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, two serologically distinct fimbriae, FIM2 and FIM3, undergo on/off phase variation independently of each other via variation in the lengths of C stretches in the promoters for their major subunit genes, fim2 and fim3. These two promoters are also part of the BvgAS virulence regulon and therefore, if in an on configuration, are activated by phosporylated BvgA (BvgA similar to P) under normal growth conditions (Bvg(+) mode) but not in the Bvg(-) mode, inducible by growth in medium containing MgSO4 or other compounds, termed modulators. In the B. pertussis Tohama I strain (FIM2(+) FIM3(-)), the fim3 promoter is in the off state. However, a high level of transcription of the fim3 gene is observed in the Bvg(-) mode. In this study, we provide an explanation for this anomalous behavior by defining a Bvg-repressed promoter (BRP), located approximately 400 bp upstream of the Pfim3 transcriptional start. Although transcription of the fim3 gene in the Bvg(-) mode resulted in Fim3 translation, as measured by LacZ translational fusions, no accumulation of Fim3 protein was detectable. We propose that Fim3 protein resulting from translation of mRNA driven by BRP in the Bvg(-) mode is unstable due to a lack of the fimbrial assembly apparatus encoded by the fimBC genes, located within the fha operon, and therefore is not expressed in the Bvg(-) mode. IMPORTANCE In Bordetella pertussis, the promoter Pfim3-15C 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 Pfim3 is in a non-permissive state (Pfim3-13C), suggesting the presence of a reciprocally regulated element upstream of Pfim3. 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.