The gene encodingStreptomyces coelicolorxanthine dehydrogenase regulator (XdhR) is divergently oriented fromxdhABC, which encodes xanthine dehydrogenase (Xdh). Xdh is required for purine salvage pathways. XdhR was previously shown to repressxdhABCexpression. We show that XdhR binds thexdhABC–xdhRintergenic region with high affinity (Kd˜ 0.5 nM). DNaseI footprinting reveals that this complex formation corresponds to XdhR binding thexdhRgene promoter at two adjacent sites; at higher protein concentrations, protection expands to a region that overlaps the transcriptional and translational start sites ofxdhABC. While substrates for Xdh have little effect on DNA binding, GTP and ppGpp dissociate the DNA–XdhR complex. Progression of cells to stationary phase, a condition associated with increased (p)ppGpp production, leads to elevatedxdhBexpression; in contrast, inhibition of Xdh by allopurinol results inxdhBrepression. We propose that XdhR is a direct target of (p)ppGpp, and that expression ofxdhABCis upregulated during the stringent response to promote purine salvage pathways, maintain GTP homeostasis and ensure continued (p)ppGpp synthesis. During exponential phase growth, basal levels ofxdhABCexpression may be achieved by GTP serving as a lower-affinity XdhR ligand.
Xanthine dehydrogenase (Xdh) functions in purine salvage pathways by converting hypoxanthine to xanthine, thereby favoring GTP synthesis. In S. coelicolor, the genes encoding Xdh are regulated by XdhR. We show that XdhR is a direct target of (p)ppGpp, which is a signaling molecule derived from GTP that accumulates during stress and starvation. We suggest that expression of xdhABC is upregulated by (p)ppGpp to promote purine salvage pathways and maintain GTP homeostasis.