Molecular characterization of an enterotoxin from Salmonella typhimurium

In this study, we describe the molecular and antigenic characteristics of a cloned enterotoxin from Salmonella typhimurium strain Q1. The full length Salmonella enterotoxin gene (stn), localized on a 2.8 kb ClaI/PstI DNA fragment, was cloned from a genomic library of Salmonella. Based on nucleotide sequence analysis, the stn gene contained 749 bp that would encode a protein having a molecular size of 29,073. The most unusual feature of the stn gene was the presence of a rare initiation codon (TTG) in lieu of the typical ATG codon, which required site-directed mutagenesis to confirm the precise initiation site. The expression of the stn gene in a bacteriophage T7 RNA polymerase/promoter system was enhanced by introducing a typical ATG start codon and an optimal Shine-Dalgarno sequence upstream of the stn gene by site-directed mutagenesis. The stn gene was located opposite the hydHG operon that regulates labile hydrogenase activity in Salmonella species and Escherichia coli. The overall amino acid sequence of the enterotoxin was quite dissimilar to any other published sequence, including cholera toxin or other adenylate cyclase-activating proteins. However, an intriguing similarity in a small region of the amino acid sequence of Stn was observed with portions of the amino acid sequences from several other protein toxins known to ADP-ribosylate host cell proteins. This region of homology may indicate a conserved motif, within the active site, that is involved in the stimulation of adenylate cyclase activity.