The nucleotide messenger (p)ppGpp is a co-repressor of the purine synthesis transcription regulator PurR in Firmicutes

The nucleotide messenger (p)ppGpp allows bacteria to adapt to fluctuating environments by reprogramming the transcriptome. Yet despite its well-recognized role in gene regulation, (p)ppGpp is only known to directly affect transcription in Proteobacteria. Here we reveal a different mechanism of gene regulation by (p)ppGpp in Firmicutes from soil bacteria to pathogens: (p)ppGpp serves as a co-repressor of the transcription factor PurR to downregulate purine biosynthesis. We identified PurR as a receptor of (p)ppGpp in Bacillus anthracis and revealed that (p)ppGpp strongly enhances PurR binding to its regulon in the Bacillus subtilis genome. A co-structure reveals that (p)ppGpp binds to a PurR pocket reminiscent of the active site of PRT enzymes that has been repurposed to serve a purely regulatory role, where the effectors (p)ppGpp and PRPP compete to allosterically control transcription. PRPP inhibits PurR DNA binding to induce transcription of purine synthesis genes, whereas (p)ppGpp antagonizes PRPP to enhance PurR DNA binding and repress transcription. A (p)ppGpp-refractory purR mutant fails to downregulate purine synthesis genes upon starvation. Our work establishes precedent of (p)ppGpp as a classical transcription co-repressor and reveals the key function of (p)ppGpp in regulating nucleotide synthesis through gene regulation, from the human intestinal tract to host-pathogen interfaces. ### Competing Interest Statement The authors have declared no competing interest.