Genome Organization of a Biologically Active Molecular Clone of the Lymphoproliferative Disease Virus of Turkeys

The lymphoproliferative disease retrovirus (LPDV) induces an acute, horizontally transmitted disease of turkeys that is often fatal. Although LPDV cannot be grown in cultured cells, it was possible to isolate molecular clones of biologically active integrated provital genomes from spleens of infected turkeys. Based upon molecular hybridization and nucleotide sequence comparisons of its pol gene, LPDV was shown to represent a distinct group of avian retroviruses most closely related to avian sarcoma-leukemia viruses. Here we report the complete nucleotide sequence of the LPDV genome as well as amino acid sequence analysis of its gag gene products. The genetic organization of LPDV is characteristic of members of the oncovirus subfamily. Further sequence comparisons of the gag gene confirmed that LPDV is most closely related to Rous sarcoma virus (RSV). However, the gag, pro, and pol open reading frames (ORFs) were in different translational phases so that the expression of their mature gene products would require the double frame-shifting mechanism utilized by simian retroviruses, mouse mammary tumor virus, and human T-cell leukemia virus. In contrast, the RSV proteinase is synthesized as part of the gag precursor. The LPDV gag gene differs from that of RSV as well as from all other retroviruses in that it encodes a unique 31,000-Da (p31) protein, located between the MA and the CA coding sequences. Four short ORFs of unknown function were present. Whether the putative products of these ORFs account for the acute nature of LPDV-induced disease remains to be determined.