Figure 4.

Model for σ^E-dependent regulation of the SsrB regulon. Membrane-targeting host defences including reactive oxygen stress and antimicrobial peptides cause an accumulation of unfolded outer membrane proteins (OMPs) and stimulate the cleavage of the anti-sigma factor RseA consequently releasing σ^E into the cytoplasm where it directs RNA polymerase to a subset of SPI-2 promoters. RpoE can positively or negatively regulate SsrB-regulated genes including integrated virulence genes unlinked with SPI-2 but has no effect on some effector genes such as sseL. This regulatory pathway may have evolved to coordinate virulence gene expression with host infection by responding to host-specific defence pathways that perturb the bacterial outer membrane.