SalmoneUa typhi, the causative agent of typhoid fever, must invade the human gastrointestinal tract and multiply within the host to cause disease. We have cloned from S. typhi Ty2 a chromosomal region that confers upon Escherichia coli HB101 the ability to invade cultured human intestinal epithelial cells. Three invasion-positive recombinant cosmids were isolated and restriction endonuclease analyses of the inserts showed a 33-kilobase region of identity. Transmission electron microscopy of epithelial cells invaded by S. typhi Ty2 or E. coli HB101 carrying an invasion cosmid showed intracellular bacteria contained within endocytic vacuoles. One of the invasion cosmids was mutagenized with transposon TnS to identify the cloned sequences that are required for the invasive phenotype. Seven of 92 independent Tn5 insertions within the common 33-kilobase region eliminated invasive ability and revealed at least four separate loci that are required for invasion. Penetration of epithelial cells by Ty2 and HB101 carrying the cloned invasion determinants was inhibited by cytochalasin B and D, indicating that epithelial cell endocytosis of S. typhi is a microframent-dependent event. The invasion cosmids were found to carry the recA and srlC genes indicating that the cloned invasion determinants are located at about 58 minutes on the S. typhi chromosome. With a segment of the cloned S. typhi invasion region used as a probe, homologous sequences were isolated from Salmonella typhimurium. Two independent S. typhimurium recombinant cosmids containing the entire 33-kilobase common region identified in S. typhi were isolated, but these cosmids did not confer upon HB101 the ability to invade epithelial cells.

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Coliphage lambda gene expression is regulated temporally by systems of termination and antitermination of transcription. The lambda-encoded N protein (pN) acting with host factors (Nus) at sites (nut) located downstream from early promoters is the first of these systems to operate during phage development. We report observations on some of the components of this complex system that, in part, address the way in which these elements interact to render RNA polymerase termination-resistant. (1) The isolation of a conditionally lethal cold-sensitive nusA mutation demonstrates that NusA is essential for bacterial growth. (2) The effect on lambda growth in a host in which the Salmonella NusA protein is overproduced suggests that NusA is essential for N-mediated antitermination in phage lambda. (3) A truncated NusA product, representing only the amino two-thirds of the native protein, is active for both bacterial growth and pN action, indicating that the carboxy end of the molecule may not be a functionally important region. (4) lambda pN can function with the heterologous nut region from Salmonella typhimurium phage P22 when lambda pN is overproduced, demonstrating that lambda pN can function with the nut regions of other lambdoid phages. (5) A single base-pair change in the lambda nutR boxA sequence that was selected to permit a lambda derivative to utilize the Salmonella NusA protein restores lambda growth in the Escherichia coli nusA1 host.