A genetic locus of enterocyte effacement (LEE) is conserved among diverse enterobacterial pathogens, including enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli O157:H7, and other bacteria that cause attaching and effacing (AE) lesions. This locus is located on the E. coli chromosome at the same site where a virulence gene block of uropathogenic E. coli is inserted. The LEE contains genes necessary for AE lesion formation and signal transduction. The eaeA gene encodes an outer-membrane adhesin essential for AE lesion formation, while eaeB is necessary for inducing signal transduction. Additional genes are required for AE lesion formation, as evidenced by transposon mutants that lack the AE phenotype. A 35-kb region of DNA encoding all necessary determinants for the AE phenotype was identified and is conserved among pathogens that produce AE lesions. This region is inserted into the E. coli chromosome at the same site as the virulence gene block of uropathogenic E. coli. The LEE is conserved among various pathogens, including EPEC, EHEC, RDEC-1, Hafnia alvei, and Citrobacter freundii biotype 4280. The LEE is located at a position downstream of the selC tRNA gene and is disrupted in uropathogenic E. coli strain 536 by the pai I locus. The LEE and pai I are distinct loci but share similar sequences at their junctions, suggesting evolutionary relatedness. The LEE is thought to have been acquired through horizontal gene transfer. The study provides evidence that the LEE encodes factors directly involved in AE lesion formation and their secretion. The findings highlight the importance of the LEE in the pathogenesis of EPEC and other enterobacterial pathogens.A genetic locus of enterocyte effacement (LEE) is conserved among diverse enterobacterial pathogens, including enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli O157:H7, and other bacteria that cause attaching and effacing (AE) lesions. This locus is located on the E. coli chromosome at the same site where a virulence gene block of uropathogenic E. coli is inserted. The LEE contains genes necessary for AE lesion formation and signal transduction. The eaeA gene encodes an outer-membrane adhesin essential for AE lesion formation, while eaeB is necessary for inducing signal transduction. Additional genes are required for AE lesion formation, as evidenced by transposon mutants that lack the AE phenotype. A 35-kb region of DNA encoding all necessary determinants for the AE phenotype was identified and is conserved among pathogens that produce AE lesions. This region is inserted into the E. coli chromosome at the same site as the virulence gene block of uropathogenic E. coli. The LEE is conserved among various pathogens, including EPEC, EHEC, RDEC-1, Hafnia alvei, and Citrobacter freundii biotype 4280. The LEE is located at a position downstream of the selC tRNA gene and is disrupted in uropathogenic E. coli strain 536 by the pai I locus. The LEE and pai I are distinct loci but share similar sequences at their junctions, suggesting evolutionary relatedness. The LEE is thought to have been acquired through horizontal gene transfer. The study provides evidence that the LEE encodes factors directly involved in AE lesion formation and their secretion. The findings highlight the importance of the LEE in the pathogenesis of EPEC and other enterobacterial pathogens.