Speaker
Description
The nucleoid-structuring protein H-NS silences segments of the enterobacterial genomes by binding to AT-rich nucleation sites and forming nucleoprotein filaments that block transcription over extended regions. H-NS:DNA filaments assemble on negatively supercoiled DNA and are stabilized by this DNA topology. In Salmonella, H-NS silences all major pathogenicity islands when the bacteria are outside their hosts. Given that H-NS is largely thought to obstruct RNA polymerase (RNAP) access to promoters, we were surprised to find that impairing Rho-dependent transcription termination leads to the constitutive activation of Salmonella pathogenicity islands (SPIs). How could termination play a role in regions that are not supposed to be transcribed? Our work on Salmonella pathogenicity island 1 (SPI-1) revealed that while H-NS efficiently represses bona fide promoters, it does not completely prevent RNAP from binding to intragenic promoter-like sequences that arise at high frequency in AT-rich regions. This is where Rho factor comes into play. Aided (or recruited) by the transcription elongation factor NusG, Rho terminates transcription originating from these spurious promoters. This function is critical for maintaining H-NS-mediated SPI-1 silencing. When Rho or NusG is defective, spurious transcripts elongate, generating positive DNA supercoiling. This supercoiling propagates and invades the negatively supercoiled DNA:H-NS filament. As positive and negative supercoils neutralize each other, the loss of negative superhelicity destabilizes the H-NS:DNA complex, leading to filament disassembly. This disruption grants RNAP access to bona fide promoters, triggering a feedforward cascade that ultimately activates the entire 48-Kb island.
