Determination by systematic deletion of the amino acids essential for catalysis by ricin A chain.

The A chain of ricin is a RNA N-glycosidase that inactivates ribosomes by depurination of a single adenosine in 28S rRNA. Of the 267 amino acids in the protein, 222 (83%) could be deleted from one or another of 74 mutants without loss of the capacity of the protein to recognize a single nucleotide from among the 7000 in ribosomes or to catalyze hydrolysis.