Polymerization Factors in Yeast

Abstract

It is now apparent that elongation of the peptide chain on the ribosomes requires the participation of several protein factors (elongation factors) which have been isolated from the soluble fraction of the cell. Three elongation factors have been obtained from bacteria (1, 2) and designated as T_u, T_s, and G. Factor T (T_u plus T_s) is involved in the binding of the aminoacyl-tRNA to the ribosome-mRNA complex in a GTP-dependent reaction (2–4). This binding reaction is preceded by the formation of a ternary complex between factor T, GTP, and aminoacyl-t-RNA (5–8). Once the aminoacyl-tRNA has been bound to the ribosome, a peptide bond is formed between the carboxyl group of the carboxy-terminal amino acid of the peptidyl-tRNA (bound to the donor site of the ribosome) and the amino group of the newly bound amino acid. This reaction is catalyzed by peptidyl transferase, a protein located in the large ribosomal subunit (9). Elongation factor G then catalyzes the translocation of the nascent peptide chain from the acceptor to the donor site of the ribosome (10). A simultaneous release of the tRNA from the donor site occurs together with the movement of the ribosome along the mRNA (5). In this way, the acceptor site is ready to bind a new aminoacyl-tRNA molecule, and a new elongation cycle starts.