Envelope gene sequence of two in vitro-generated mink cell focus-forming murine leukemia viruses which contain the entire gp70 sequence of the endogenous nonecotropic parent.

The mink cell focus-forming (MCF) class of recombinant murine leukemia viruses (CI-1 to 4) were isolated from iododeoxyuridine-induced C3H/MCA 5 cells in culture and molecularly cloned. These genomes included infectious (CI-3) and defective (CI-4) recombinants. A total of 2,408 nucleotides of CI-3 virus DNA, including the MCF envelope gene, were sequenced and compared with ecotropic, dual-tropic, and xenotropic sequences. The extent of recombinational exchange in CI-3 was from 145 nucleotides 3' of the splice acceptor site for the envelope mRNA to nucleotide 1,722, between the end of gp70 and the beginning of Prp15E. Thus, the entire gp70 sequence of the endogenous nonecotropic parent was present in this recombinant. The nature and location of the recombinant junctions were consistent with a mechanism involving DNA exchange during reverse transcription. Comparison of the substituted sequence in CI-3 with that of Moloney MCF virus suggests a very close relationship, if not identity, between the endogenous dual-tropic proviruses from which they were derived. A nonidentity of xenotropic and MCF gp70s was observed, suggesting that xenotropic murine leukemia viruses are not the nonecotropic parent of the env gene of MCF murine leukemia viruses. The replication-defective virus CI-4 had a 684-nucleotide deletion present in the env gene, eliminating the hydrophobic regions within the gp70 carboxy end and the p15E amino end. This sequence was bordered by an 11-nucleotide direct repeat in CI-3 viral DNA.