The molecular nature of heat-labile enterotoxin (LT) of Escherichia coli

ALTHOUGH Escherichia coli is part of the natural flora of the mammalian gut, certain strains cause a cholera-like disease in humans and young farm animals 1,2 . These E. coli strains carry at least two plasmid-mediated determinants of virulence 3,4 . One class of plasmids, called Ent, codes for the synthesis of one or more enterotoxins 3 . These toxins disrupt the fluid balance in the gut resulting in a net efflux of fluid and electrolytes from the gut epithelial cells 1 . The heat-labile toxin (LT) and cholera toxin have been shown to be identical in their modes of action 5 and also to be immunologically similar 6,7 . Although cholera toxin has been purified and characterised 8 , it has proved difficult to purify LT and it is not yet fully characterised. Molecular weight estimates for LT range from 20,000 to over 100,000 (refs. 9–11). We report here investigations of the molecular nature of LT carried out by studying an LT gene isolated using recombinant DNA technology 12 . Using E. coli minicells 13 , the proteins encoded by the LT gene were identified and the structure of LT was found to be similar to that of cholera toxin.