Fig. 1.

Examples of fimbrial gene regulation involving invertible DNA elements, DNA methylation, and DNA-protein as well as protein-protein interactions. (A) The expression of the E. coli type 1 fimbrial gene cluster (fim) is dependent on the orientation of the fimS invertible DNA fragment flanked by two inverted repeat sequences (IR). Recombination at these sites is mediated by the fimbria-specific recombinases FimB and FimE. In addition, the global regulators IHF and Lrp facilitate fim gene transcription, whereas H-NS binding inhibits gene expression, depending on the orientation of the fragment. (B) E. coli pap gene expression is dependent upon the methylation status of two GATC sites upstream of the pap operon. Lrp and Dam compete for binding at these sites, and the pap-encoded protein PapI is a positive regulator of gene expression. (C) Expression of the Salmonella type 1 fimbrial gene cluster (fim) is controlled by two fim-encoded positive activators (FimZ and FimY) and one negative regulator (FimW). FimZ is a DNA binding protein and also interacts with FimW. The Fim regulators also control expression of the three regulatory genes via feedback loops. In addition, Lrp has been shown to bind to the promoter region of fimZ to affect its expression.