![]() Next, the addition of the amino acid to the growing polypeptide requires peptide bond formation, which involves close approximation of the 3’ ends of the aminoacyl and the peptidyl site tRNAs in the peptidyl transferase center of the large subunit, and large scale movement of the ribosomal subunits relative to each other to form a hybrid state. In this step, GTP-bound elongation factor (eEF1A GTP in eukaryotes or EF-Tu GTP in bacteria) delivers an aminoacyl-tRNA to the A site of the ribosome, where base pairing interactions between the anticodon of the tRNA and the 3 bases of the codon in the mRNA trigger hydrolysis of GTP, release of the deacylated tRNA from the E site of the ribosome, release of the GDP-bound elongation factor and further accommodation of the tRNA (a second proofreading step that depends upon codon-anticodon base pairing). First, addition of a new amino acid begins with the delivery, recognition and accommodation of an aminoacyl-tRNA into the A site of the ribosome. Bacteria specific components are shown in parentheses. The top diagram shows an elongating ribosome, including the 3 sites for tRNA binding (A, P, and E) that span the large and small ribosomal subunits the nascent polypeptide (which is attached to the P site tRNA and exits through the exit tunnel in the large subunit) and the mRNA (which enters and exits through the small subunit, moving 5’ to 3’).
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