Splicing of intervening sequences introduced into an infectious retroviral vector.

The v-src gene was removed from Rous sarcoma virus DNA and replaced with either a cDNA or genomic clone of human alpha-chorionic gonadotropin. Transfection of the recombinant retrovirus genomes into normal chicken fibroblasts produced nontransforming recombinant virus in high titer. Neither helper virus nor selective conditions were needed. Cells infected with the recombinant viruses expressed RNA containing the gonadotropin sequences in levels equivalent to those of term human placenta (approximately 0.5% of poly A+). Essentially every fibroblast in the culture was infected; the infected cells contained approximately one recombinant provirus each. The gonadotropin intervening sequences were removed precisely from the recombinant genomes that contained them, creating recombinants carrying a perfect cDNA copy of the original genomic insert. However, the intervening sequences were removed inefficiently such that several viral replicative cycles were necessary before all genomes had been processed completely. The implications of these observations to the transduction of viral oncogenes and the creation of processed pseudogenes are discussed.