VACCINIA NPH-I, A DEXH-BOX ATPASE, IS THE ENERGY COUPLING FACTOR FOR MESSENGER-RNA TRANSCRIPTION TERMINATION

Vaccinia virus RNA polymerase terminates transcription in response to a specific signal UUUUUNU in the nascent RNA. Transduction of this sig nal to the elongating polymerase requires a trans-acting viral termina tion factor (VTF/capping enzyme), and is coupled to the hydrolysis of ATP. Recent studies suggest that ATP hydrolysis is catalyzed by a nove l termination protein (factor X), which is tightly associated with the elongation complex. Here, we identify factor X as NPH-I (nucleoside t riphosphate phosphohydrolase-I), a virus-encoded DNA-dependent ATPase of the DExH-box family. We report that NPH-I serves two roles in trans cription (1) it acts in concert with VTF/CE to catalyze release of UUU UUNU-containing nascent RNA from the elongation complex, and (2) it ac ts by itself as a polymerase elongation factor to facilitate readthrou gh of intrinsic pause sites. A mutation (K61A) in the GxGKT motif of N PH-I abolishes ATP hydrolysis and eliminates the termination and elong ation factor activities. Related DExH proteins may have similar roles at postinitiation steps during cellular mRNA synthesis.