Nucleotide-mediated allosteric regulation of bifunctional Rel enzymes

Bifunctional Rel stringent factors, the most broadly distributed class of RSHs, are ribosome-associated enzymes that transfer a pyrophosphate group from ATP onto the 3' of GTP or GDP to synthesize (p)ppGpp and also catalyse the 3' pyrophosphate hydrolysis of the alarmone to degrade it. The precise regulation of these enzymes seems to be a complex allosteric mechanism, and despite decades of research, it is unclear how the two opposing activities of Rel are controlled at the molecular level. Here we show that a stretch/recoil guanosine-switch mechanism controls the catalytic cycle of T. thermophilus Rel (RelTf). The binding of GDP/ATP stretches apart the NTD catalytic domains of RelTf (RelTtNTD) activating the synthetase domain and allosterically blocking the hydrolase active site. Conversely, binding of ppGpp unlocks the hydrolase domain and triggers recoil of both NTDs, which partially buries the synthetase active site and precludes the binding of synthesis precursors. This allosteric mechanism acts as an activity switch preventing futile cycles of alarmone synthesis and degradation.