Localized modulation of DNA supercoiling, triggered by theShigellaanti-silencer VirB, is sufficient to relieve H-NS-mediated silencing

Abstract

ABSTRACTIn Bacteria, nucleoid structuring proteins govern nucleoid dynamics and regulate transcription. InShigella spp., at ≤ 30 °C, the histone-like nucleoid structuring protein (H-NS) transcriptionally silences many genes on the large virulence plasmid. Upon a switch to 37 °C, VirB, a DNA binding protein and key transcriptional regulator ofShigellavirulence, is produced. VirB functions to counter H-NS-mediated silencing in a process called transcriptional anti-silencing. Here, we show that VirB mediates a loss of negative DNA supercoils from our plasmid-borne, VirB-regulatedPicsP-lacZreporter,in vivo. The changes are not caused by a VirB-dependent increase in transcription, nor do they require the presence of H-NS. Instead, the VirB-dependent change in DNA supercoiling requires the interaction of VirB with its DNA binding site, a critical first step in VirB-dependent gene regulation. Using two complementary approaches, we show that VirB:DNA interactionsin vitrointroduce positive supercoils in plasmid DNA. Subsequently, by exploiting transcription-coupled DNA supercoiling, we reveal that a localized loss of negative supercoils is sufficient to alleviate H-NS-mediated transcriptional silencing, independently of VirB. Together, our findings provide novel insight into VirB, a central regulator ofShigellavirulence and more broadly, a molecular mechanism that offsets H-NS-dependent silencing of transcription in bacteria.