Regulation of influenza virus RNA polymerase activity by cellular and viral factors.

An in vitro RNA synthesis system mimicking replication of genomic influenza virus RNA was developed with nuclear extracts prepared from influenza virus-infected HeLa cells using exogenously added RNA templates. The RNA synthesizing activity was divided into two complementing fractions, i.e. the ribonucleoprotein (RNP) complexes and the fraction free of RNP, which could be replaced with RNP cores isolated from virions and nuclear extracts from uninfected cells, respectively. When nuclear extracts from uninfected cells were fractionated by phosphocellulose column chromatography, the stimulatory activity for RNA synthesis was further separated into two distinct fractions. One of them, tentatively designated RAF (RNA polymerase activating factor), stimulated RNA synthesis with either RNP cores or RNA polymerase and nucleocapsid protein purified from RNP cores as the enzyme source. In contrast, the other, designated PRF (polymerase regulating factor), functioned as an activator only when RNP cores were used as the enzyme source. Biochemical analyses revealed that PRF facilitates dissociation of RNA polymerase from RNP cores. Of interest is that virus-coded non-structural protein 1 (NS1), which has been thought to be involved in regulation of replication, counteracted PRF function. Roles of cellular factors and viral proteins, NS1 in particular, are discussed in terms of regulation of influenza virus RNA genome replication.