2. Protein Synthesis—Peptide Chain Elongation

Abstract

This chapter discusses the process of peptide chain elongation. For studies on the mechanism of polypeptide chain elongation extensive use has been made of the model system in which polyuridylic acid, poly(U), directs the formation of polyphenylalanyl-tRNA. The requirements for this in vitro system are high salt washed ribosomes, Phe-tRNA, poly (U), guanosine triphosphate, Mg2+, and NH4+ ions, a sulfhydryl reagent such as dithiothreitol and three protein factors which are present in the supernatant fraction of lysed cells. In peptide chain elongation, Protein growth is accomplished by a cyclic process involving aminoacyl-tRNA binding, peptidyl transfer, peptidyl tRNA translocation, and exposure of a new triplet codon through movement of the ribosome on mRNA. These factors have been recently designated as elongation factor thermo unstable (EF-Tu), elongation factor thermo stable (EF-Ts), and elongation factor G (EF-G), the new symbols being intended to replace the various designations used for the factors in different laboratories. When isolated from the soluble fraction of the cell, EF-Tu and EF-Ts are associated, and this complex is referred to as EF-T. The respective factors from various bacterial species may be interchanged in the partial reactions of peptide chain elongation as well as in the overall polymerization reaction. The amount of EF-T and EF-G in the bacterial cell is a significant percentage of the total soluble proteins. The EF-G content of Escherichia coli cells has been estimated under different growth conditions to be 2-3 % and 6% and the EF-T content as 2% and 3%. The relative amount of each factor compared to ribosomes remains constant at different growth rates, suggesting that the synthesis of the polypeptide chain elongation factors is coordinated with that of ribosomes.