Mutation in NS2, a nonstructural protein of influenza A virus, extragenically causes aberrant replication and expression of the PA gene and leads to generation of defective interfering particles.

Several consecutive undiluted passages of infectious virus are usually required to obtain defective interfering particles of influenza virus. In contrast, a reassortant (Wa-182) of influenza A/WSN, which we isolated, whose NS gene was replaced with the NS gene of A/Aichi, was readily converted to defective interfering form after only a single high-multiplicity infection. The defective interfering particles of Wa-182 were devoid of the PA gene (RNA segment 3) but possessed several species of subgenomic RNAs of the PA gene origin. Such aberrant replication of the PA gene was shown to be caused by an extragenic effect of the NS gene of Wa-182, because, when the NS gene of Wa-182 was singly transferred to the wild-type A/Ann Arbor/6/60 virus, the recipient showed exactly the same features. Analysis of nucleotide sequence demonstrated that the NS gene of Wa-182 contained three point mutations relative to the wild-type NS gene that resulted in two amino acid substitutions in the nonstructural protein NS2, suggesting that the mutation in NS2 protein affected the normal replication of the PA gene of Wa-182. The results also suggest that the NS2 protein plays an important role in the synthesis of intact genome RNAs.