A Novel Bvg-Repressed Promoter Causes vrg -Like Transcription of fim3 but Does Not Result in the Production of Serotype 3 Fimbriae in Bvg ⁻ Mode Bordetella pertussis

In Bordetella pertussis , the promoter P fim3 -15C for the major fimbrial subunit gene fim3 is activated by the two-component system BvgAS in the Bvg + mode but not in the Bvg − mode. However, many transcriptional profiling studies have shown that fim3 is transcribed in the Bvg − mode even when P fim3 is in a nonpermissive state (P fim3 -13C), suggesting the presence of a reciprocally regulated element upstream of P fim3 . Here, we provide evidence that BRP is the cause of this anomalous behavior of fim3 . Although BRP effects vrg -like transcription of fim3 in the Bvg − mode, it does not lead to stable production of FIM3 fimbriae, because expression of the chaperone and usher proteins FimB and FimC occurs only in the Bvg + mode.