# Stringent Response in Mycobacteria: From Biology to Therapeutic Potential
## 3 Mycobacterial Stringent Response and Its Role in Survival during Stress  
Alarmones (p)ppGpp control cellular processes by affecting DNA replication, transcription, and translation, and thus, bring about timely changes in bacterial physiology. In this regard, there are excellent reviews that provide detailed information on other bacterial species such as _E. coli_[26; 53; 54; 55; 56; 57; 58; 59]. _M. tuberculosis_ faces several stresses, such as oxidative, nitrosative, and nutrient stress upon infecting the host. However, it overcomes these potentially fatal stresses and successfully establishes chronic infection [38; 60; 61]. The adaptation to such stresses requires a large-scale transcriptional reprogramming, which eventually lets _M. tuberculosis_ not only infect the macrophages but also survive for years in granuloma [38; 61; 62]. The hallmark of stringent response is downregulation of rRNA and ribosomal protein synthesis with concomitant upregulation of amino acid biosynthetic operons to supply necessary amino acids for survival [63; 64]. The _M. tuberculosis_ strain H37 Rv, like other bacteria, also shows these signature transcription changes upon nutrient deprivation. Microarray analysis of H37 Rv and H37 Rv\(\Delta\)_r_l_M_t_h_ mutant strains, starved for six hours, showed differential expression of several genes [65]. The study found that 54 genes encoding ribosomal proteins were downregulated in the parental H37 Rv strain compared to H37 Rv\(\Delta\)_r_l_M_t_h_ mutant. Moreover, the parental H37 Rv strain also showed the downregulation of 5 genes involved in transcription and 16 genes involved in protein synthesis. Late-log phase cultures of H37 Rv\(\Delta\)_r_l_M_t_h_ showed at least five-fold more ribosomes per unit protein when compared to the parental H37 Rv strain. Thus, in the absence of (p)ppGpp, _M. tuberculosis_ fails to regulate the synthesis rRNA and ribosomal protein, which are needed for adaptation to the stationary phase. This aspect of mycobacterial stringent response is similar to _E. coli_. However, some aspects of stringent response are mycobacteria specific such as CarD-based regulation and inorganic polyphosphate (polyP)-based regulation. CarD is a conserved essential transcriptional regulator found in actinobacteria and its depletion in _M. tuberculosis_ and _M. smgematis_ impaired the stringent response [66]. Stringent response in mycobacteria is also regulated by a feedback loop between (p)ppGpp and inorganic polyphosphate [67; 68; 69; 70]. PolyP is synthesized and degraded by polyphosphate kinases (PPK) and exopolynophastases (PPX), respectively. A signaling cascade between two-component system MprAB, alternative sigma factor SigE and Rel protein governs the levels of (p)ppGpp and polyP in mycobacteria. PolyP functions as a phosphate donor to MprB, a stress-responsive histidine kinase. Subsequently, the phosphorylated MprB transfers its phosphoryl group to MprA, a response regulator. The phosphorylated MprA then activates the transcription of alternative sigma factor SigE, which eventually upregulates the transcription of _relA_ gene [68; 71; 72; 73]. Thus, apart from carbon starvation, nutrient starvation and hypoxia, phosphate starvation can also trigger the stringent response in mycobacteria. In the absence of stringent response, the long-term survival of both _M. tuberculosis_ and _M. smgematis_ during stress conditions is impaired [24; 74; 75]. H37 Rv\(\Delta\)_r_l_M_t_h_ mutant exhibited a slower growth rate than the parental H37 Rv _M. tuberculosis_ in synthetic media and failed to survive long-term starvation (4 months). Moreover, the H37 Rv\(\Delta\)_r_l_M_t_h_ mutant failed to survive the oxygen limitation and increased temperature of 42 \({}^{\circ}\)C, and lost viability sooner than the parental _M. tuberculosis_ H37 Rv strain [74].