The amino-acid sequence of two non-toxic mutants of diphtheria toxin: CRM45 and CRM197

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Abstract
The amino-acid sequences of two diphtheria toxin-related, non-toxic proteins, CRM45 and CRM197, were deduced from the complete sequence of their genes: tox 45 and tox 197. CRM45 lacks the last 149 C-terminal amino-acid residues, but is otherwise identical to diphtheria toxin: a single C→T transition introduces an “ochre” (TAA) termination signal in tox 45, after the codon for threonine-386. A single G→A transition was also found in tox 197, leading to the substitution of glycine-52, present in the wild-type toxin, with glutamic acid in CRM197. This aminoacid change is responsible for the loss of the NAD:EF2 ADP-riboayltransferase activity in CRM197, due moat probably to an alteration of the NAD+ binding site.