Models for the binding of the sarcin–ricin loop (SRL) of 28S ribosomal RNA to ricinAchain (RTA) suggest that several surface exposed arginine residues surrounding the active site cleft make important interactions with the RNA substrate. The data presented in this study suggest differing roles for these arginyl residues. Substitution of Arg48 or Arg213with Ala lowered the activity of RTA 10-fold. Furthermore, substitutionofArg213withAsp lowered theactivityofRTA 100-fold. The crystal structure of thisRTAvariant showed it to have an unaltered tertiary structure, suggesting that the positively charged state of Arg213 is crucial for activity
