Puromycin reactivity of pretranslocation ribosomes incubated with EF-G in the presence of various nucleotides and phosphate analogues. Deacylated tRNAMet was bound to the P site, and N-acetyl-[3H]Tyr-tRNATyr was bound to the A site of the ribosome. Bar graphs indicate the fraction of ribosome-bound N-acetyl-[3H]Tyr-tRNATyr, measured by a filter-binding assay, that is puromycin reactive in pretranslocation ribosomes in the absence of EF-G (a) or pretranslocation ribosomes incubated with EF-G in the presence of various nucleotides and phosphate analogues (as indicated) (b–l). EF-G was added with GTP (b, j), in the absence of any nucleotides (c), with GDP (d), with GDP and KF (e), with GDP and sodium meta vanadate (f), with GDPNP (g), with GDP and beryllium fluoride (h, k) or with GDP and aluminum fluoride (i, l). In (j–l) pretranslocation ribosomes were pre-incubated with viomycin (0.5 mM). Error bars show standard deviations calculated from four to six independent measurements.