The changes in rRNA stability observed in the terminal loop of Helix 88 and in ES31 are intriguing. Chemical protection experiments revealed the terminal loop of Helix 88 is involved in a kissing loop interaction with the terminal loop of Helix 22, and this interaction is apparent in the X-ray crystal and cryo-EM structures (4, 49) . Increased lability at A2779 and A2780 was tRNA H84 L11 S18 P-Site H84 L11 S18 C C Figure 6 . Model: the P-site loop acts as a sensor of the occupancy status of the P-site. (Left) When the large subunit P-site is unoccupied by tRNA, the L11 P-site loop is able to extend into this space leaving the distal loop of H84 partially deprotected from chemical attack. This conformation is favored by the 54-7A mutant of L11B. (Right panel) Occupation of the P-site by peptidyl-tRNA displaces the L11 P-site loop, causing it to tightly retract from the P-site and interact with H84, resulting in increased protection of the H84 terminal loop from chemical attack. H84 likely moves toward the P-site loop slightly, increasing the exposure of C2675 to the surrounding solvent. This conformation is favored by the L11B 51-4A mutant.