A subgenomic mRNA transcript of the coronavirus mouse hepatitis virus strain A59 defective interfering (DI) RNA is packaged when it contains the DI packaging signal

In infected cells, only the genomic RNA of the coronavirus mouse hepatitis virus strain A59 (MHV-A59) is packaged into the virions. In this study, we show that a subgenomic (sg) defective interfering (DI) RNA can be packaged into virions when it contains the DI RNA packaging signal (DI RNA-Ps). However, the sg DI RNA is packaged less efficiently than the DI genomic RNA. Thus, while specificity of packaging of RNAs into MHV-A59 virions is determined by the DI RNA-Ps, efficiency of packaging is determined by additional factors. Coronaviruses are enveloped positive-strand RNA viruses, and during replication of the 28- to 32-kb genome, a 39-co-terminal nested set of mRNAs from which the different viral proteins are translated is produced (5, 17). Only the genomic RNA is detected in the virions of the murine coronavirus mouse hepatitis virus strain A59 (MHV-A59) (14), although trace amounts of mRNA 7 of strain JHM have been detected in purified virions (7). The specific encapsidation of MHV genomic RNA is in keeping with the identification of a defec-tive interfering RNA packaging signal (DI RNA-Ps) in th