Structure of the Rho transcription terminator: Mechanism of mRNA recognition and helicase loading

In bacteria, one of the major transcriptional termination mechanisms requires a RNA/DNA helicase known as the Rho factor. We have determined two structures of Rho complexed with nucleic acid recognition site mimics in both free and nucleotide bound states to 3.0 Angstrom resolution. Both structures show that Rho forms a hexameric ring in which two RNA binding sites-a primary one responsible for target mRNA recognition and a secondary one required for mRNA translocation and unwinding-point toward the center of the ring. Rather than forming a closed ring, the Rho hexamer is split open, resembling a lock washer in its global architecture. The distance between subunits at the opening is sufficiently wide (12 Angstrom) to accommodate single-stranded RNA. This open configuration most likely resembles a state poised to load onto mRNA and suggests how related ring-shaped enzymes may be breached to bind nucleic acids.