Use of Phenylalanyl-tRNA—Guanosine-5′-Triphosphate—TI_u Complex in Peptide Bond Formation

Abstract

IT has been demonstrated^1–10 that in bacterial systems a complex, designated complex II, that is composed of aminoacyl-tRNA, GTP, and the heat labile transfer factor TI_U^* serves as an intermediate in the binding of aminoacyl-tRNA to ribosomes and in the elongation of polypeptide chains. When Phe-tRNA from complex II is bound to a ribosome-poly U complex carrying AcPhe-tRNA at the donor site, most of the Phe-tRNA bound to the ribosomes interacts with the AcPhe-tRNA giving rise to AcPhe-Phe-tRNA at the acceptor site^6,10,12,13. Phe-tRNA from complex II is also bound at the acceptor site of a ribosome–poly U complex having an unoccupied donor site, but in this case very little dipeptide is formed^9,10. In both instances, approximately one GTP from complex II is cleaved for each Phe-tRNA bound to the ribosomes^9,10,14,15. Lack of dipeptide formation is also observed in the presence of the antibiotic, sparsomycin^6,13,16, or when the GTP analogue 5′-guanylmethylene diphosphonate is substituted for GTP^12,13. Our results indicate that almost none of the Phe-tRNA from complex II bound to the acceptor sites before the addition of AcPhe-tRNA to donor sites is incorporated in peptide chain.