An amino acid change in the non-structural NS2 protein of an influenza A virus mutant is responsible for the generation of defective interfering (DI) particles by amplifying DI RNAs and suppressing complementary RNA synthesis.

The mutated non-structural NS2 protein of an influenza A virus mutant, Wa-182, has been shown to be responsible for the production of defective interfering (DI) particles lacking the PA gene after a single cycle high-multiplicity infection. Using a subclone of Wa-182, A3/e-3, that inherited the Wa-182 phenotype but contained only a marginal amount of DI RNAs derived from the PA gene, we showed that replication of the PA genome RNA was suppressed primarily at the step of complementary RNA (cRNA) synthesis. On the other hand, the small amounts of DI RNA species present in the stock of A3/e-3 were shown to be replicated efficiently. These findings suggested that the suppression of cRNA synthesis of the PA gene was caused by preferential amplification of the DI RNAs. The suppression of PA gene cRNA synthesis subsequently resulted in suppression of both virion RNA synthesis and secondary transcription of the PA gene. Such aberrant replication of the PA gene was found to be attributable to an amino acid change in the NS2 protein at position 32, from isoleucine to threonine. These results suggest that the NS2 protein plays a role in promoting normal replication of the genomic RNAs by preventing the replication of short-length RNA species.