FIGURE 1.

Principles of modulating the activity of the PT center. (A) When the A site on the 50S subunit is empty, for example prior to accommodation of aa-tRNA, water (blue) is prevented from nucleophilic attack on the pept-tRNA by a specific conformation of active-site residues (black spiked element). (B) Upon accommodation of aa-tRNA, the PT center rearranges into an active conformation and positions pept-tRNA for the nucleophilic attack of aa-tRNA (nucleophile is indicated red). (C) Hydrolysis of pept-tRNA during termination of protein synthesis is catalyzed by release factor (pale green), which may induce an active conformation of the PT center similar to that brought about by aa-tRNA. Additionally, the release factor orients and/or activates the hydrolytic water molecule.