Polypeptide chain initiation in E. coli: sulfhydryl groups and the function of initiation factor F2.

The activity of the chain initiation factor F(2) in promoting the messenger-dependent binding of formylmethionyl-transfer RNA (fMet approximately 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 F(2) whereby "active" sulfhydryl groups become sheltered. Experiments on the time course of the binding reaction, with and without preincubation of F(2) with GTP, and gel filtration experiments with (3)H-labeled GTP lend support to this suggestion.