= Ribosomal P site
The \b[ribosomal P site], or peptidyl site, is the <ribosome> binding position associated with the <transfer RNA> carrying the growing <peptide> before a peptide-transfer step. During initiation, the <initiator transfer RNA> is placed there directly, paired with the <start codon> of the <messenger RNA>. Its decoding contacts involve the <small ribosomal subunit>, while its acceptor end extends toward the <peptidyl transferase center> in the <large ribosomal subunit>.<Large-Scale Movements of IF3 and tRNA during Bacterial Translation Initiation by Tanweer Hussain and others>[Highlights and Introduction, initiator-tRNA accommodation in the P site]{ref}
For an original chain-length example, let a P-site <peptidyl-tRNA> carry four <amino acid residues>, and let an <aminoacyl-tRNA> at the <ribosomal A site> carry one. Forming the next <peptide bond> transfers the four-residue chain onto the incoming residue: the five-residue product is initially attached to the A-site carrier, leaving the P-site carrier deacylated. Subsequent movement positions the carriers for another cycle.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Substrate-analogue structures and peptide-bond synthesis]{ref}
Thus “P site” names a position in a changing reaction cycle. It does not imply that a <peptide> remains permanently attached to whichever <transfer RNA> first occupied that position.
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