Autoactivation and environmental regulation of bfpT expression, the gene coding for the transcriptional activator of bfpA in enteropathogenic Escherichia coli

Expression of bfpA, the gene coding for the structural subunit of the bundle-forming pili (BFP) in enteropathogenic Escherichia coli(EPEC), requires the product of bfpT(also called perA), a member of the AraC family of transcriptional regulators. Here, we show that bfpT-cat fusions were not expressed in a bfpT(-) or in a non-EPEC strain, unless a functional bfpT was present, indicating that an autoregulatory mechanism is involved in expression. Further experiments with bfpT-cat fusions and primer extension analysis showed that bfpT is transcribed from a conventional sigma-70 promoter and that it is expressed throughout the growth curve. It is regulated in response to the ammonium concentration, temperature and growth media, in the same proportions as those described previously for bfpA. In addition, bfpT and bfpA expression was also modulated by osmolarity, but was not affected by pH, iron excess or limitation. Deletion analysis of the bfpT upstream region revealed that a DNA segment of 81 bp, extending upstream from the transcriptional start site, contained all the sequence elements required for maximal expression of bfpT. Furthermore, it shares significant homology with a bfpA upstream AT-rich region, which has been shown to be involved in the BfpT-dependent regulation of bfpA. Interestingly, ammonium repression was observed only when bfpT-cat or bfpA-cat expression was complemented in an EPEC background, whereas low-temperature regulation was observed in both EPEC and non-EPEC strains. This suggests that specific regulatory elements are present in EPEC, while others are shared with non-pathogenic E. coli.