A Model of the tRNA Binding Sites on the Escherichia Coli Ribosome

In the course of the elongation cycle of translation, three different functional forms of tRNA bind to mRNA-programmed ribosomes. Two of them, aminoacyl-tRNA and peptidyl-tRNA, participate in the formation of the peptide bond while accommodated in the A (acceptor) and P (peptidyl) sites. A third form, deacylated tRNA, a product of the transpeptidation reaction, is transferred from the P site to the E (exit) site during translocation before it leaves the ribosome. Since binding of tRNA to the ribosome is essential for protein synthesis, numerous studies have been designed to delineate the topography of the ribosomal A, P and E sites (for review see Cooperman, 1980; Ofengand et al., 1986; Cooperman, 1987).