FIGURE 4.

Mutations in key ribosome-tRNA interaction sites aided in identifying and stabilizing hybrid state intermediates on the ribosome. The 3′-CCA termini of A and P site tRNA make critical contacts with the A and P loops of 23S rRNA in the peptidyl transferase center of the 50S subunit.⁹²–⁹⁴ (A) Here the CCA termini shown in red, of the A- (right) and P- (left) site tRNA are shown in a surface-rendered crystal structure of the peptidyl transferase center.¹⁵ rRNA is shown in tan, with the A and P loops highlighted in brown. Protein components (mainly L27) are shown in blue. (B) Mutations G2553C in the A loop and G2252C in the P loop interrupted base-pairing interactions between C75 and C74 of the A- and P-site tRNA, respectively. Mutation in the A loop promoted formation of the A/P hybrid state, giving rise to an A/P-P/E hybrid state configuration. The P loop mutation promoted formation of the previously unidentified A/A-P/E hybrid state, which leave the 50S P site vacant.⁸⁵