Transcriptional autoregulation by quorum sensing Escherichia coli regulators B and C (QseBC) in enterohaemorrhagic E. coli (EHEC)

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Abstract
The cell-to-cell communication system referred to as quorum sensing (QS) is based on the principle that bacteria secrete hormone-like compounds referred to as autoinducers. Upon reaching a threshold concentration, these autoinducers interact with transcription factors to regulate gene expression. We previously reported that enterohaemorrhagic Escherichia coli (EHEC), which is responsible for outbreaks of bloody diarrhoea, utilizes a QS system to regulate gene transcription. We have also previously shown that the quorum sensing E. coli regulators B and C (QseBC) may act as a two-component system to transcriptionally regulate the expression of flagella and motility. Here, using a reverse transcription polymerase chain reaction (RT-PCR), we show that qseBC are transcribed in an operon. Furthermore, using a qseBC::lacZ transcriptional fusion, we observed that QseB autoactivates its own transcription. In addition, the transcriptional start site of the qseBC promoter responsive to QseBC was mapped, and single-copy and multicopy deletion analyses were performed to determine the minimal region necessary for QseB transcriptional activation. These data allowed us to map an additional transcriptional start site for the qseBC promoter which may allow for a basal level of QseBC expression. Finally, electrophoretic mobility shift assays, competition experiments and DNase I footprints were performed and demonstrated that QseB directly binds to two sites in its own promoter. These results indicate that QseB may act to autoregulate its own transcription through binding to low- and high-affinity sites found in its promoter.