The chaperone Tsr2 regulates Rps26 release and reincorporation from mature ribosomes to enable a reversible, ribosome-mediated response to stress.

Although ribosome assembly is quality controlled to maintain protein homeostasis, different ribosome populations have been described. How these form, especially under stress conditions that affect energy levels and stop the energy-intensive production of ribosomes, remains unknown. Here, we demonstrate how a physiologically relevant ribosome population arises during high Na, sorbitol, or pH stress via dissociation of Rps26 from fully assembled ribosomes to enable a translational response to these stresses. The chaperone Tsr2 releases Rps26 in the presence of high Na or pH in vitro and is required for Rps26 release in vivo. Moreover, Tsr2 stores free Rps26 and promotes reincorporation of the protein, thereby repairing the subunit after the Na stress subsides. Our data implicate a residue in Rps26 involved in Diamond Blackfan Anemia in mediating the effects of Na. These data demonstrate how different ribosome populations can arise rapidly, without major energy input and without bypass of quality control mechanisms.