Retrovirus transduction: generation of infectious retroviruses expressing dominant and selectable genes is associated with in vivo recombination and deletion events

We describe the generation of infectious retroviruses containing foreign genes by an in vivo recombination-deletion mechanism. Cotransfection into mouse cells of chimeric plasmids carrying a murine retrovirus 5' long terminal repeat and either the thymidine kinase (tk) gene of herpesvirus or the dominant selectable bacterial gene for neomycin resistance (neo), along with a clone of Moloney murine leukemia virus, results in the generation of infectious thymidine kinase or neomycin-resistant viruses. Expression of the selectable marker in these viruses can be regulated by the homologous transcriptional promoter of the gene, by the promoter contained within the Friend spleen focus-forming virus long terminal repeat, or by the simian virus 40 early region promoter. In all cases, the rescued viruses appeared to arise by recombination in vivo with Moloney murine leukemia virus sequences, resulting in the acquisition of the Moloney 3' long terminal repeat and variable amounts of the 3' adjacent Moloney genome. In two of the thymidine kinase constructs where tk was inserted 200 base pairs downstream from the long terminal repeat, the rescued viruses acquired a large part of the murine leukemia virus genome, including the region involved in packaging genomic RNA into virions. The generation of infectious neomycin-resistant virus is associated with deletions of simian virus 40 splicing and polyadenylation sequences. These results demonstrate that nonhomologous recombination and deletion events can take place in animal cells, resulting in the acquisition or removal of cis-acting sequences required for, or inhibitory to, retrovirus infectivity.