Abstract
To probe the genetic basis of disease specificity of nondefective murine type C viruses, we are constructing recombinants in vitro between molecular clones of Friend murine leukemia virus (Fr-MuLV) and Moloney murine leukemia virus (Mo-MuLV). Fr-MuLV induces erythroleukemias when injected into newborn NFS mice, whereas Mo-MuLV almost invariably induces T-cell lymphomas. We find that a recombinant whose genome is derived primarily from Fr-MuLV but which has 621 nucleotides of Mo-MuLV information at its 3' end induces almost exclusively thymic lymphomas. The sequences derived from Mo-MuLV include 99 nucleotides encoding the carboxyl terminus of Prp15E, the origin of DNA +-strand synthesis, all of the U3 region, and 36 nucleotides of the R portion of the long terminal repeat. When the segment of Mo-MuLV was removed and replaced with the comparable segment from Fr-MuLV, the virus was again erythroblastosis-inducing. These results, in conjunction with studies from other laboratories [Laimins, L. A., Khoury, G., Gorman, C., Howard, B. & Gruss, P. (1982) Proc. Natl. Acad. Sci. USA 79, 6453-6457], suggest that transcriptional signals in U3 may determine tissue tropism and hence influence disease specificity ("targeting") of murine leukemia viruses.