RIBOSOME-CATALYZED PEPTIDE-BOND FORMATION WITH AN A-SITE SUBSTRATE COVALENTLY-LINKED TO 23S RIBOSOMAL-RNA

In the ribosome, the aminoacyl-transfer RNA (tRNA) analog 4-thio-dT-p- C-p-puromycin crosslinks photochemically with G2553 of 23S ribosomal R NA (rRNA). This covalently linked substrate reacts with a peptidyl-tRN A analog to form a peptide bond in a peptidyl transferase-catalyzed re action. This result places the conserved 2555 loop of 23S rRNA at the peptidyl transferase A site and suggests that peptide bond formation c an occur uncoupled from movement of the A-site tRNA. Crosslink formati on depends on occupancy of the P site by a tRNA carrying an intact CCA acceptor end, indicating that peptidyl-tRNA, directly or indirectly, helps to create the peptidyl transferase A site.