Molecular cloning of biologically active Rauscher spleen focus-forming virus and the sequences of its env gene and long terminal repeat

Rauscher and Friend spleen focus-forming viruses (R- and F-SFFVs) cause similar progressive erythroleukemias dependent upon a virus-encoded membrane glycoprotein. Moreover, these SFFV glycoproteins are immunologically related to each other and to the recombinant-type glycoproteins encoded by the env genes of dual tropic murine leukemia viruses. To better understand these diseases and the viral origins, we isolated a pathogenically active molecular clone of R-SFFV proviral DNA, sequenced its 3'-terminal 2,163-base-pair (bp) region, and compared these sequences with previously determined sequences of F-SFFV. The 516-bp R-SFFV long terminal repeat is highly homologous to those of F-SFFV and Friend murine leukemia virus, although only the latter contains a 65-bp direct repeat in its U3 region. The env gene of R-SFFV encodes a glycoprotein with 408 amino acids that is identical in its basic domain organization to the glycoprotein of F-SFFV. Thus, the junctions between the dual tropic-related and ecotropic sequences occur at the same nucleotide, and both SFFV env genes contain identical 585-bp deletions in their ecotropic domains and single-bp insertions which cause premature terminations at the same amino acid in their ecotropic p15E domains. Consistent with their independent origins, however, the env sequences of R- and F-SFFV are distinctive in both their 5' dual tropic-related and 3' ecotropic-related domains. Furthermore, there are several consistent amino acid differences between the polycythemic F-SFFV sequences and the anemia-inducing R-SFFV sequence. The striking similarities of the independently formed F- and R-SFFV env genes imply that all of the glycoprotein domains arranged in a precise organization may be required for its leukemogenic activity.