2 HIGHLY RELATED REGULATORY PROTEINS, SHIGELLA-FLEXNERI VIRF AND ENTEROTOXIGENIC ESCHERICHIA-COLI RNS, HAVE COMMON AND DISTINCT REGULATORY PROPERTIES

The Rns protein of enterotoxigenic Escherichia roll (ETEC) and the Vir F protein of Shigella flexneri are members of the AraC family of trans cription regulators. Rns is required for positive activation of the CS 1 fimbrial genes, while VirF is a positive regulator of an invasion ge ne regulon. The amino acid sequences of the proteins are 36% identical , and both proteins activate transcription in response to increases in temperature. Here, we show that Rns is capable of complementing fully a null mutation in the S. flexneri virF gene. However, the VirF prote in cannot replace Rns as an activator of CS1 gene expression in ETEC. This failure is not due to the absence from ETEC of a co-factor requir ed by VirF since it also occurs when the CS1 system is moved into an S . flexneri genetic background. Nor is it a function of growth medium c omposition or a failure in virF gene expression. Instead, these findin gs point to important differences in the mechanisms by which these rel ated transcription factors regulate gene expression in Gram-negative p athogens. (C) 1998 Federation of European Microbiological Societies. P ublished by Elsevier Science B.V.