HilD-mediated transcriptional cross-talk between SPI-1 and SPI-2

Abstract

The acquisition of new genetic traits by horizontal gene transfer and their incorporation into preexisting regulatory networks have been essential events in the evolution of bacterial pathogens. An example of successful assimilation of virulence traits is Salmonella enterica, which acquired, at distinct evolutionary times, Salmonella pathogenicity island 1 (SPI-1), required for efficient invasion of the intestinal epithelium and intestinal disease, and SPI-2, essential for Salmonella replication and survival within macrophages and the progression of a systemic infection. A positive regulatory cascade mainly composed of HilD, HilA, and InvF, encoded in SPI-1, controls the expression of SPI-1 genes, whereas the two-component regulatory system SsrA/B, encoded in SPI-2, controls expression of SPI-2 genes. In this study, we report a previously undescribed transcriptional cross-talk between SPI-1 and SPI-2, where the SPI-1-encoded regulator HilD is essential for the activation of both the SPI-1 and SPI-2 regulons but at different times during the stationary phase of growth in Luria-Bertani medium. Our data indicate that HilD counteracts the H-NS-mediated repression exerted on the OmpRdependent activation of the ssrAB operon by specifically interacting with its regulatory region. In contrast, HilD is not required for SPI-2 regulon expression under the in vitro growth conditions that are thought to resemble the intracellular environment. Our results suggest that two independent SPI-2 activation pathways evolved to take advantage of the SPI-2-encoded information at different niches and, in consequence, in response to different growth conditions.H-NS ͉ OmpR ͉ microbial pathogenesis ͉ transcriptional regulation ͉ salmonella

A novel PhoP-regulated locus encoding the cytolysin ClyA and the secreted invasin TaiA of Salmonella enterica serovar Typhi is involved in virulence

“…Regulation of the taiA invasin by PhoP may seem to contradict its well-accepted function as a regulator of virulence genes involved in intracellular survival, such as the SPI-2 T3SS. However, using LB broth, a condition not usually recognized to support expression of genes involved in intracellular survival, it has been shown that pag genes, controlled by PhoP, and some SPI-2 effectors are expressed (Belden & Miller, 1994;Bustamante et al, 2008). Moreover, it was recently shown that two genes encoded on SPI-1 (orgBC) are activated by PhoP during growth in LB NaCl, a condition known to induce SPI-1 genes (Aguirre et al, 2006).…”

Dual RNA-seq of pathogen and host

“…1.2). For example, HilD, the master activator of SPI-1 genes has been shown to regulate transcription of the ssrA/B operon, encoding the key regulator of SPI-2 (Bustamante et al, 2008;Martinez et al, 2014).…”

Integration host factor alleviates H-NS silencing of the Salmonella enterica serovar Typhimurium master regulator of SPI1, hilA

“…Different studies have shown that low oxygen, high osmolarity, the exponential phase of growth and slightly alkaline pH induce hilA expression and consequently the invasion genes (Bajaj et al, 1996;Jones & Falkow, 1994;Chubiz et al, 2010) Other recent studies have shown that, when Salmonella cells are grown in aerated Luria-Bertani (LB) medium at 37 uC, hilA induction occurs at the onset of the stationary phase (Song et al, 2004;Mangan et al, 2006;Bustamante et al, 2008). These culture conditions have been used to further analyse the effects of modulators such as ppGpp (Song et al, 2004) and integration host factor (IHF) (Mangan et al, 2006), and the regulatory cross-talk between SPI1 and SPI2 (Bustamante et al, 2008). Most likely, low oxygen/exponential growth phase and aeration/ early stationary phase mimic different environmental inputs that Salmonella cells detect when growing in natural environments, and lead to hilA induction.…”