DNA recombination is sufficient for retroviral transduction

Oncogenic retroviruses carry coding sequences that are transduced from cellular protooncogenes. Natural transduction involves two nonhomologous recombinations and is thus extremely rare. Since transduction has never been reproduced experimentally, its mechanism has been studied in terms of two hypotheses: (i) the DNA model, which postulates two DNA recombinations, and (ii) the RNA model, which postulates a 5' DNA recombination and a 3' RNA recombination occurring during reverse transcription of viral and protooncogene RNA. Here we use two viral DNA constructs to test the prediction of the DNA model that the 3' DNA recombination is achieved by conventional integration of a retroviral DNA 3' of the chromosomal protooncogene coding region. For the DNA model to be viable, such recombinant viruses must be infectious without the purportedly essential polypurine tract (ppt) that precedes the 3' long terminal repeat (LTR) of all retroviruses. Our constructs consist of a ras coding region from Harvey sarcoma virus which is naturally linked at the 5' end to a retroviral LTR and artificially linked at the 3' end either directly (construct NdN) or by a cellular sequence (construct SU) to the 5' LTR of a retrovirus. Both constructs lack the ppt, and the LTR of NdN even lacks 30 nucleotides at the 5' end. Both constructs proved to be infectious, producing viruses at titers of 10(5) focus-forming units per ml. Sequence analysis proved that both viruses were colinear with input DNAs and that NdN virus lacked a ppt and the 5' 30 nucleotides of the LTR. The results indicate that DNA recombination is sufficient for retroviral transduction and that neither the ppt nor the complete LTR is essential for retrovirus replication. DNA recombination explains the following observations by others that cannot be reconciled with the RNA model: (i) experimental transduction is independent of the packaging efficiency of viral RNA, and (ii) experimental transduction may invert sequences with respect to others, as expected for DNA recombination during transfection.