An incoherent feedforward loop formed by SirA/BarA, HilE and HilD is involved in controlling the growth cost of virulence factor expression by Salmonella Typhimurium.

Deyanira Pérez-Morales,Paige Teehan,Yvonne Rosenstein,Miguel A De La Cruz,Víctor H Bustamante,Jessica Nava-Galeana,Paul Babitzke,Erika I Melchy-Pérez,Helen Yakhnin,Roberto Rosales-Reyes,William Navarre

doi:10.1371/journal.ppat.1009630

Abstract

An intricate regulatory network controls the expression of Salmonella virulence genes. The transcriptional regulator HilD plays a central role in this network by controlling the expression of tens of genes mainly required for intestinal colonization. Accordingly, the expression/activity of HilD is highly regulated by multiple factors, such as the SirA/BarA two-component system and the Hcp-like protein HilE. SirA/BarA positively regulates translation of hilD mRNA through a regulatory cascade involving the small RNAs CsrB and CsrC, and the RNA-binding protein CsrA, whereas HilE inhibits HilD activity by protein-protein interaction. In this study, we show that SirA/BarA also positively regulates translation of hilE mRNA through the same mentioned regulatory cascade. Thus, our results reveal a paradoxical regulation exerted by SirA/BarA-Csr on HilD, which involves simultaneous opposite effects, direct positive control and indirect negative control through HilE. This kind of regulation is called an incoherent type-1 feedforward loop (I1-FFL), which is a motif present in certain regulatory networks and represents a complex biological problem to decipher. Interestingly, our results, together with those from a previous study, indicate that HilE, the repressor component of the I1-FFL reported here (I1-FFLSirA/BarA-HilE-HilD), is required to reduce the growth cost imposed by the expression of the genes regulated by HilD. Moreover, we and others found that HilE is necessary for successful intestinal colonization by Salmonella. Thus, these findings support that I1-FFLSirA/BarA-HilE-HilD cooperates to control the precise amount and activity of HilD, for an appropriate balance between the growth cost and the virulence benefit generated by the expression of the genes induced by this regulator. I1-FFLSirA/BarA-HilE-HilD represents a complex regulatory I1-FFL that involves multiple regulators acting at distinct levels of gene expression, as well as showing different connections to the rest of the regulatory network governing Salmonella virulence.

Highlights

Pathogenic bacteria have developed diverse regulatory mechanisms to control the appropriated spatiotemporal expression of virulence genes
We identified a novel and sophisticated regulatory mechanism that is involved in the fine-tuned control of the expression level and activity of the transcriptional regulator HilD, for the appropriate balance between the growth cost and the virulence benefit generated by the expression of tens of Salmonella genes
incoherent type-1 feedforward loop (I1-feedforward loop (FFL)) are present in regulatory networks of diverse organisms, from bacteria to humans, and represent a complex biological problem to decipher

Summary

Introduction

Pathogenic bacteria have developed diverse regulatory mechanisms to control the appropriated spatiotemporal expression of virulence genes. Both activation and repression regulatory mechanisms are essential for bacteria to colonize different niches of hosts. Typhimurium) contains a large number of virulence genes and a very complex regulatory network that control their expression. Two groups of genes mainly govern Salmonella virulence; those located in Salmonella Pathogenicity Island 1 (SPI-1) and those located in SPI-2. Both SPI-1 and SPI-2 encode a Type III Secretion System (T3SS), several effector proteins and chaperones, as well as transcriptional regulators [1,2]. Many other Salmonella virulence genes are located in distinct genomic regions, with their biological function being related to that of SPI-1 or SPI-2 [2,4,5]

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Results

Discussion

Conclusion