Résumé : Amino acid starvation is sensed by Escherichia coli RelA and Bacillus subtilis Rel through monitoring the aminoacylation status of ribosomal A-site tRNA. These enzymes are positively regulated by their product—the alarmone nucleotide (p)ppGpp—through an unknown mechanism. The (p)ppGpp-synthetic activity of Rel/RelA is controlled via auto-inhibition by the hydrolase/pseudo-hydrolase (HD/pseudo-HD) domain within the enzymatic N-terminal domain region (NTD). We localize the allosteric pppGpp site to the interface between the SYNTH and pseudo-HD/HD domains, with the alarmone stimulating Rel/RelA by exploiting intra-NTD autoinhibition dynamics. We show that without stimulation by pppGpp, starved ribosomes cannot efficiently activate Rel/RelA. Compromised activation by pppGpp ablates Rel/RelA function in vivo, suggesting that regulation by the second messenger (p)ppGpp is necessary for mounting an acute starvation response via coordinated enzymatic activity of individual Rel/RelA molecules. Control by (p)ppGpp is lacking in the E. coli (p)ppGpp synthetase SpoT, thus explaining its weak synthetase activity.