SUMMARYA 12.4 kbp HindIII chromosomal DNA fragment harbouring an apparently intact 9-2 kbp endogenous marine teukaemia virus (MuLV)-related proviral genome was isolated from an RFM/Un strain mouse by molecular cloning and designated pRFM # 6. Nucleotide sequence analysis revealed the following characteristic features in the pRFM # 6 provirus: a distinct 200 bp sequence in the long terminal repeat (LTR) mid-U3 region, a primer binding site for glutamine tRNA, a 3' pol region encoding an 'endonuclease' protein of 390 amino acids, and the mink cell focusforming virus type-specific sequence at the 5' portion of the env gene. The 699 bp 5' LTR and 700 bp 3' LTR of pRFM # 6 provirus were identical except for three base changes in the U3 'enhancer' region. At the cell-provirus DNA junction, 4 bp direct repeats were present. The proviral genome was found at the same chromosomal DNA site in BALB/c, AKR, C3H, CBA and RFM strain mice, but not in NFS/N or C57BL/6 strain mice.

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The effect of the Fv‐1 mouse gene restriction of N‐ and B‐tropic mouse leukemia viruses was examined by determining the fate of the viruses in heterokaryons of permissive and non‐permissive cells. Virus expression in cultures of fused cells was analyzed by simultaneous autoradiography and fluorescent antibody assay, which permitted the detection of virus‐induced proteins specifically in heterokaryons or cells of either parental type. Heterokaryons were found to dominantly restrict both N‐ and B‐tropic virus, but not NB‐tropic virus which infects either cell type with equal efficiency. Fusion of permissive cells with non‐permissive cells at increasing intervals after infection showed that the restriction affects some virus function within at least 12 h of infection. These results indicate that the Fv‐1 gene locus, or gene product, affects some post‐penetration events required for virus protein synthesis.

We molecularly cloned unintegrated viral DNA of the BALB/c endogenous Ntropic and B-tropic murine leukemia retroviruses and in vitro passaged N-tropic Gross (passage A) murine leukemia retroviruses. Recombinant genomes were constructed in vitro by exchanging homologous restriction enzyme fragments from Nor B-tropic parents and subsequent recloning. Infectious virus was recovered after transfection of these recombinant genomes into NIH-3T3 cells and cocultivation with the Fv-1 nonrestrictive SC-1 cells. XC plaque assays of recombinant virus progeny on Fv-1" and Fv-lb cells indicated that the Fv-1 host range was determined by sequences located between the BamHI site in the p30 region of the gag gene (1.6 kilobase pairs from the left end of the map) and the HindIII site located in the pol gene (2.9 kilobase pairs from the left end of the map).

We analyzed 15 recombinant DNA clones of the unintegrated closed circular DNA intermediate of the BALB/c endogenous ecotropic murine leukemia virus WN1802N. Thirteen of these clones had an insert which corresponded to the complete murine leukemia virus genome. Of these, six contained a single long terminal repeat (LTR) and seven contained two LTRs. The viral genomes in nine clones had an LTR of 520 base pairs (bp), one had an LTR of 570 bp, three had an LTR of 600 bp, and one had an LTR of 670 bp. Restriction endonuclease analysis demonstrated that the size variability resides in the U3 region. Seven of eight clones which yielded infectious virus by DNA transfection had the 520-bp LTR, and the other had a 600-bp LTR. More detailed examination of plasmid subclones of three isolates with different-sized LTRs revealed that the approximate position which varies in the U3 region corresponds to the 72-bp repeat region of Moloney sarcoma virus. Possible consequences of these variations are discussed.