Conformational changes in ribosomes during protein synthesis.

In a purified system containing poly(U) and ribosomal subunits from Escherichia coli, and purified transfer factors T and G, the active ribosomal complex passes through a cycle of contraction and expansion with the addition of each amino acid; aminoacyl-tRNA binding catalyzed by T produces the stable compact state, corresponding to the 70S conformation, whereas translocation with G expands the ribosome to a less stable 60S form. It is also shown that formation of the first peptide bond must be preceded by translocation with G. These findings are consistent with a model of chain initiation in the absence of initiation factors in which deacylated tRNA(Phe) bound to the P site signals translocation by G and GTP as soon as the 60S initiation complex has been converted to the 70S form by the enzymatic binding of Phe-tRNA.