Analysis of HilC/D-dependent invF promoter expression under different culture conditions

In Salmonella enterica serovar Typhimurium, many of the genes required for intestinal penetration and invasion of host cells are encoded within the Salmonella pathogenicity island 1 (SPI1). The expression of invF, which is a positive transcriptional activator of SPI1, is controlled by HilA-dependent (invF-1) and HilC/D-dependent (invF-2) promoters. Transcriptional analysis of invF revealed that the invF-2 promoter (PinvF-2) was not activated when cells were grown in standing culture conditions (which are known to induce SPI1) and that hilD mutation decreased the expression of PinvF-2 only in shaking culture conditions. In the absence of invF-1 promoter (PinvF-1), PinvF-2 promoted InvF production and sipC expression (which is regulated by InvF) in shaking culture conditions. An analysis of the transcription patterns of plasmids harboring the lacZY reporter gene under various PinvF-2 derivatives with truncations or mutations revealed that the downstream region of the PinvF-2 transcription start site (i.e., +148 to +363) plays a role in repressing PinvF-2 in standing culture and in HilD-dependent activation of PinvF-2 in shaking culture conditions. The expression of invH overlaps with PinvF-2, but they are transcribed in opposite directions. However, invH expression did not influence PinvF-2 activity. This suggests that independent regulation of the two invF promoters allows Salmonella to respond quickly to environmental changes.

► The invF-2 promoter was not activated under standing culture conditions.
► From +148 to +363 of the PinvF-2 transcription start site was required.
► Independent regulation of two invF promoters is required for Salmonella virulence.