Functional expression of influenza A viral nucleoprotein in cells transformed with cloned DNA

Simian cells permissive for influenza A virus infection were stably transformed with a full-length cloned influenza A nucleoprotein gene under the control of an inducible metallothionein promoter and linked to a dihydrofolate reductase gene to facilitate cell selection. Transformed cells synthesized a virus-specific nucleoprotein which was indistinguishable from the nucleoprotein synthesized in virus-infected cells with respect to molecular weight and intracellular localization. It was estimated that transformed cells produced only 1% of the amount of nucleoprotein synthesized in simian cells infected with influenza A virus. Nonetheless, when transformed cells were infected with influenza virus mutants which synthesized temperature-sensitive nucleoprotein, protein expressed by the cloned gene was able to complement the synthesis of plus-strand and minus-strand viral RNA for one mutant and only plus-strand synthesis for another mutant. This indicated that the influenza A nucleoprotein expressed in the transformed cells exhibited functional activity.