# The Link between Purine Metabolism and Production of Antibiotics in _Streptomyces_
## 4 Link Between Purine Salvage and Production of Guanosine Nucleotide Second Messengers
### Cellular Levels of GTP and (p)ppGpp are Inter-Dependent  
_De novo_ purine biosynthesis involves the assembly of the purine ring on phosphoribosyl-pyrophosphate (PRPP) to generate inosine monophosphate (IMP), which is the precursor to both AMP and GMP (Figure 1) [43; 44]. Purine bases or nucleotides, including those acquired from exogenous sources, may be inter-converted using salvage pathways. Activity of a number of enzymes in these pathways has been reported to affect the generation of (p)ppGpp, including hypoxanthine guanine phosphoribosyl transferase (HGPRT), which salvages guanine, xanthine, and hypoxanthine nucleobases by converting them to the corresponding mononucleotides, IMP dehydrogenase (GuaB), which catalyzes the committed step in _de novo_ GMP biosynthesis, GMP synthetase (GuaA), and xanthine dehydrogenase (Xdh). Xdh converts hypoxanthine to xanthine, thus favoring the conversion of adenine to guanine, and it can divert purines away from salvage pathways and towards degradation by catalyzing the oxidation of xanthine to urate [45]. For example, \(guaA\) and \(guaB\) mutants of the Lyme disease pathogen _Borrelia burgdorferi_ fail to infect a mouse model, illustrating that purine metabolic enzymes play an important role in virulence [46]. Lacking enzymes involved in _de novo_ purine biosynthesis, this bacterium in addition depends on transport proteins to salvage purines from the host [47]. Both (p)ppGpp and c-di-GMP play a vital role during starvation and survival in the tick host, and failure to synthesize these messengers reduces persistence [48; 49]. In _Sinorhizlobium meliloti_, (p)ppGpp synthesis is increased upon carbon or nitrogen starvation, concomitant with an increase in the transcript levels of _xdh_. This indicates that the _xdh_ gene product is linked to synthesis of (p)ppGpp during starvation [50]. Consistent with this interpretation, _xdh_ expression is also increased in stationary phase in _S. coelicolor_, whereas inhibition of Xdh with allopurinol attenuates (p)ppGpp production [51]. In _Listeria monocytogenes_, both _hgprt_ and _relA_ mutants are incapable of synthesizing (p)ppGpp and fail to cause infection in mice [52]. Evidently, the enzymes of purine metabolism are critical for maintaining the requisite levels of guanosine nucleotide second messengers.  
A feedback response in which (p)ppGpp negatively controls activity of purine metabolic enzymes also exists. As noted above, _S. griseus_ GuaB is inhibited by ppGpp, which accounts for reduced cellular levels of GTP during stringent response (Figure 3) [28]. In _B. subtilis_, (p)ppGpp inhibits several GTP biosynthesis enzymes, but primarily HGPRT and Gmk, which converts GMP to GDP, in order to regulate the GTP levels in the cell [9]; in absence of (p)ppGpp, excess GTP leads to cell death [18]. _Staphylococcus aureus_ HGPRT and Gmk are also direct targets for (p)ppGpp, suggesting a conserved regulatory mechanism [10]. By contrast, Gmk is not specifically inhibited by (p)ppGpp in _Enterococcus faecalis_, whereas HGPRT is [53]. In addition, HPRT/GPRT were recently confirmed as physiologically relevant targets of (p)ppGpp in _E. coli_ as well, suggesting that direct regulation of purine metabolic enzymes by (p)ppGpp is not unique to Gram-positive species [8]. That (p)ppGpp targets different enzymes in various bacterial species peaks to distinct mechanisms for control of GTP homeostasis.