Ribosomal tolerance and peptide bond formation.

In the ribosome, the decoding and peptide bond formation sites are composed entirely of ribosomal RNA, thus confirming that the ribosome is a ribozyme. Precise alignment of the aminoacylated and peptidyl tRNA 3'-ends, which is the major enzymatic contribution of the ribosome, is dominated by remote interactions of the tRNA double helical acceptor stem with the distant rims of the peptidyl transferase center. An elaborate architecture and a sizable symmetry-related region within the otherwise asymmetric ribosome guide the A --> P passage of the tRNA 3'-end by a spiral rotatory motion, and ensures its outcome: stereochemistry suitable for peptide bond formation and geometry facilitating the entrance of newly formed proteins into their exit tunnel.