Fv-1 host restriction of Friend leukemia virus: analysis of unintegrated proviral DNA.

The murine gene Fv-1 predominantly controls the outcome of infection by murine ecotropic retroviruses. The inhibition of virus replication by the Fv-1 gene product has been determined to be at an early stage in virus replication. Mechanistically, its effect appears to be on the accumulation of unintegrated proviral DNA or its integration or both. We investigated the synthesis of unintegrated proviral DNA, using several clones of B-, N-, or NB-tropic Friend murine leukemia virus. Our results indicate that the accumulation of B-tropic proviral DNA in NIH cells may be inhibited at either the level of linear (form III) or covalently closed circular DNA (form I), depending upon the degree of restriction of the clone of virus used. We confirmed that there is an effect of the Fv-1 gene on the accumulation of form I DNA of either B- or N-tropic Friend murine leukemia virus. However, the decrease in infectious centers effected by the Fv-1 gene did not correlate quantitatively with the effect on form I proviral DNA produced by N-tropic Friend murine leukemia virus in nonpermissive cells. Lastly, we demonstrated in nonpermissively infected NIH cells that a rapidly migrating doublet of viral DNA is formed.