A plasmid-encoded regulatory gene, rns, required for expression of the CS1 and CS2 adhesins of enterotoxigenic Escherichia coli.

To be virulent, enterotoxigenic Escherichia coli (ETEC) must produce a toxin and a pilus-like structure that mediates specific attachment to host tissue. Expression of two of these specific adherence structures, CS1 and CS2, requires the presence of a plasmid in an ETEC strain of a particular serotype and biotype. We show here that this plasmid does not contain the structural gene for a pilin protein, as previously believed. Instead we have identified a plasmid-encoded gene called rns that is required for expression of CS1 or CS2 colonization factor antigens and for adhesion. The rns gene, defined by two separately isolated insertion mutations, produces a 26-kDa protein when transcribed and translated in vitro. At the protein level the rns gene product is homologous to AraC, a positive regulator of the arabinose operon of enteric bacteria, and to RhaR and RhaS, which regulate the rhamnose operon of E. coli. The homology of the Rns protein to AraC is localized to regions that are believed to bind to DNA. Moreover, the sequence of one of these homologous regions is consistent with a DNA binding helix-turn-helix motif. The average G + C content of E. coli DNA is 50%; yet the rns gene contains only 28% G + C, suggesting that it was acquired from some other organism.