Nonpermissive infection of L cells by an avian reovirus: restricted transcription of the viral genome.

Avian reovirus multiples in chicken embryo fibroblasts. Although the avian virus adsorbs to L cells and is uncoated therein, it does not multiply. In the nonpermissive infection of L cells with the avian reovirus only four of the genomic segments of the viral genome are transcribed, L1, M3, S3, and S4, and these are the same segments that have been designated previously as early functions in the permissive infection of L cells with type 3 reovirus. When L cells are co-infected with avian reovirus and type 3 virus all ten segments of the avian viral genome are transcribed, although there is no synthesis of avian viral double-stranded RNA. Type 3 reovirus multiplies almost normally in this mixed infection. The most likely explanation is that a cellular repressor blocks transcription of the six late segments of the avian viral genome and that this repressor is removed by the co-infection with type 3 virus. A second block prevents replication of the viral genome.