Construction and Expression of the Genes for Neurotoxins and Non-Toxic Components in C. Botulinum Types C and E

Abstract

Botulinum toxins are classified into seven groups (type A to G) based on their antigenicity. Type C 1 (or C) toxin exists in large molecular sizes of 12S (300 kDa) and 16S (500 kDa) in culture supernatants or in an acid condition, and type E toxin exists in 12S size (Fig. 1). These large (progenitor) toxins are formed by association of the 7S neurotoxin (150 kDa) with a nontoxic component(s). In an alkaline condition, the 12S and 16S toxins dissociate into neurotoxin and nontoxic components. The neurotoxin is produced as a single polypeptide chain and is separated into two fragments, designated as the heavy (H) chain (100 kDa) and the light (L) chain (50 kDa), by reduction of a disulfide bond. On the contrary, the molecular constitution of nontoxic components is not clear. The nontoxic component of the 16S toxin shows hemagglutinating activity, but that of 12S toxin does not. It has been postulated⁾ that the nontoxic component of 16S toxin is made up by conjugation of the nontoxic component of 12S toxin (designated as nontoxic-nonHA) with hemagglutinin (HA), and that this conjugation is not separated in an alkaline condition. Molecular mass (Mr) of nontoxic-nonHA component is approximately 120 – 140 kDa in any progenitor toxins, but that of HA is not clear because the preparation of HA alone has not yet been isolated. Suzuki et al. ² reported that Mr. of the nontoxic component of type C 16S toxin was 240 kDa and that it was dissociated into five components (27, 35, 55, 115, and 120 kDa), indicating that HA consisted of several subcomponents. Ohishi et al. ³ clarified that the nontoxic components were necessary to maintain the oral toxicity or to cause food poisoning because they prevented the neurotoxin from degradation by gastric juice at a low pH.