HOMOLOGOUS AND NONHOMOLOGOUS RETROVIRAL PECOMBINATIONS ARE BOTH INVOLVED IN THE TRANSFER BY INFECTIOUS PARTICLES OF DEFECTIVE AVIAN-LEUKOSIS VIRUS-DERIVED TRANSCOMPLEMENTING GENOMES

We previously described avian leukosis virus-based packaging cell line s that produce stocks of retroviral vectors in which replication-compe tent viruses were not detectable. However, following infection of targ et cells with these retroviral stocks, we recently obtained colonies r esulting from the transmission of recombinant genomes. Here, we have a nalyzed their genetic structure and shown that (i) each of them result s from recombination between the packaging- and integration-defective transcomplementing genomes and the retroviral vector; (ii) recombinati on probably occurred during the reverse transcription step, involving strand switching of the reverse transcription growing point from the i nfectious retroviral vector to the transcomplementing RNA; and (iii) s equence identity and nonhomologous sequences were both used for the st rand snitching.