*2.2.7 Joint regulation between SPI-1 and SPI-4*

The functional relatedness between SPI-1 and SPI-4 is reflected by their coregulation by the same set of key regulators, for example, a transcriptional activator SprB encoded within SPI-1 and regulated by HilA under similar environmental conditions; SprB directly activates SPI-4 gene expression and weakly represses SPI-1 gene expression through HilD [101].
