The structural basis for terminator recognition by the Rho transcription termination factor.

The E. coli Rho protein disengages newly transcribed RNA from its DNA template, helping terminate certain transcripts. We have determined the X-ray crystal structure of the RNA-binding domain of Rho complexed to an RNA ligand. Filters that screen both ligand size and chemical functionality line the primary nucleic acid-binding site, imparting sequence specificity to a generic single-stranded nucleic acid-binding fold and explaining the preference of Rho for cytosine-rich RNA. The crystal packing reveals two Rho domain protomers bound to a single RNA with a single base spacer, suggesting that the strong RNA-binding sites of Rho may arise from pairing of RNA-binding modules. Dimerization of symmetric subunits on an asymmetric ligand is developed as a model for allosteric control in the action of the intact Rho hexamer.