Widespread gene regulator Psu inhibits transcription termination factor rho by forced hyper-oligomerization

Many bacteriophages modulate the host transcription machinery for efficient expression of their own genomes. Phage P4 polarity suppression protein, Psu, is a building block of the viral capsid and inhibits the hexameric transcription termination factor, {rho}, by presently unknown mechanisms. We elucidated cryogenic electron microscopy structures of {rho}-Psu complexes, showing that Psu dimers laterally clamp two inactive, open {rho} rings and promote their expansion to higher-oligomeric states. Systematic ATPase, nucleotide binding and nucleic acid binding studies revealed that Psu hinders {rho} ring closure and traps nucleotides in their binding pockets on {rho}. Structure-guided mutagenesis in combination with growth, pull-down and termination assays further delineated the functional {rho}-Psu interfaces. Bioinformatic analyses suggested that, in addition to guarding its own genome against {rho}, Psu enables expression of diverse phage-defense systems commonly found in P4-like mobile genetic elements across bacteria. Thus, Psu is a widespread gene regulator that inhibits {rho} via forced hyper-oligomerization.