The Stringent Response Induced by Phosphate Limitation Promotes Purine Salvage in Agrobacterium fabrum

Abstract

Agrobacterium fabrum induces tumor growth in susceptible plant species. The upregulation of virulence genes that occurs when the bacterium senses plant-derived compounds is enhanced by acidic pH and limiting inorganic phosphate. Nutrient starvation may also trigger the stringent response, and purine salvage is among the pathways expected to be favored under such conditions. We show here that phosphate limitation induces the stringent response, as evidenced by production of (p)ppGpp, and that the xdhCSML operon encoding the purine salvage enzyme xanthine dehydrogenase is upregulated ∼15-fold. The xdhCSML operon is under control of the TetR family transcription factor XdhR; direct binding of ppGpp to XdhR attenuates DNA binding, and the enhanced xdhCSML expression correlates with increased cellular levels of (p)ppGpp. Xanthine dehydrogenase may also divert purines away from salvage pathways to form urate, the ligand for the transcription factor PecS, which in the plant pathogen Dickeya dadantii is a key regulator of virulence gene expression. However, urate levels remain low under conditions that produce increased levels of xdhCSML expression, and neither acidic pH nor limiting phosphate results in induction of genes under control of PecS. Instead, expression of such genes is induced only by externally supplemented urate. Taken together, our data indicate that purine salvage is favored during the stringent response induced by phosphate starvation, suggesting that control of this pathway may constitute a novel approach to modulating virulence. Because bacterial purine catabolism appears to be unaffected, as evidenced by the absence of urate accumulation, we further propose that the PecS regulon is induced by only host-derived urate.

Streptomyces: implications and interactions in plant growth promotion

“…Production of these metabolites involves the actions of some genes that exist majorly in clusters. It is in these clusters that regulatory proteins and phosphorylated guanosine nucleotide phosphate (ppGpp) (which is an important protein for metabolite synthesis) are encoded (Sivapragasam et al 2017 ). Nutritional stress activates alarmone ppGpp which regulates antibiotic production.…”

The Link between Purine Metabolism and Production of Antibiotics in Streptomyces

“…Consistent with this interpretation, the catabolism of xanthine to urate appears to be disfavored in Agrobacterium 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 Δ xdhR strain does not express whi genes, which are important for sporulation, likely on account of increased (p)ppGpp synthesis and dysregulated GTP homeostasis.…”

Impaired purine homeostasis plays a primary role in trimethoprim‐mediated induction of virulence genes in Burkholderia thailandensis

“…Under such conditions, purine salvage may be favored, including reducing the conversion of xanthine to urate, as urate formation siphons purines away from the salvage pathway and precludes recycling. Increased purine salvage has also been reported during the starvation-induced stringent response, another condition that disfavors de novo purine biosynthesis, where upregulation of genes encoding Xdh is linked to production of the signaling molecules guanosine 5′-triphosphate-3′-diphosphate (pppGpp) and guanosine 5′-diphosphate-3′-diphosphate (ppGpp) (Sivapragasam et al, 2017;Sivapragasam and Grove, 2016;Sivapragasam and Grove, 2019).…”

“…The enzyme that converts xanthine to urate is xanthine dehydrogenase (Xdh), whereas xanthine can be produced by the actions of either Xdh or guanine deaminase (Figure 2a). Allopurinol, a competitive inhibitor of Xdh, is clinically utilized for treatment of gout, which is caused by accumulation of uric acid crystals (Pacher et al, 2006), and it functions in several bacterial species to block Xdh activity (Sivapragasam et al, 2017;Sivapragasam and Grove, 2016;Truglio et al, 2002). Allopurinol not only inhibits conversion of xanthine to urate, but also an adverse effect of allopurinol in humans is the accumulation of xanthine (Jezewska, 1973;Torres et al, 2016).…”