TRANSLATION OF GENETIC MESSAGE .7. ROLE OF INITIATION FACTORS IN FORMATION OF CHAIN INITIATION COMPLEX WITH ESCHERICHIA COLI RIBOSOMES

Washing of Escherichia coli Q13 ribosomes with 1.0 m ammonium chloride yields, in addition to the known initiation factors F1and F2, a third factor F3. All three factors are required for translation of natural messenger RNA (mRNA), e.g., phage RNA, in the cell-free E. coli system. They function in formation of the polypeptide chain initiation complex. Incubation of purified ribosomes with 32P-labeled Qβ phage RNA and methionine-14C-labeled formylmethionyl-transfer RNA, at low magnesium ion concentrations, followed by zonal centrifugation analysis in sucrose gradients, showed that formation of the initiation complex involves at least two steps: (1) The F3-dependent binding of mRNA to the ribosome, and (2) the F2-dependent binding of formylmethionyl-transfer RNA to the mRNA-ribosome complex to form the initiation complex proper. © 1968.