INTERFERENCE BY A NONDEFECTIVE VARIANT OF INFLUENZA-A VIRUS IS DUE TOENHANCED RNA-SYNTHESIS AND ASSEMBLY

Mouse-adapted influenza A virus, FM-MA, interferes with the replicatio n of wild-type strains on co-infection. The interference phenotype was previously mapped to FM-MA segment 2 encoding a mutant PB1 protein, t he catalytic component of the RNA polymerase complex. To identify the point at which FM-MA interferes with wild-type A/HK/1/68 (HK), the rel ative levels of transcription and genome replication of the PB1, NP an d M1 genes were determined for FM-MA and KK viruses in co-infected cel ls using RT-PCR. All stages of HK macromolecular synthesis (primary an d secondary transcription, genomic RNA: complementary RNA and protein synthesis) were suppressed relative to FM-MA. Infection with HK virus alone resulted in the accumulation of similar or greater amounts of RN A at late times post-infection relative to FM-MA thus indicating that the presence of FM-MA specifically compromised HK transcription and re plication in co-infected cells. However early in infection FM-MA was t en times more active in mRNA transcription than HK or its parental str ain FM. FM-MA's ability to interfere was primarily due to an increased capacity for primary transcription. FM-MA genomes were also selective ly assembled into progeny virus from cells co-infected with KK and FM- MA, a step which was distinct from the capacity for enhanced RNA synth esis. This suggests that interference of HK growth by FM-MA in mixed i nfections results from two distinct events: a preferential synthesis o f FM-MA-specific macromolecules which is then augmented by a preferent ial assembly of FM-MA genomes. (C) 1998 Elsevier Science B.V. All righ ts reserved.