Transcription termination factor Rho of Escherichia coli has an ATP-dependent RNA.DNA helicase activity that presumably facilitates RNA transcript release from the elongation complex. This helicase activity is unidirectional (5' to 3') and is stoichiometric, with one RNA molecule released per Rho hexamer in vitro. A simple RNA tracking model postulates that after Rho's initial binding, it translocates preferentially toward the 3' end of the RNA. Nitrocellulose filter binding studies combined with RNase H cleavage are inconsistent with this simple tracking model. Instead, they support a model in which Rho forms tight primary binding interactions with the recognition region of the RNA and remains bound there while transient secondary RNA binding interactions coupled to ATP hydrolysis serve to scan along the RNA to contact the RNA.DNA helix. This "tethered tracking" model is consistent with other properties of Rho factor, including the presence of two classes of RNA binding sites on the Rho hexamer and the 1:1 stoichiometry in the Rho helicase assay
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