Disruption of influenza virus packaging signals results in various misassembled genome complexes

The eight different genomic vRNAs of influenza A virus are typically packaged into virions as a (7+1) complex of seven ribonucleoproteins (vRNPs) around a central vRNP. Mutations in the packaging signals of these vRNAs are predicted to result in virions lacking specific vRNPs. How-ever, whether these vRNPs are replaced by vRNP-like cellular RNAs is an open question. Here, we report that mutations in two packaging signals of influenza A/SC35M virus, which cause the near-complete loss of four vRNAs from the total virion population, result in several virion sub-populations with unusual vRNP content. Half of the virions analyzed are empty or contain incomplete genome complexes with one to four vRNPs. Unexpectedly, most other virions contain atypical (7+1) complexes consisting of multiple copies of the four efficiently packaged vRNAs, but no cellular RNAs. This diversity of misassembled genome complexes suggests that packaging sig-nals coordinate a modular genome assembly process.