Puromycin as inhibitor of translation. (a) Puromycin inhibits translation by binding to the A site and mimicking the 3′ end of an aminoacyl-tRNA. The puromycin participates in the peptidyl transfer reaction, but because it is not bound to a tRNA, it is not anchored to the ribosome. (b) Crystal structure showing puromycin bound to the peptidyl transferase center of the bacterial 50S ribosomal subunit.