Dominance of cold-adapted influenza A virus over wild-type viruses is at the level of RNA synthesis.

Cold-adapted (ca) influenza A virus is dominant over wild-type (wt) influenza A viruses in mixed infections of MDCK cells. Since the inhibition of the growth of wt viruses occurs at or before the level of protein synthesis, the effect of coinfection by ca virus on RNA synthesis of wt viruses was investigated. RNA from single and mixed infections of ca and wt viruses was analyzed by hybridization with positive and negative sense oligonucleotide probes capable of distinguishing the RNAs of the two viruses. Primary and secondary transcription of mRNA and replication of vRNA from an early (NP) and a late (M) gene were quantitated. Although all stages of RNA synthesis were reduced, the key inhibition of wt RNA synthesis in coinfections with ca virus was at the level of vRNA replication. The inhibition of wt RNA synthesis occurred in mixed infections without any corresponding reduction of vRNA or mRNA synthesis by ca virus. Mechanisms by which ca virus may inhibit wt virus RNA synthesis are proposed based on the role of the products of gene segment 7 of the ca virus, the gene known to be responsible for the dominance phenotype.