Rho-dependent termination and ATPases in transcript termination.

Transcription factor Rho is a ring-shaped, homohexameric protein that causes transcript termination through actions on nascent RNAs that are coupled to ATP hydrolysis. The Rho polypeptide has a distinct RNA-binding domain (RNA-BD) of known structure as well as an ATP-binding domain (ATP-BD) for which a structure has been proposed based on homology modeling. A model is proposed in which Rho first makes an interaction with a nascent RNA on a C-rich, primarily single-stranded rut region of the transcript as that region emerges from the exit site of RNA polymerase. A subsequent step involves a temporary release of one subunit of the hexamer to allow the 3' segment of the nascent transcript to enter the central channel of the Rho ring. Actions of the Rho structure in the channel on the 3' segment that are coupled to ATP hydrolysis pull the RNA from its contacts with the template and RNA polymerase, thus causing termination of its synthesis.