Generation of novel, biologically active Harvey sarcoma viruses via apparent illegitimate recombination.

NIH 3T3 cells transfected with Harvey sarcoma virus (HSV) DNA may acquire deleted proviruses (Goldfarb and Weinberg, J. Virol. 38:125-135, 1981). Such proviruses lack the right end of the wild-type HSV DNA genome corresponding to the 3'-proximal portion of the viral RNA. As expected, the RNA transcripts of these deleted HSV (delHSV) proviruses lacked sequences normally found at the 3' end of wild-type HSV RNA. Since frequently these delHSV RNA transcripts were longer than wild-type HSV RNA, we suggest that transcription proceeded through the deleted provirus and continued into flanking nonviral sequences. When delHSV-transformed cells were infected with Moloney murine leukemia virus (M-MLV), delHSV RNA was pseudotyped into new virus particles, demonstrating that the 3'-proximal sequences of wild-type HSV RNA are not essential for virion RNA encapsidation. Cells which carried a delHSV genome and were infected with M-MLV helper released very low titers of highly transmissible sarcoma virus. The inability to rescue high titers of sarcoma virus from these cells reflected the necessary presence of the deleted 3'-terminal sequences for normal efficient transmission of the sarcoma virus genome (Goldfarb and Weinberg, J. Virol. 38:125-135, 1981). The small amount of highly transmissible sarcoma virus rescuable from delHSV-transformed cells originated via genetic recombination between del HSV and the M-MLV helper used for the sarcoma virus rescue. The recombinant sarcoma virus genomes reacquired a competent 3' genomic end from the parental M-MLV genome, which restored efficient transmissibility. The locations of sites for recombination between the delHSV and M-MLV genomes appeared to be nonrandom. These sites were in genomic regions where the parental genomes bore no detectable sequence homology. Structural mapping of these recombinant sarcoma virus genomes indicated that the HSV transformation gene lies within 2.0 kilobases of the RNA 5' end. Based upon our genetic recombination studies, we suggest a model to explain how leukemia viruses can recombine with cellular sequences to generate novel defective viruses.