Ribosome association primes the stringent factor Rel for recruitment of deacylated tRNA to ribosomal A-site

In the Gram-positive Firmicute bacterium Bacillus subtilis, amino acid starvation induces synthesis of the alarmone (p)ppGpp by the multi-domain RelA/SpoT Homolog factor Rel. This bifunctional enzyme is capable of both synthesizing and hydrolysing (p)ppGpp. To detect amino acid deficiency, Rel monitors the aminoacylation status of the ribosomal A-site tRNA by directly inspecting the tRNAs CCA end. Here we uncover the molecular mechanism of Rel-mediated stringent response. Off the ribosome, Rel assumes a closed conformation which has predominantly (p)ppGpp hydrolysis activity. This state does not specifically inspect tRNA and the interaction is only moderately affected by tRNA aminoacylation. Once bound to the vacant ribosomal A-site, Rel assumes an open conformation, which primes its TGS and Helical domains for specific recognition and recruitment of cognate deacylated tRNA to the ribosome. The tRNA locks Rel on the ribosome in a hyperactivated state that processively synthesises (p)ppGpp while the hydrolysis is suppressed. In stark contrast to non-specific tRNA interactions off the ribosome, tRNA-dependent Rel locking on the ribosome and activation of (p)ppGpp synthesis are highly specific and completely abrogated by tRNA aminoacylation. Binding pppGpp to a dedicated allosteric site located in the N-terminal catalytic domain region of the enzyme further enhances its synthetase activity.