Thymotropism of murine leukemia virus is conferred by its long terminal repeat.

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Abstract
Several murine leukemia viruses (MuLV) replicate efficiently in the thymus (T+) of the mouse; others are unable to replicate (T-) in this organ. To map the region of the viral genome harboring the sequences responsible for this thymotropic phenotype, viral DNA recombinants were constructed in vitro between cloned infectious viral DNA from T- BALB/c N-tropic MuLV and from T+ BALB/c B-tropic MuLV or AKR Gross passage A MuLV (N- and B-tropic refer to the Fv-1 host range of MuLV). Infectious recombinant MuLV, recovered from murine cells microinjected with these recombinant DNA, were injected into newborn mice to test their ability to replicate in the thymus. The long terminal repeat from the T+ BALB/c B-tropic or AKR Gross passage A MuLV genome was sufficient to allow replication of recombinant MuLV in the thymus. Evidently, the U3 tandem direct repeat was responsible for this effect. These results suggest a new role for the U3 long terminal repeat in the replication of MuVL in specific differentiated target cells.