Sieves in Sequence

The enzymes that load amino acids onto their respective tRNAs in preparation for protein synthesis must do so with high accuracy. Now a structure of the synthase for isoleucine reported by Nureki et al . in this issue shows how this is accomplished. Fersht describes in his commentary that the structure confirms a proposal he made previously that the enzyme uses a double-sieve method for ensuring that isoleucine and not the smaller amino acid valine is used in the reaction. Two biochemical steps serve the same function as would the use of two different-size mesh screens in sorting sand.