Further characterization of mouse hepatitis virus RNA-dependent RNA polymerases

Two temporally and enzymatically distinct RNA-dependent RNA polymerase activities associated with membranes of the mouse hepatitis virus (MHV)-infected cells have been identified previously [Brayton et al., J. Virol.42, 847–853 (1982)]. In this paper, the subcellular distribution and functions of these two polymerases were examined. Fractionation of the postnuclear membranes by sucrose gradient sedimentation showed that the early polymerase activity (detected at 1 hr p.i.) was homogeneous, while the late polymeras e (6 hr p.i.) was associated with two distinct membrane fractions. The early polymerase synthesized a single RNA species of viral genomic size and negative sense. In contrast, the light peak of the late polymerase synthesized genomic-sized RNA of positive sense, while the heavy peak of the activity synthesized positive-sensed genomic and subgenomic mRNAs. These findings suggest that the light peak of the late polymerase represents a replication complex while the heavy peak represents a transcription complex. They also establish the essential features of the mode of replication of MHV.