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.