Identification and characterization of mutants with increased expression of hilA, the invasion gene transcriptional activator of Salmonella typhimurium.

Induction of invasion gene transcription and expression of the invasive phenotype of Salmonella strains are regulated by environmental conditions. Experimental evidence indicates that oxygen, pH, and osmotic conditions need to closely resemble those of the host intestinal lumen for invasion gene activation. The hilA gene, encoded on Salmonella pathogenicity island 1 (SPI-1), is a transcriptional activator which is required for invasion and whose expression is modulated by oxygen, pH, and osmolarity. Additionally, hilA is regulated by genetic elements encoded on SPI-1 (hilC/sirC/sprA and hilD), as well as by elements which reside outside of SPI-1 (phoP/phoQ and sirA), although how environmental signals modulate hilA is unknown. In an effort to further characterize the Salmonella invasion gene regulon, we have created and preliminarily characterized 18 Tn5 insertions which result in upregulation of a hilA::lacZY fusion. We have classified the mutations based on location and phenotype into three classes. Six class 1 and six class 2 mutants have insertions in SPI-1 near the invasion gene orgA or the invasion gene regulator hilD, respectively. Six class 3 mutants reside outside of SPI-1 in four different loci. The class 2 and 3 mutations induce overexpression of an episomal hilA::lacZY fusion and significantly increase S. typhimurium invasion of HEp-2 cells in a standard invasion assay. These data implicate new regions of SPI-1 as being involved in the regulation of invasion by S. typhimurium and identify new invasion gene regulators located outside of SPI-1.