Capture of a cellular transcriptional unit by a retrovirus: mode of provirus activation in embryonal carcinoma cells.

The expression of murine leukemia provirus in embryonal carcinoma (EC) cells is blocked by a mechanism still incompletely understood. The blockage is not overcome by deleting a large portion of the enhancer region (in U3) in recombinant retroviruses (M-MuLVneo delta Enh). This confirms the presence of negative elements outside the viral 82-bp repeats. However, a few sites in the genomes of EC cells permit M-MuLVneo delta Enh proviral expression. One such site, identified in PCC4, PCC3, and LT, was studied. The complete analysis of the mechanism of activation by Northern (RNA) blotting, cloning, and sequencing of partial cDNA copies of the viral transcript and of the site of integration establishes that viral transcripts are initiated from an upstream host-cell promoter and are spliced from a host donor to a cryptic viral acceptor at position 542 in the Moloney murine leukemia virus (M-MuLV) genome. In consequence, the mature transcripts are host cell-virus fusion transcripts from which M-MuLV sequences, including the cis-active negative elements of the 5' long terminal repeat-containing region, are absent. The provirus integrates apparently randomly into any of the three most proximal introns of the transcriptional unit. The host cell promoter contains a TATA box and 14 potential SpI binding sites included in a 1.0-kb GC-rich island. These elements promote gene expression of recombinant vectors in EC and differentiated cells. The mechanism described points to a mechanism by which retroviruses can be transcribed from upstream nonviral elements and can acquire host genes by 5' annexation of exons.