An ATPase component of the transcription elongation complex is required for factor-dependent transcription termination by vaccinia RNA polymerase.

Vaccinia virus RNA polymerase terminates transcription in response to a specific signal UUUUUNU in the nascent transcript. Transduction of this signal to the elongating polymerase requires a virus-encoded termination factor, VTF. The existence of a second termination factor was suggested by the finding that transient exposure of purified elongation complexes to heparin rendered them refractory to VTF-induced termination. Loss of termination competence correlated with the removal of several polypeptide components of the elongation complex. We present the identification of factor X, an activity that restored VTF responsiveness to heparin-stripped ternary complexes. We propose that factor X, which has an associated DNA-dependent ATPase activity, mediates the requirement for ATP hydrolysis during transcription termination.