The peptidyltransferase center of Escherichia coli ribosomes: binding sites for the cytidine 3'-phosphate residues of the aminoacyl-tRNA 3'-terminus and the interrelationships between the acceptor and donor sites.

The substrate specificity of the acceptor site of peptidyltransferase of Escherichia coli 70 S ribosomes was investigated in Ac-Phe-tRNA . poly(U) . 70 S ribosome (system A) and tRNC-A-Phe . poly(U) . C-A-C-C-A-Phe . 70 S ribosome (system B) systems by using C-C-A-Gly, C-C-A-Phe, C-A-Gly and C-A-Phe as analogs of the 3'-terminus of aminoacyl-tRNA. It was found that an addition of CP residue to C-A-Gly and C-APhe resulted in an increase of the acceptor activity in system A; the increase is more remarkable for C-A-Gly than for C-A-Phe, while the acceptor activities of C-C-A-Gly and C-C-A-Phe are roughly similar. On the other hand, dramatically increased binding affinities of C-C-A-Phe and C-C-A-Gly relative to C-A-Phe and C-A-Gly for the A site of peptidyltransferase were observed in system B using an inhibition assay; C-C-A-Phe binds much more strongly than C-C-A-Gly. The results indicate the important role of the third CP residue and the aminoacyl moiety of the 3'-terminus of aminoacyl-tRNA in the interaction with the acceptor site of peptidyltransferase, as well as the existence of cooperative effects between A and P sites of peptidyltransferase. These effects, depending on an occupancy of P site, may significant the specificity of the peptidyltransferase A site.