Complementation and analysis of an NP mutant of influenza virus.

Cell lines were constructed so as to express the influenza A virus nucleoprotein (NP) at levels approximating 5% of the total NP made throughout virus infection. Two types of cell lines were analyzed. One cell line (NP-5) expresses only the NP while another cell line was constructed which expresses the three viral polymerase proteins in addition to the NP (3PNP-4). Both cell lines were able to complement the growth of an NP mutant, ts56, at the non-permissive temperature. The 3PNP-4 cell line, constructed by transfecting a cell line already expressing the three polymerase proteins, continued to be able to complement viral PB2 mutants. In addition, sequence analysis was performed on the NP gene segment of A/WSN/33 and ts56 viruses. This analysis revealed that the mutant phenotype exhibited by ts56 at non-permissive temperature is due to a single serine to asparagine change (at codon 332) within the protein.