Peptide chain termination: effect of protein S on ribosomal binding of release factors

THE PROTEIN FACTOR S, PREVIOUSLY SHOWN TO STIMULATE POLYPEPTIDE CHAIN TERMINATION IN BACTERIAL EXTRACTS, HAS TWO EFFECTS UPON THE COMPLEX FORMED BETWEEN RIBOSOMES, RELEASE FACTOR, AND TERMINATOR (TRINUCLEOTIDE) CODON: (1) in the absence of GTP or GDP, S stimulates formation of an [R.UAA.ribosome] intermediate, and (2) in the presence of GTP or GDP, S participates in dissociation of this intermediate. Factor S can stimulate fMet release from [fMet-tRNA(f).AUG.ribosome] intermediates in either the presence or absence of GTP or GDP. A model is proposed which relates the in vitro effects of S +/- GTP (or GDP) on fMet release to the effects of S +/- GTP (or GDP) on the binding and dissociation of R factor from ribosomes.