Exotoxin A of Pseudomonas aeruginosa (PA toxin) inhibits intracellular protein synthesis in mammalian cells by a very similar if not identical mechanism to diphtherial toxin (Iglewski et al., 1976; Iglewski & Kabat, 1975; Iglewski et al., 1977). This inhibition of protein synthesis requires NAD and results in the inactivation of elongation factor 2 (EF-2). Specifically, PA toxin or its fragment A (Vasil et al., 1977; Chung & Collier, 1977) like the fragment A of diphtherial toxin (Collier, 1975) catalyses the transfer of the adenosine diphosphate ribose moiety (ADP-ribose) of NAD on to EF-2. The resultant ADP-ribose-EF-2 complex is inactive in protein synthesis. There are also similarities between PA toxin and diphtherial toxin with regard to structure-function relationships (Vasil et al., 1977; Chung & Collier, 1977). PA toxin (mol. wt 71 500) and diphtherial toxin (mol. wt 63000) are both single polypeptides and are found in culture supernatants in a toxic but nonenzymically active form (proenzyme). Each toxin consists of an enzymically active component (ADP-ribosyl transferase) necessary for intracellular inhibition of protein synthesis and a carrier component termed fragment B (binding) which is thought to be responsible for making initial contact with a cell surface receptor. It should be noted that PA toxin can be separated into A and B fragments, but it is not clear whether this type of fragmentation is actually required for expression of toxicity of PA toxin as it appears to be for diphtherial toxin (Vasil et al., 1977; Leppla, 1976).

The A fragments of PA toxin and diphtherial toxin have been isolated and characterized (Collier, 1975) but the relative insolubility of the B fragments in the absence of dispersing agents has limited the study of these components (Collier, 1975; Vasil et al., 1977). It was recently reported (Zanen et al., 1976) that the B fragment of diphtherial toxin remains soluble after extensive dialysis against a 0.02 M-borate buffer. The fragment B of diphtherial toxin so obtained can compete with the intact toxin for the specific receptors on susceptible cells (Zanen et al., 1976; Everse et al., 1977). If the B fragment is added in sufficient quantities it can reduce the cytotoxicity and/or the inhibition of cellular protein synthesis caused by diphtherial toxin (Everse et al., 1977; Zanen et al., 1976). Similar competition studies have also been done using a non-toxic protein (CRM 197) which cross-reacts with diphtherial toxin and contains an inactive fragment A and a functional fragment B (Uchida et al., 1973). Thus far, no one has successfully isolated the B fragment of PA toxin, and non-toxic CRM proteins (similar to CRM 197) are not yet available. While the available literature pertaining to the action of diphtherial toxin on intact cells and experimental animals is extensive (see review by Solotorovsky & Johnson, 1970), there is only a limited amount of information regarding the cellular and animal specificities of* Present address: Department of Microbiology and Immunology, University of Colorado School of Medicine, Denver, Colorado 80262, USA