Fig. (3) Poly(U)-dependent poly(Phe) synthesis activity of E. coli 70S ribosomes reconstituted with rps (L7/L12)₂ or (L7)₄. Purified E. coli 30S subunits (0.65 µM) and purified 50S subunits (0.65 µM) untreated or depleted from rp bL12 were incubated for 10 min at 37°C in the presence of 10 µM purified rps (L7/L12)₂ or (L7)₄ in 15 µl of 50 mM Tris-HCl pH 7.5, 30 mM KCl, 30 mM NH₄Cl, 10 mM MgCl_2, 1mM DTT, before adding all the components required for poly(Phe) synthesis, as described in Materials and Methods. The activity (expressed as the amount (pmol) of poly-[¹⁴C]Phe synthesized) was determined after 12 min incubation at 30°C in 35µl total reaction mixture.