EVIDENCE SUPPORTING A TETHERED TRACKING MODEL FOR HELICASE ACTIVITY OF ESCHERICHIA-COLI RHO-FACTOR

Transcription termination factor Rho of Escherichia coli has an ATP-de pendent RNA DNA helicase activity that presumably facilitates RNA tran script 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 postul ates that after Rho's initial binding, it translocates preferentially toward the 3' end of the RNA. Nitrocellulose filter binding studies co mbined with RNase H cleavage are inconsistent with this simple trackin g model. Instead, they support a model in which Rho forms tight primar y binding interactions with the recognition region of the RNA and rema ins bound there while transient secondary RNA binding interactions cou pled to ATP hydrolysis serve to scan along the RNA to contact the RNA DNA helix. This ''tethered tracking'' model is consistent with other p roperties 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.