 Outside of the ribosome, EF-Tu complexed with GTP (EF-Tu • GTP) complexes with aa-tRNA to form a stable EF-Tu • GTP • aa-tRNA ternary complex.EF-Tu • GTP binds all correctly-charged aa-tRNAs with approximately identical affinity, except those charged with initiation residues and selenocysteine.This can be accomplished because although different amino acid residues have varying side-chain properties, the tRNAs associated with those residues have varying structures to compensate for differences in side-chain binding affinities.