Evidence that a new enterotoxin of Escherichia coli which activates adenylate cyclase in eucaryotic target cells is not plasmid mediated.

Escherichia coli SA53 produces a new enterotoxin that has a biological activity similar to that of E. coli heat-labile enterotoxin (LT) but is not neutralized by antiserum against LT or cholera enterotoxin. Strain SA53 contained two plasmids, pRB1 (69.2 +/- 4.3 megadaltons) and pRB2 (57.6 +/- 5.3 megadaltons). Studies were undertaken to determine whether either plasmid was required for production of the LT-like toxin. We isolated a derivative of SA53 lacking both plasmids and confirmed that radioactively labeled pRB1 and pRB2 DNAs failed to hybridize to total DNA digests of the cured strain. The new enterotoxin was still produced by the cured strain, demonstrating that the gene(s) encoding the toxin was not located on pRB1 or pRB2 and was most likely on the bacterial chromosome. Although sonic extracts from SA53 contained no detectable LT antigen, plasmid pRB1 DNA did contain sequences with partial homology to the LT-A and LT-B genes. No sequence homology with LT genes was detected with pRB2 DNA. When the enterotoxin plasmid pCG86 was introduced into a rifampin-resistant derivative of SA53, LT was produced. Thus, plasmid-coded LT could be produced in the E. coli SA53 host, and the sequences homologous to LT in pRB1 were cryptic.