The stringent response and Mycobacterium tuberculosis pathogenesis

Abstract

During infection, the host restrains Mycobacterium tuberculosis (Mtb) from proliferating by imposing an arsenal of stresses. Despite this onslaught of attacks, Mtb is able to persist for the lifetime of the host, indicating that this pathogen has substantial molecular mechanisms to resist host-inflicted damage. The stringent response is a conserved global stress response in bacteria that involves the production of the hyperphosphorylated guanine nucleotides ppGpp and pppGpp (collectively called (p)ppGpp). (p)ppGpp then regulates a number of cellular processes to adjust the physiology of the bacteria to promote survival in different environments. Survival in the presence of host-generated stresses is an essential quality of successful pathogens, and the stringent response is critical for the intracellular survival of a number of pathogenic bacteria. In addition, the stringent response has been linked to virulence gene expression, persistence, latency and drug tolerance. In Mtb, (p)ppGpp synthesis is required for survival in low nutrient conditions, long term culture and during chronic infection in animal models, all indicative of a strict requirement for (p)ppGpp during exposure to stresses associated with infection. In this review we discuss (p)ppGpp metabolism and how this functions as a critical regulator of Mtb virulence.

Figure 1.

Diagram of the full-length Rel_Mtb protein illustrating functional domains. The N-terminal domain (NTD) contains the catalytic domains and the C-terminal domain (CTD) contains the regulatory domains. The CTD contains two structural domains: the TGS domain (named for its initial identification in ThrRS, GTPase and SpoT) and the ACT domain (named for its initial identification in aspartate kinase, chorismite mutase and TyrA). The numbers designate the amino acid positions that form the borders of each domain.

Figure 2.

Sequence alignment of GMK homologs from Mycobacterium tuberculosis (Mtb), Streptomyces coelicolor and Bacillus subtilis. Yellow boxes indicate residues that interact with pppGpp and are conserved in all three species. Red boxes indicate residues that interact with pppGpp and are polymorphic in Mtb.

Figure 3.

Signaling network involving Rel_Mtb, (p)ppGpp and polyP in Mtb. The network shows the proteins and molecules comprising the signaling cascades that positively and negatively regulate (p)ppGpp and polyP accumulation. Polyphosphate kinase 1 (PPK1), Polyphosphate kinase 2 (PPK2), Polyphosphatase 1 (PPX1) and Polyphosphatase 2 (PPX2) are shown. Green arrows indicate positive regulation. Red lines indicate negative regulation.

Figure 4.

Summary of the effects of (p)ppGpp in Mtb. The RelMtb synthetase domain transfers the 5΄-β,γ-pyrophosphate from ATP to the 3΄-OH of GDP or GTP to synthesize (p)ppGpp, which can then be hydrolyzed by the RelMtb hydrolase domain. (p)ppGpp accumulates in response to a number of stresses, including starvation and oxidative stress, leading to alterations in purine metabolism, gene expression and polyP metabolism. It is also possible that there are other unidentified targets of (p)ppGpp in Mtb. These effects of (p)ppGpp accumulation contribute to enhanced virulence, persistence and stress responses.