= Ribosomal A site
The \b[ribosomal A site], or aminoacyl site, is the <ribosome> position that accepts an incoming <aminoacyl-tRNA> during <biological translation>. Its <anticodon> is tested against a <messenger RNA> <codon> at the <small ribosomal subunit>, while its amino-acid-bearing end reaches the <peptidyl transferase center> of the <large ribosomal subunit>. The A site therefore spans coordinated decoding and chemical positions rather than being a hole in only one subunit.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Substrate-analogue complexes and A-site/P-site chemistry]{ref}
For an original chain-transfer example, a <peptidyl-tRNA> at the <ribosomal P site> carries eight <amino acid residues>, and an A-site <aminoacyl-tRNA> carries one. <Peptidyl transfer> produces a nine-residue chain attached to the A-site carrier. Movement of the <ribosome> then relocates the carriers and presents a new <codon> for the next incoming <transfer RNA>. Entry, bond formation and movement are distinct stages.
Occupancy also depends on the stage of <biological translation>. During bacterial initiation, <bacterial initiation factor IF1> binds in the small-subunit A site instead of an elongating <aminoacyl-tRNA>. A primary <X-ray crystallography> study found IF1 occluding that position and changing nearby <16S ribosomal RNA> bases.<Crystal structure of an initiation factor bound to the 30S ribosomal subunit by A. P. Carter and others>[Abstract, A-site occlusion and A1492/A1493 displacement]{ref} This explains why observing a bound molecule at the A site does not by itself identify an elongation-ready complex.
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