B-187. HilD Counteracts the Repression Exerted by H-NS on the ssrAB Operon to Induce Expression of the SPI-2 Regulon in Salmonella enterica Serovar Typhimurium

L. C. Martinez, J. L. Puente, V. H. Bustamante;
Inst. de Biotecnología, Cuernavaca, Morelos, MEXICO.

Salmonella infections cause two clinical entities in humans and animals: typhoid fever and gastroenteritis. Salmonella Pathogenicity Islands 1 and 2 (SPI-1 and SPI-2), both encoding a type 3 secretion system, effector and chaperone proteins and transcriptional regulators, are required for the invasion of epithelial host cells and for bacterial replication/survival inside macrophages, respectively. A regulatory cascade comprising the SPI-1-encoded regulators HilD, HilA and InvF induces expression of the SPI-1 regulon in response to conditions resembling the intestinal environment, such as mid exponential growth in Luria-Bertani (LB) rich media. The two-component system SsrA/B, encoded in SPI-2, controls the expression of the SPI-2 regulon in response to intracellular environmental signals, such as low concentrations of magnesium and phosphate that are mimicked in vitro using minimal media. We have recently shown that the expression of the SsrA/B system, and thus of the SPI-2 regulon, is regulated by an alternative HilD-dependent pathway when Salmonella reaches late stationary phase growing in LB, but not in minimal media. Here, we determined that growth conditions that affect expression of hilD, similarly affect the expression of hilA and ssrAB, and, in consequence, the expression of the SPI-1 and SPI-2 regulons. Analysis of the expression of cat transcriptional fusions containing different regions of the ssrAB locus, as well as of the SsrAB proteins by western blot, revealed that the genes encoding the SsrA/B system are transcribed as an operon. Furthermore, we determined that HilD positively regulates the expression of the SPI-2 regulon by counteracting the H-NS-mediated repression on the ssrAB promoter and, consistently, that both HilD and H-NS bind to the regulatory region of ssrAB. These results expand our knowledge about the role of HilD in coordinating two different regulatory cascades, that allow the transcriptional transition between the expression of the SPI-1 and SPI-2 regulons in response to changing environmental signals, which could be important for initiating the adaptation to the intracellular environment.