The codon specificity of eubacterial release factors is determined by the sequence and size of the recognition loop

RF1 recognition loop residue substitutions affect codon recognition and specificity. (A) The RF1 SPFD recognition loop variants. Substitutions are shown in black. (B) The codon-dependent peptidyl-tRNA hydrolysis activities of the SPFD recognition loop variants at the standard stop codons UAG, UAA, and UGA, and the sense codon UGG. The RF variants were assayed and the data analyzed as described for Figure 1C. (C) Codon specificity of the RF1 SPFD(ItoR) recognition loop. The SPFD(ItoR) variant was assayed with the codons indicated and the data analyzed as for Figure 1C. Statistical significances, (**) P < 0.01, (***) P < 0.001 (Student's two-tailed t-test), were determined by comparison to the activity of RF1-SPFD(ItoR) with no codon.