SummaryThe bundle‐forming pili (BFP) of enteropathogenicEscherichia coli(EPEC) are required for the development of circumscribed colonies of bacteria attached to the surfaces of cultured epithelial cells, a process termed the localized adherence (LA) phenotype. Similar lesions are evident in jejunal biopsies from EPEC‐infected children. BFP production is not constitutive, but instead occurs upon transfer of bacteria from nutrient broth to tissue culture media, indicating that the expression of BFP may be environmentally regulated. To learn more about how BFP protein expression is induced during epithelial‐cell adherence,bfpA‐cattranscriptional fusions and northern blot analyses were employed to monitorbfpAexpression as a function of environmental signals and growth kinetics.bfpAexpression was found to be regulated at the transcriptional level, and to require a separate locus on the EPEC adherence factor (EAF) plasmid. Expression occurred selectively during exponential‐growth phase and was greatest between 35 and 37°C, and in the presence of calcium. Ammonium (20 mM) significantly reducedbfpAmRNA and protein expression and the development of the LA phenotype. Analysis of thebfpAupstream sequence and identification of the transcription initiation site revealed a conventional σ70‐dependent promoter and an AT‐rich tract that might affect promoter activity. Taken together, these findings further support the pathog
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