mRNA A site E site 5s位 A位 chain E位 mRNA-binding Figure 20-2 Important Binding Sites on the Prokaryotic Ribosome This model of ribosome structure shows the A (aminoacyl)and P(peptidyl) mRNA sites as cavities on the ribosome where charged (amino acid-carryingltRNA nolecules bind during polypeptide synthesis. The more recently postulated E exit) site is the site from which discharged tRNAs leave the ribosome. The mRNA-binding site binds a particular nucleotide sequence near the 5 end of the mRNA, placing the mRNA in the proper position for the translation of its first codon (a)The diagrammatic representation of a ribosome that is used in this chapter. The pair of horizontal dashed lines indicate where the mRNA molecule lies. (b)A more realistic representation. The binding sites are all located at or near the interface between the large and small subunits27