FIG. 4.

Model of the SirA regulatory cascade. SirA directly activates the csrB, hilA, and hilC promoters. HilA and HilC activate the invF gene, the product of which activates genes encoding secreted effectors, including sopB. HilA also activates genes encoding the structural apparatus of TTSS-1 (data not shown). The csrB RNA directly binds and antagonizes the activity of CsrA. CsrA directly increases FlhDC expression posttranscriptionally. CsrA and fliZ also affect multiple levels of the HilA regulon through undefined mechanisms.