Tale of Twin Bifunctional Second Messenger (p)ppGpp Synthetases and Their Function in Mycobacteria

Abstract

M. tuberculosis, an etiological agent of tuberculosis, requires a long treatment regimen due to its ability to respond to stress and persist inside the host. The second messenger (p)ppGpp-mediated stress response plays a critical role in such long-term survival, persistence, and antibiotic tolerance which may also lead to the emergence of multiple drug resistance. In mycobacteria, (pp)pGpp molecules are synthesized predominantly by two bifunctional enzymes-long RSH-Rel and short SAS-RelZ. The long RSH-Rel is a major (p)ppGpp synthetase and hydrolase. How it switches its activity from synthesis to hydrolysis remains unclear. Rel_Mtb mutant has been reported to be defective in biofilm formation, cell wall function, and persister cell formation. The survival of such mutants has also been observed to be compromised in infection models. In M. smegmatis, short SAS-RelZ has RNase HII activity in addition to (pp)Gpp synthesis activity. The RNase HII function of RelZ has been implicated in resolving replication-transcription conflicts by degrading R-loops. However, the mechanism and regulatory aspects of such a regulation remain elusive. In this article, we have discussed (p)ppGpp metabolism and its role in managing the stress response network of mycobacteria, which is responsible for long-term survival inside the host, making it an important therapeutic target.

Figure 1

Domain architecture of Rel and RelZ proteins involved in the synthesis and hydrolysis of alarmones (pp)pGpp. Rel is composed of a catalytic and regulatory domain with opposite (hydrolase and synthetase) enzymatic activities. (p)ppGpp binds to the regulatory domain at CTD and may show negative feedback loop. RelZ has N-terminal RNaseHII domain and the (pp)pGpp synthetase domain. Synthetase domain of RelZ are responsible for the synthesis of (pp)pGpp and hydrolyze RNA: DNA hybrid in addition to R-loops.

Figure 2

Rel, RelZ, and their pleiotropic role in the management of bacterial stress response and housekeeping functions.

Figure 3

(p)ppGpp enables biofilm formation and mediates bacterial persistence. Persistent cell survives lethal antibiotic treatment and replenish the population upon return of favorable conditions.

Figure 4

Schematic representation of the regulation of the TA system influencing vital processes in mycobacteria. The pathways associated with replication (left) and translation (right) are targeted predominantly by free active toxins in stress conditions in mycobacteria.