Amino-acid sequence of fragment A, an enzymically active fragment from diphtheria toxin.

The amino-acid sequence of Fragment A from diphtheria toxin is reported. Fragment A (molecular weight, Mr, 21,145) is the major enzymically active fragment produced upon activation of the intact toxin (Mr about 60,000) by limited tryptic digestion and reduction. It, or a similar fragment, is believed responsible for the inhibition of protein synthesis in animal cells exposed to the toxin. Fragment A, which corresponds to the amino terminus of the toxin, is shown here to consist of three major forms (190, 192, and 193 residues) resulting from cleavage by trypsin adjacent to any of three closely spaced arginine residues. All three forms are enzymically active.