SlyA and HilD counteract H-NS-mediated repression on the ssrAB virulence operon of Salmonella Typhimurium and thus promote its activation by OmpR

Abstract

H-NS-mediated repression of acquired genes and the subsequent adaptation of regulatory mechanisms that counteract this repression have played a central role in the Salmonella pathogenicity evolution. The Salmonella pathogenicity island 2 (SPI-2) is an acquired chromosomal region containing genes necessary for Salmonella to colonize and replicate in different niches of hosts. The ssrAB operon, located in SPI-2, encodes the two-component system SsrA-SsrB, which positively controls the expression of the SPI-2 genes, but also other many genes located outside SPI-2. Several regulators have been involved in the expression of ssrAB, such as the ancestral regulators SlyA and OmpR, and the acquired regulator HilD. In this study, we show how SlyA, HilD and OmpR coordinate to induce the expression of ssrAB in different growth conditions. We found that when S. Typhiumurium is grown in the nutrient-rich lysogeny broth (LB), SlyA and HilD additively counteract H-NS-mediated repression on ssrAB; whereas, in N-minimal medium (N-MM), SlyA antagonizes H-NS-mediated repression on ssrAB independently of HilD. Interestingly, our results indicate that OmpR is required for the expression of ssrAB independently of the growth conditions, even in the absence of the repression by H-NS. Therefore, our data supports two mechanisms adapted for the expression of ssrAB in different growth conditions. One involves the additive action of SlyA and HilD, whereas the other involves SlyA, but not HilD, to counteract H-NS-mediated repression on ssrAB, thus favoring, in both cases, the activation of ssrAB by OmpR.

IMPORTANCE The global regulator H-NS represses the expression of acquired genes and thus avoids possible detrimental effects on bacterial fitness. Regulatory mechanisms are adapted to induce expression of the acquired genes in particular niches, to get a benefit from the information encoded in the foreign DNA, as for pathogenesis. Here we show two mechanisms that were integrated for the expression of virulence genes in Salmonella Typhimurium. One involves the additive action of the regulators SlyA and HilD, whereas the other involves SlyA, but not HilD, to counteract H-NS-mediated repression on the ssrAB operon, thus favoring its activation by the OmpR regulator. To our knowledge, this is the first report involving the coordinated action of two regulators to counteract H-NS-mediated repression.