SEGMENT-SPECIFIC NONCODING SEQUENCES OF THE INFLUENZA-VIRUS GENOME RNA ARE INVOLVED IN THE SPECIFIC COMPETITION BETWEEN DEFECTIVE INTERFERING RNA AND ITS PROGENITOR RNA SEGMENT AT THE VIRION ASSEMBLY STEP

The generation of influenza A virus defective interfering (DI) particl es was studied by using an NS2 mutant which produces, in a single cycl e of virus replication, a large amount of DI particles lacking the PA polymerase gene. The decrease in PA gene replication has been shown to occur primarily at the cRNA synthesis step, with preferential amplifi cation of PA DI RNA species present in a marginal amount in the virus stock In addition, at the assembly step the PB DI RNAs were preferenti ally incorporated into virions, resulting in selective reduction in th e packaging of the PA gene into virions, Similarly, in cells dually in fected,with the NS2 mutant and wild-type viruses, packaging of the wil d-type PA gene was also greatly suppressed, In contrast, incorporation of other RNA segments, i.e., the PB2 and NS genes, was not affected, suggesting that the PA DI RNAs competed only with the PA gene in a seg ment-specific manner, Experiments involving rescue of recombinant chlo ramphenicol acetyltransferase (CAT) RNA Ranked by the noncoding region s of the PA (PA/CAT RNA) and PB2 (PB2/CAT RNA) genes into viral partic les showed that only PA/CAT RNA was not rescued by infection with the NS2 mutant virus containing the PA DI RNAs, However, recombinant PA/CA T RNA in which either the 3' or 5' noncoding region was replaced with that of the PB2 gene was rescued by the NS2 mutant,These results sugge st that the noncoding regions of the PA gene are responsible for the c ompetition with PA DI RNA species at the virus assembly step and that coexistence of the both noncoding regions would be a prerequisite for this phenomenon. Decreased packaging of the progenitor RNA by the DI R NA, in addition to the suppression of cRNA synthesis, is likely involv ed in the production of DI particles.