Hierarchy in the expression of the locus of enterocyte effacement genes ofenteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) elicit changes in host cell morpho logy and cause actin rearrangement, a phenotype that has commonly been refe rred to as attaching/effacing (AE) lesions. The ability of EPEC to induce A E lesions is dependent upon a type III protein secretion/translocation syst em that is encoded by genes clustered in a 35.6 kb DNA segment, named the l ocus of enterocyte effacement (LEE). We used transcriptional fusions betwee n the green fluorescent protein (gfp) reporter gene and LEE genes rorf2, or f3, orf5, escJ, escV and eae, together with immunoblot analysis with antibo dies against Tir, intimin, EspB and EspF, to analyse the genetic regulation of the LEE. The expression of all these LEE genes was strictly dependent u pon the presence of a functional integration host factor (IHF). IHF binds s pecifically upstream from the ler (orf1) promoter and appears to activate e xpression of ler, orf3, orf5 and rorf2 directly. The ler-encoded Ler protei n was involved in activating the expression of escJ, escV, tir, eae, espB a nd espF. Expression of both IHF and Ler was needed to elicit actin rearrang ement associated with AE lesions. In conclusion, IHF directly activates the expression of the ler and rorf2 transcriptional units, and Ler in turn med iates the expression of the other LEE genes.