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 bundl e-forming pili (BFP) in enteropathogenic Escherichia coli(EPEC), requires t he product of bfpT(also called perA), a member of the AraC family of transc riptional regulators. Here, we show that bfpT-cat fusions were not expresse d in a bfpT(-) or in a non-EPEC strain, unless a functional bfpT was presen t, indicating that an autoregulatory mechanism is involved in expression. F urther experiments with bfpT-cat fusions and primer extension analysis show ed 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 prop ortions as those described previously for bfpA. In addition, bfpT and bfpA expression was also modulated by osmolarity, but was not affected by pH, ir on excess or limitation. Deletion analysis of the bfpT upstream region reve aled that a DNA segment of 81 bp, extending upstream from the transcription al start site, contained all the sequence elements required for maximal exp ression of bfpT. Furthermore, it shares significant homology with a bfpA up stream AT-rich region, which has been shown to be involved in the BfpT-depe ndent regulation of bfpA. Interestingly, ammonium repression was observed o nly when bfpT-cat or bfpA-cat expression was complemented in an EPEC backgr ound, whereas low-temperature regulation was observed in both EPEC and non- EPEC strains. This suggests that specific regulatory elements are present i n EPEC, while others are shared with non-pathogenic E. coli.