Determination of the mechanism of viral interference manifested by a mouse-adapted strain of influenza A virus.

Abstract

Mouse-adapted influenza A virus, FM-MA, interferes with the replication of wild-type strains on coinfection of MDCK cells to the extent that $>$99% of the infectious progeny virus are FM-MA. The interference phenotype was previously mapped to FM-MA segment 2 which encodes the PB1 polymerase protein with a single amino acid substitution relative to the parent, FM, at position 538. To identify the point at which FM-MA interferes with wild-type A/HK/1/68 (HK), the relative levels of FM-MA and HK transcription and genome replication from PB1, NP and M1 genes were determined in coinfected MDCK cells using RT-PCR. All stages of FM-MA macromolecular synthesis (primary and secondary transcription, genomic RNA, complementary RNA and protein) were enhanced relative to HK, a phenotype which mapped to FM-MA segment 2. The kinetics of viral RNA synthesis in single or mixed infections indicated not only that the presence of FM-MA specifically compromised HK transcription and replication in coinfected cells but also that FM-MA's ability to interfere was due in part to its capacity for increased primary transcription relative to HK. FM-MA genomes were also selectively assembled into progeny virus from cells coinfected with HK and FM-MA, a step which was distinct from the capacity for enhanced RNA synthesis. This suggests that interference of wild-type virus growth by FM-MA in mixed infections resulted from two distinct events: a preferential synthesis of FM-MA-specific macromolecules which was then augmented by a preferential assembly of FM-MA genomes. The ability of FM-MA to selectively amplify its own genes in cells coinfected with HK did not depend on recognition of an FM-MA-specific promoter by the mutant polymerase. Alternate explanations for this selectivity are proposed.