Generation of AKR mink cell focus-forming viruses: a conserved single-copy xenotrope-like provirus provides recombinant long terminal repeat sequences.

AKV and AKR mink cell focus-forming virus-specific probes from the envelope and long terminal repeat (LTR) regions were prepared for study of the structure of recombinant proviruses in tumor tissues of AKR mice. The results showed that (i) all somatically acquired proviruses possessed, besides a recombinant gp70 gene, an altered U3 LTR; (ii) in a substantial portion of the somatically acquired AKR mink cell focus-forming proviruses, the LTR comprised sequences derived from the same xenotropic-like provirus; (iii) this U3 LTR donating parental provirus (Xeno-dL) was present only once per genome equivalent in several mouse strains; (iv) in the strains containing the Xeno-dL provirus, the provirus was present in the same chromosomal site; (v) restriction analysis of the Xeno-dL revealed that the mink cell focus-forming gp70 sequences were derived from a parental provirus, different from Xeno-dL. Therefore, at least two non-ecotropic parents participate in the generation of leukemogenic AKR mink cell focus-forming viruses: a xenotropic-like virus, Xeno-dL, donating U3 LTR sequences, and another xenotropic-like virus or viruses providing gp70 sequences.