Fig. (6) Enzymatic binding of [³H]Phe-tRNA^Phe to the ribosomal A-site of poly(U)-programmed ribosomes reconstituted with wild-type or mutant rps bL12. The 100% activity corresponds to the enzymatic binding of [³H]Phe-tRNA^Phe to the wild-type ribosomes obtained by the combination of equimolar amounts of the E. coli small (30S) and large (50S) ribosomal subunits in the presence of EF-Tu•GTP. Non enzymatic binding was deduced (Materials and Methods).