci-associated RNAs. Some regulatory processes may be executed within the condensates, such as post-transcriptional regulation by CsrA or/and foci-associated sRNAs. Alternatively, these compartments may store or sequester molecules, or function as RNA and/or protein degradation centers. The decay rate and processing of focus-associated RNAs might be executed by the foci-associated RNase E, which is also anchored to the inner membrane. The association of RNase E with the foci might be driven by RNAs that bridge CsrA and RNase E. Created using BioRender. Rosenshine, I. (2025) https://BioRender.com/k60n247.