NOTES Properties of Human Parainfluenza Virus Type 3 RNA Polymerase/Replicase Activity In Vitro: Consensus with Other Negative-Stranded RNA Viruses

A cell-free system supporting transcription, replication, and nucleocapsid assembly of the genome RNA of human parainfluenza virus type 3 (HPF3) is described. Cytoplasmic extracts from infected CV-1 or BHK cells catalyzed the transcription of the entire HPF3 genome, the replication of genome RNA, and the assembly of this RNA into nucleocapsidlike structures. Newly replicated RNA was resistant to micrococcal nuclease digestion and was stable in CsCI gradients, exhibiting the density of authentic HPF3 nucleocapsids. After fractionation of the extracts, the nucleocapsid-containing pellet fraction synthesized viral mRNAs. Reconstitution with the soluble protein fraction was necessary for genome RNA replication and nucleocapsid assembly. Human parainfluenza virus type 3 (HPF3) is a member of the paramyxovirus group of viruses. The HPF3 virion pos-sesses a core composed of a helical nucleocapsid containing the viral negative-stranded RNA genome and three viral proteins, the nucleocapsid protein (NP), the phosphoprotein (P), and the RNA polymerase (L) (15). The RNA genome is tightly associated with NP, forming a stable, RNase-resis-tant nucleocapsid. P and L are less tightly associated wit