The viral RNA polymerase H4L subunit is required for Vaccinia virus early gene transcription termination.

Vaccinia virus early gene transcription is catalyzed by a multisubunit virion form of RNA polymerase that possesses a unique subunit, H4L. Prior studies from this laboratory showed that the NH(2)-terminal domain of H4L, containing amino acids 1-195, interacts with the COOH-terminal end of nucleoside triphosphate phosphohydrolase I (NPH I), an ATPase that is employed in early gene transcription termination. Carboxyl-terminal deletion mutations of NPH I lose both the ability to mediate transcription termination and binding to H4L, providing evidence that the interaction between NPH I and H4L is required for termination.

Interaction between nucleoside triphosphate phosphohydrolase I and the H4L subunit of the viral RNA polymerase is required for vaccinia virus early gene transcript release.

Signal-dependent termination is restricted to early poxvirus genes whose transcription is catalyzed by the virion form of RNA polymerase. Two termination factors have been identified. Vaccinia termination factor/capping enzyme is a multifunctional heterodimer that also catalyzes the first three steps of mRNA cap formation and is an essential intermediate gene transcription initiation factor.