Cyclic dinucleotides (CDNs) play central roles in bacterial homeostasis and virulence as nucleotide second messengers. Bacterial CDNs also elicit immune responses during infection when they are detected by pattern recognition receptors in animal cells. Here, we performed a systematic biochemical screen for bacterial signaling nucleotides and discovered a broad family of cGAS / DncV-like nucleotidyltransferases (CD-NTases) that use both purine and pyrimidine nucleotides to synthesize an exceptionally diverse range of CDNs. A series of crystal structures establish CD-NTases as a structurally conserved family and reveal key contacts in the active-site lid that direct purine or pyrimidine selection. CD-NTase products are not restricted to CDNs and also include an unexpected class of cyclic trinucleotide compounds. Biochemical and cellular analysis of novel signaling nucleotides demonstrate that these molecules activate distinct host receptors and thus may modulate the interaction of both pathogens and commensal microbiota with their animal and plant hosts.

Summary Cyclic GMP-AMP synthase (cGAS) recognition of cytosolic DNA is critical for immune responses to pathogen replication, cellular stress, and cancer. Existing structures of the mouse cGAS-DNA complex provide a model for enzyme activation, but do not explain why human cGAS exhibits severely reduced levels of cyclic GMP-AMP (cGAMP) synthesis compared to other mammals. Here we discover that enhanced DNA-length specificity restrains human cGAS activation. Using reconstitution of cGAMP signaling in bacteria, we mapped the determinant of human cGAS regulation to two amino acid substitutions in the DNA-binding surface. Human-specific substitutions are necessary and sufficient to direct preferential detection of long DNA. Crystal structures reveal why removal of human substitutions relaxes DNA-length specificity, and explain how human-specific DNA interactions favor cGAS oligomerization. These results define how DNA-sensing in humans adapted for enhanced specificity, and provide a model of the active human cGAS-DNA complex to enable structure-guided design of cGAS therapeutics. eTOC Blurb The structure of the human cGAS–DNA complex reveals regulatory adaptations that balance enzymatic activity with DNA-length sensitivity, and additional features important for drug design.

Intracellular pathogens are responsible for much of the world-wide morbidity and mortality due to infectious diseases. To colonize their hosts successfully, pathogens must sense their environment and regulate virulence gene expression appropriately. Accordingly, on entry into mammalian cells, the facultative intracellular bacterial pathogen Listeria monocytogenes remodels its transcriptional program by activating the master virulence regulator PrfA. Here we show that bacterial and host-derived glutathione are required to activate PrfA. In this study a genetic selection led to the identification of a bacterial mutant in glutathione synthase that exhibited reduced virulence gene expression and was attenuated 150-fold in mice. Genome sequencing of suppressor mutants that arose spontaneously in vivo revealed a single nucleotide change in prfA that locks the protein in the active conformation (PrfA*) and completely bypassed the requirement for glutathione during infection. Biochemical and genetic studies support a model in which glutathione-dependent PrfA activation is mediated by allosteric binding of glutathione to PrfA. Whereas glutathione and other low-molecular-weight thiols have important roles in redox homeostasis in all forms of life, here we demonstrate that glutathione represents a critical signalling molecule that activates the virulence of an intracellular pathogen.