Identification of a signal in a murine retrovirus that is sufficient for packaging of nonretroviral RNA into virions

A region near the 5' end of Moloney murine leukemia virus (MoMLV) is required for packaging of viral RNA into virions. Retroviral vectors based on MoMLV have been constructed that are also efficiently packaged into virions despite removal of most of the interior region of the parental virus. To further localize sequences which are sufficient for packaging, we inserted various fragments from an MoMLV-based retroviral vector into a nonretroviral transcription unit, transfected these constructs into retrovirus-packaging cells, and measured packaging of RNA transcribed from these constructs into virions. Transcripts from some of these constructs were packaged at least as well as those from the parental vector or MoMLV itself. Sequences extending into the gag region, but not the long terminal repeat or tRNA-binding sequences, were required for efficient RNA packaging. RNAs transcribed from constructs which did not contain an insert, or in which the orientation of the insert was reversed, were not packaged at detectable levels. These studies define sequences which are necessary and sufficient for encapsidation of murine leukemia virus RNA into virions.