 Once the correctly charged tRNA has been placed in the A site and has rotated into the peptidyl transferase center, peptide bond formation takes place. This reaction is catalyzed by RNA, specifically the 23S rRNA component of the large subunit of ribosome. Early evidence for this came from experiments in which it was shown that a large subunit that had been stripped off of its proteins was still able to carry out peptide bond formation. Early experiments were seen that large subunit whose proteins were removed, even then this large subunit is able to carry out peptide bond formation which means that no protein is participating in this reaction to catalyze this reaction. Proof that the peptidyl transferase is entirely composed of RNA has come from the high-resolution three-dimensional structure of the ribosome which reveals that no amino acid is situated closer than 18 angstrom from the active site. Most catalysis requires distances in the range of 1 to 3 angstrom, it is clear that the peptidyl transferase center is a ribozyme that is it is an enzyme composed of RNA. How does the 23S rRNA catalyze peptide bond formation? Means what is the mechanism of this catalysis? The exact mechanism remains to be determined but some answers to this question are beginning to emerge. First, base pairing between the 23S rRNA and the CCA ends of the tRNAs in the A and the P sites help to position the alpha amino group of the amino acid tRNA to attack the carbonyl group of the growing polypeptide attached to the peptidyl tRNA. These interactions are also likely to stabilize the amino acid tRNA after accommodation. Because close proximity of substrates is rarely sufficient to generate high levels of catalysis it is hypothesized that other elements of the ribosomal rRNA change the chemical environment of the peptidyl transferase active site. So, substrates close proximity may not be sufficient to get catalysis so this means that there are some other factors of the ribosomal rRNA which change this chemical environment and this change leads to the catalysis. For example, it has been proposed that nucleotides in the peptidyl transferase center accept a hydrogen from the alpha amino group of the amino acid tRNA making the associated nitrogen a strong nucleophile so this is a common mechanism used by many proteins and enzymes to stimulate nucleophilic attack of carbonyl groups.