Environmental Regulation of Yersinia Pathophysiology

Abstract

Hallmarks of Yersinia pathogenesis include the ability to form biofilms on surfaces, the ability to establish close contact with eukaryotic target cells and the ability to hijack eukaryotic cell signaling and take over control of strategic cellular processes. Many of these virulence traits are already well-described. However, of equal importance is knowledge of both confined and global regulatory networks that collaborate together to dictate spatial and temporal control of virulence gene expression. This review has the purpose to incorporate historical observations with new discoveries to provide molecular insight into how some of these regulatory mechanisms respond rapidly to environmental flux to govern tight control of virulence gene expression by pathogenic Yersinia.

Keywords: RovA; acidity; c-di-GMP; cAMP; extracytoplasmic stress; metabolism; temperature; transition metals.

Figure 1

Prominent Yersinia virulence factors. Yersinia pestis and enteropathogenic Y. pseudotuberculosis and Y. enterocolitica vary greatly in their pathogenicity and in aspects of their pathogenesis. This is reflected by the different repertoire of proven and potential virulence factors in their respective armories. In particular, Y. pestis has acquired additional plasmid DNA that encodes for factors that enable colonization and transmission via the flea vector and survival in blood. It is also apparent that the regulatory circuitry of Y. pestis has been rewired in ways that drive elevated in vivo expression of critical virulence associated factors. On the flip side, Y. pestis has lost flagella-mediated motility and cell-adhesive capacities that are otherwise critical for survival of the enterics both in the environment and in the GI tract, respectively. Yet commonalities between all three pathogens exist, such as the prominent virulence plasmid-encoded Ysc-Yop type III secretion system responsible for promoting an extracellular infection niche, along with other systems responsible for distributing de novo synthesized proteins into other extracytoplasmic compartments or even realized free from the bacteria.

Figure 2

Thermoregulation of Ysc-Yop type III secretion by Yersinia. Thermoregulation of Ysc-Yop type III secretion is mediated through control of the transcriptional activator, LcrF. (A) At ambient temperature, transcription of the yscW-lcrF operon is inhibited by the YmoA DNA binding protein. Furthermore, post-transcriptional inhibition occurs as a result of stem-loop formation of mRNA within the intergenic region between the two alleles. (B) De-regulation occurs at elevated temperature because YmoA affinity for the yscW-lcrF operon promoter is dramatically diminished, and this promotes operon transcription. Moreover, elevate temperature resolves the stem-loop structure in mRNA transcripts, so that translation into LcrF can proceed. This results in a positive auto-regulatory cascade that enhances lcrF transcription. Accumulated LcrF can then transcriptionally activate responsive ysc and yop promoters. This illustration is inspired in part from Bohme et al. (2012) and initial artistic work of Tiago Costa.

Figure 3

A network of diverse regulatory inputs controls RovA transcriptional output in Yersinia. Available RovA is strictly controlled by cascade regulation at both the transcriptional and post-transcriptional levels in response to multiple environmental cues. The strongest influence on RovA production is through two opposing pathways. The first is an auto amplification loop, which in turn is responsive to thermo-regulated proteolysis of RovA by ClpXP and Lon proteases. The second is via RovM that is principally mediated by the prominent Csr and Crp pathways responsive to carbon and glucose availability. Other regulatory pathways are known, but the extent to which they alter RovM or RovA levels is less clear. In the diagram, induction of RovA expression is indicated by an arrow, while repression is indicated by a blunted line. For simplicity, information concerning whether the pathway is direct or indirect has been omitted on the basis that this is not always defined.

Figure 4

Sensing of noxious extracytoplasmic stress by the bacteria envelope of Yersinia. The molecular basis for the activation of four prominent extracytoplasmic stress sensing sentinels are displayed, along with a summary of their respective phenotypic effects in pathogenic Yersinia. Maintaining the outer membrane (OM) are the RopE-, Cpx-, and Rcs-pathways. (A) Outer membrane protein misfolding initiates digestion of the anti-RpoE factor, RseA, through successive proteolytic actions of the DegS, RseP, and ClpXP proteases. Free RpoE is released into the cytoplasm, and when engaged with core RNA polymerase (RNAP), can establish controlled transcription of a large RpoE-regulon. The Cpx two-component system (B) and the Rcs phosphorelay system (C) both rely upon sensor kinase autophosphorylation (CpxA and RscC, respectively) to initiate the transduction of phosphate through to the cytoplasmic cognate response regulator (CpxR and RcsB, respectively). CpxA activation requires the DegP-dependent release of inhibitory CpxP, while RcsC activation might utilize an RcsF-dependent pathway. Inorganic phosphate can also be donated from unstable high-energy metabolic intermediates. Active phosphorylated response regulators dimerize and in concert with the house-keeping RNAP holoenzyme, target specific responsive promoters to influence transcriptional output. RcsB transcriptional control sometimes requires partners with RcsA. (D) Controlling the integrity of the cytoplasmic membrane (CM) are the phage shock proteins (psp). Secretin complex mislocalization to the cytoplasmic membrane risks dissipating the proton motive force (PMF). This is prevented by the PspB and PspC proteins actively sequestering the anti-PspF factor, PspA, to free up the PspF transcription factor to initiate promoter targeting and transcriptional output by the house-keeping RNAP holoenzyme.

Figure 5

Metabolic intermediates regulate Yersinia virulence. Understanding the role of nucleotide-based second messengers in the regulation of Yersinia survival and virulence is still in its infancy. As determined from many studies in E. coli systems, three major regulatory molecules are known: (A) c-di-GMP, (B) cAMP and its receptor Crp, and (C) ppGpp and pppGpp [collectively known as (p)ppGpp]. Primary activation signals control diguanylate cyclase (GGDEF domain containing proteins) to generate c-di-GMP (A). Various effector molecules then interact with c-di-GMP to regulate gene expression, including Hms-mediated biofilm formation. Deactivating signals stimulate phosphodiesterase activity of proteins containing either EAL or HD-GYP domains, and this degradation pathway ensures that c-di-GMP levels are stringently controlled. Glucose availability and the phosphoenolpyruvate (PEP)—carbohydrate phosphotransferase system (PTS) control cAMP production by adenylate cyclase (AC) (B). Upon cAMP production when glucose is limiting, cAMP-CRP complexes form and this enhances RNAP holoenzyme binding to vast numbers of sensitive promoters including several secreted Yersinia virulence factors. Finally, various forms of starvation stimulates RelA-dependent and SpoT/acyl carrier protein (ACP)-dependent synthesis of (p)ppGpp (C). By direct binding to the RNA polymerase, (p)ppGpp can influence sigma factor competition for core RNAP, and through this affects transcription of a plethora of genes that in Yersinia includes the prominent Ysc-Yop plasmid-encoded T3SS.