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 Cla I/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.