# The Link between Purine Metabolism and Production of Antibiotics in _Streptomyces_
## 4 Link Between Purine Salvage and Production of Guanosine Nucleotide Second Messengers
### Production of Xdh As a Mechanism to Promote Purine salvage  
Control of cellular GTP levels is thought to be a primary function of (p)ppGpp in Gram-positives, guarding against excess GTP accumulation during balanced growth and effecting a decrease during the stringent response [9; 18; 53]. However, excessive depletion of GTP is also deleterious, and mechanisms exist that favor synthesis of guanosine nucleotides. In _S. coelicolor_, xanthine dehydrogenase regulator (XdhR) represses expression of the _xdh_ operon; XdhR binds ppGpp (and with lower affinity to GTP), a result of which is attenuated DNA binding and increased expression of the _xdhABC_ gene cluster during the stringent response [51]. This mode of regulation would bias purine salvage pathways towards production of guanosine nucleotides, particularly during stringent response when _de novo_ pathways are downregulated and the bacteria are dependent upon salvage pathways. The \(\sim\)0.2 mM affinity of ppGpp for XdhR suggests that _xdh_ expression (and hence GTP synthesis) will be induced when (p)ppGpp accumulates to mM concentrations; under these conditions, (p)ppGpp-mediated repression of target enzymes in the purine biosynthesis pathways would be efficient, and production of Xdh may avert excessive depletion of GTP. If (p)ppGpp preferentially inhibits GuaB, as reported  
Figure 3: Summary of link between guanosine metabolism and antibiotic production. During vegetative growth, cellular concentrations of GTP are high, but maintained by (p)ppGpp-mediated inhibition of GTP biosynthesis, most likely direct inhibition of GuaB. Under these conditions, c-di-GMP is produced, and it binds BldD to form a complex, which represses _bld_ and _xhi_ genes. Under stress conditions, (p)ppGpp accumulates, resulting in efficient inhibition of GTP biosynthesis. However, high levels of (p)ppGpp also mediate an increase in Xdh production, averting excessive depletion of GTP. Reduced production of c-di-GMP results in derepression of _bld_ and _whi_ genes, leading to morphological differentiation. The accumulation of (p)ppGpp generally leads to production of secondary metabolites, including antibiotics. The role of BldD in antibiotic production remains to be fully elucidated. in _S. griseus_[28], then GTP production would depend on synthesis of XMP _via_ xanthine (Figure 1), rationalizing the need for (p)ppGpp-dependent control of _xdh_ expression (Figure 3). The _xdhR-xdhABC_ gene locus is not conserved in _B. subtilis_, where instead expression of _xdh_ is induced in the presence of hypoxanthine [54]. By contrast, hypoxanthine is not a ligand for _S. coelicolor_ XdhR [51]. This may reflect that _B. subtilis_ Xdh has a primary role in purine catabolism, whereas _S. coelicolor_ Xdh activity is more important for GTP homeostasis. Consistent with this interpretation, the catabolism of xanthine to urate appears to be disfavored in _Agrobacteriumarium fabrum_ during the stringent response when (p)ppGpp accumulates and _xdh_ expression is increased; _A. fabrum_ encodes a locus that is homologous to the _xdhR-xdhABC_ gene cluster found in _Streptomyces_ spp. [55]. Furthermore, an _S. coelicolor xdhR_ mutant, which overproduces Xdh, is defective in forming aerial mycelium and spores and it overproduces the antibiotic ACT [56; 57; 58]; the _xdhR_ strain does not express _whi_ genes, which are important for sporulation, likely on account of increased (p)ppGpp synthesis and dysregulated GTP homeostasis. _A. fabrum_ XdhR binds c-di-GMP with even higher affinity than ppGpp [51]. As noted above, c-di-GMP is also involved in morphological differentiation and antibiotic production in _Streptomyces_ spp., and it activates DNA binding by the key regulator BldD. _S. coelicolor_ XdhR likewise responds to c-di-GMP by attenuated DNA binding ([PERSON] and [PERSON], unpublished), which may favor synthesis of GTP to sustain production of secondary messengers that derive from GTP.