FIGURE 2 The ribosome does not proofread aminoacylated tRNAs. (a) The ¹⁴C-labeled Cys residue of Cys-tRNA^Cys is converted to an Ala residue by Raney nickel, which removes the sulfur from cysteine. (b) Polypeptide synthesis with and without poly(UG) mRNA, using correctly charged [¹⁴C]Cys-tRNA^Cys, or correctly charged [¹⁴C]Ala-tRNA^Ala (as control), or mischarged [¹⁴C]Ala-tRNA^Cys. The results in the presence of UGU codons (dark purple columns on the right) show incorporation of labeled Cys with Cys-tRNA^Cys (left); negligible incorporation of labeled Ala with Ala-tRNA^Ala (middle); and incorporation of labeled Ala with Ala-tRNA^Cys (right).