Polypeptide Chain Initiation in E. coli: Sulfhydryl Groups and the Function of Initiation Factor F2

The activity of the chain initiation factor F₂ in promoting the messenger-dependent binding of formylmethionyl-transfer RNA (fMet∼tRNA_f) to purified E. coli ribosomes is inhibited by sulfhydryl-binding reagents, such as N-ethylmaleimide or p-hydroxymercuribenzoate, but prior incubation with guanosine triphosphate (GTP) or with ribosomes largely prevents this inhibition. The effect of GTP suggests that it forms a complex with F₂ whereby "active" sulfhydryl groups become sheltered. Experiments on the time course of the binding reaction, with and without preincubation of F₂ with GTP, and gel filtration experiments with ³H-labeled GTP lend support to this suggestion.