An interbacterial toxin inhibits target cell growth by synthesizing (p)ppApp

Bacteria have evolved sophisticated mechanisms to inhibit the growth of competitors(1). One such mechanism involves type VI secretion systems, which bacteria can use to directly inject antibacterial toxins into neighboring cells. Many of these toxins target cell envelope integrity, but the full range of growth inhibitory mechanisms remains to be determined(2). Here, we discover a novel type VI secretion effector, Tas1, in the opportunistic pathogen Pseudomonas aeruginosa. A crystal structure of Tas1 reveals similarity to enzymes that synthesize (p)ppGpp, a broadly conserved signaling molecule in bacteria that modulates cell growth rate, particularly in response to nutritional stress(3). Strikingly, however, we find that Tas1 does not synthesize (p)ppGpp, and instead pyrophosphorylates adenosine nucleotides to produce (p)ppApp at rates of nearly 180,000 per min. Consequently, delivery of Tas1 into competitor cells drives the rapid accumulation of (p)ppApp, depletion of ATP, and widespread dysregulation of essential metabolic pathways, resulting in target cell death. Collectively, our findings reveal a new mechanism for interbacterial antagonism and demonstrate, for the first time, a physiological role for the metabolite (p)ppApp in bacteria.