Starvation sensing by mycobacterial RelA/SpoT homologue through constitutive surveillance of translation

Abstract

The stringent response, which leads to persistence of nutrient-starved mycobacteria, is induced by activation of RelA/SpoT homologue (Rsh) upon entry of a deacylated-tRNA in a translating ribosome. However, the mechanism by which Rsh finds such ribosomes in vivo remains unclear. Here we show that conditions inducing ribosome hibernation cause loss of intracellular Rsh in a Clp protease-dependent manner. The loss is also observed in non-starved cells when Rsh-ribosome interaction is abolished by mutation in Rsh, indicating that interaction with translating ribosomes is necessary for physiological stability of Rsh. The cryo-EM structure of the Rsh-bound 70S ribosome in a translation initiation complex reveals hitherto unknown interactions between the ACT domain of Rsh and components of the ribosomal L7/L12-stalk base, suggesting that the aminoacylation status of A-site tRNA is surveyed during the first cycle of elongation. Altogether, we propose a surveillance model of Rsh activation that originates from its constitutive interaction with translating ribosomes.