Site‐directed mutagenesis of ricin A chain Trp 211 to Phe

Oligonucleotide-directed site-specific mutagenesis has been used to make a conservative change in the putative active site of the catalytic subunit of the toxin ricin. The substitution of phenylalanine for tryptophan at position 211 of the A chain reduces but does not abolish catalytic activity, as assayed by inhibition of cell-free protein synthesis.