In protein synthesis, peptide bond formation requires that the tRNA carrying the amino acid (A site tRNA) contact the tRNA carrying the growing peptide chain (P site tRNA) at their 3′ termini. Two models have been proposed for the orientations of two tRNAs as they would be bound to the mRNA in the ribosome. Viewing the tRNA as an upside down L, anticodon loop pointing down, acceptor stem pointing right, and calling this the front view, the R (Rich) model would have the back of the P site tRNA facing the front of the A site tRNA. In the S (Sundaralingam) model the front of the P site tRNA faces the back of the A site tRNA. Models of two tRNAs bound to mRNA as they would be positioned in the ribosomal A and P sites have been created using MC-SYM, a constraint satisfaction search program designed to build nucleic acid structures. The models incorporate information from fluorescence energy transfer experiments and chemical crosslinks. The models that best answer the constraints are of the S variety, with no R conformations produced consistent with the constraints.
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