Bvr-1, a restriction locus of a type C RNA virus in the feline cellular genome: identification, location, and phenotypic characterization in cat X mouse somatic cell hybrids.

Somatic cell hybrids were constructed between BALB/c-RAG mouse cells and feline lymphoma cells by the hypoxanthine-aminopterin-thymidine selection scheme. RAG cells spontaneously produce an endogenous B-tropic type C virus. Cat-mouse hybrids preferentially segregate feline chromosomes and retain murine chromosomes-demonstrable by karyotypic and isozyme analyses. Despite the presence of the complete mouse genome, including the viral genome, virus production was diminished to 1-5% of the levels observed in RAG parents based upon particle-associated RNA-dependent DNA polymerase (reverse transcriptase) activity in the culture fluid. Thirty-seven hybrids made on four different occasions had suppressed virus levels, and no hybrids expressed parental virus levels. Reverse selection experiments on 6-thioguanine demonstrated that a restriction gene, tentatively named Bvr-1, was linked to the feline structural genes for hypoxanthine phosphoribosyltransferase (IMP:pyrophosphate phosphoribosyltransferase; EC 2.4.4.8) and glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase; EC 1.1.1.49) in cats, probably on the X-chromosome. The genetic mode of action of Bvr-1 is trans dominant in restriction of murine leukemia virus. The restriction locus results in a block late in virus maturation but prior to release, since expression of antigens for viral structural proteins and matrue budding particles is apparent on surfaces of restriced hybrid cells but not in high-speed pellets from culture fluid of restricted cells.