Host restriction of Friend leukemia virus: synthesis and integration of the provirus.

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Abstract
Host restriction of exogenous infection by murine leukemia viruses is controlled in vitro predominantly by the murine Fv-1 locus. The mechanism of this host restriction was investigated by comparing the early events in the replication of N-tropic vs. B-tropic Friend leukenia virus in [mouse fibroblast] NIH 3T3 cells. These cells, which are Fv-1nn in type, are permissive for the N-tropic strain, but nonpermissive for the B-tropic strain, which replicates permissively in [mouse fibroblast] Balb/c 3T3 cells. The synthesis, intracellular location and molecular form of virus-specific DNA early in replication was studied using molecular hybridization with a virus-specific DNA probe. In the permissive infection, viral DNA rapidly becomes integrated with cellular DNA. In the nonpermissive infection, although almost equal amounts of positive and negative strand viral DNA are synthesized, integration of the provirus does not occur.