The main transcriptional regulator of the Salmonella pathogenicity island 1 (SPI1) gene expression. Activates the expression of invasion genes by a direct action at their promoters and also indirectly by increasing the level of InvF. Also binds upstream of prgH and directly activates the expression of prgHIJK operon. Expressed in response to both environmental conditions and genetic regulatory factors. Transcription is subject to complex control and is stimulated by the SPI1-encoded HilC and HilD.