Date Published: April 24, 2019
Publisher: Public Library of Science
Author(s): Chien-Che Hung, Colleen R. Eade, Michael I. Betteken, Paulina D. Pavinski Bitar, Elaine M. Handley, Staci L. Nugent, Rimi Chowdhury, Craig Altier, Andreas J. Baumler.
Virulence functions of bacterial pathogens are often energetically costly and thus are subjected to intricate regulatory mechanisms. In Salmonella, invasion of the intestinal epithelium, an essential early step in virulence, requires the production of a multi-protein type III secretion apparatus. The pathogen mitigates the overall cost of invasion by inducing it in only a fraction of its population. This constitutes a successful virulence strategy as invasion by a small number is sufficient to promote the proliferation of the non-invading majority. Such a system suggests the existence of a sensitive triggering mechanism that permits only a minority of Salmonella to reach a threshold of invasion-gene induction. We show here that the secondary structure of the invasion regulator hilD message provides such a trigger. The 5’ end of the hilD mRNA is predicted to contain two mutually exclusive stem-loop structures, the first of which (SL1) overlaps the ribosome-binding site and the ORF start codon. Changes that reduce its stability enhance invasion gene expression, while those that increase stability reduce invasion. Conversely, disrupting the second stem-loop (SL2) represses invasion genes. Although SL2 is the energetically more favorable, repression through SL1 is enhanced by binding of the global regulator CsrA. This system thus alters the levels of hilD mRNA and is so sensitive that changing a single base pair within SL1, predicted to augment its stability, eliminates expression of invasion genes and significantly reduces Salmonella virulence in mice. This system thus provides a possible means to rapidly and finely tune an essential virulence function.
The success of bacterial pathogens relies upon a fine balance: They must rapidly induce functions dedicated to virulence in response to signals of the host, but withstand the often immense associated fitness costs that the production of these virulence proteins entails. This need is particularly acute for enteric pathogens, including Salmonella, as they survive within the intestine in competition with a vast number and diversity of bacterial species. Pathogens thus achieve this balance by several means. They may do so by evolving to place virulence under the control of existing global regulators. They may coordinate virulence gene expression as a part of tightly controlled, integrated regulatory mechanisms that respond to host signals. They may additionally induce virulence in only a select portion of a population sufficient to cause disease.
Here we have described a sensitive mechanism for the control of Salmonella invasion, an essential virulence function. The data presented suggest a simple but elegant model (Fig 7): The message of the invasion activator HilD is capable of assuming two alternative and mutually exclusive secondary structures. Formation of the first, SL1, sequesters the ribosome-binding site and start codon, reducing message stability and presumably preventing translation. Formation of the second (SL2), however, liberates these sites from the secondary structure and instead promotes the expression of hilD. SL2 is energetically favored, and thus in the absence of additional regulatory components, it should predominate. SL1, however, binds to CsrA, further stabilizing it and shifting the balance of control towards the repression of hilD when in the presence of this global regulatory protein. This balance can thus be altered by the activity of the BarA/SirA two-component regulator, which induces the expression of the regulatory RNAs CsrB and CsrC. These RNAs bind CsrA, titrating it from its target within SL1, allowing SL2 to form and increasing invasion gene expression through enhanced translation of HilD.