ABSTRACT
The DTB fragment is located at the carboxyl-terminus of the molecule, and contains a second disulfide bond between cysteine residues 461 and 471. The most commonly used method to introduce mutations at random into the DT gene is to expose the corynephage-carrying tox to N-methyl-N'-nitro-N-nitrosoguanidine (NG) sometimes after induction of lysogeny by exposure to ultraviolet (UV) light. CRMs with no enzymatic activity, or with reduced activity when compared on an equimolar basis with wild-type DT have been isolated; in general, the ability of a mutant to ADP-ribosylate EF-2 is related to its toxicity to sensitive cells. The position and number of arginine residues in a polypeptide has been shown to be crucial for its ability to interact in the correct manner with lipid membranes; it is possible that joint membrane insertion of the carboxyl-terminal end of DTA and the amino-terminal end of DTB initiates the translocation process, but only if the A fragment contains a single C-terminus arginine residue.
