The Integrity of the Sarcin/Ricin Domain of 23 S Ribosomal RNA Is Not Required for Elongation Factor-independent Peptide Synthesis

The elongation stage of protein synthesis consists of repeated cycles of the binding of aminoacyl-tRNA, peptide bond formation, and translocation. The process is normally catalyzed by the elongation factors Tu and G; however, the reactions can proceed, at least in prescribed and limited circumstance, in the absence of the elongation factors, a finding that strongly implies that the chemistry of protein synthesis is inherent in the ribosome. The sarcin/ricin domain in 23 S rRNA, the site of inactivation of ribosomes by ribotoxins, is where the elongation factors bind. The question that arises is whether the sarcin/ricin domain is necessary for factor-independent peptide synthesis. The answer is that it is not. The disruption of the sarcin/ricin domain by covalent modification with either sarcin or pokeweed antiviral protein did not affect factor-independent peptide synthesis; nor did lethal mutations of nucleotides that abolish the binding of elongation factors. The results imply that the sole function of the sarcin/ricin domain is to provide a binding site for the elongation factors and, hence, to facilitate the elongation reactions. The results also raise the possibility of the co-evolution of the sarcin/ricin domain and the elongation factors.