Molecular cloning and characterization of endogenous feline leukemia virus sequences from a cat genomic library

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Abstract
Recombinant bateriophage .lambda. clones from a cat genomic library derived from placental DNA of a specific pathogen-free cat were screened to identify endogenous feline leukemia virus (FeLV) sequences. Restriction endonuclease mapping of 4 different clones indicates that there are a number of similarities among them, notably the presence of a 6.0-6.4 kilobase (kbp) EcoRI hybridizing fragment containing portions of sequences homologous to the gag, pol, env and long terminal repeat-like elements of the infectious FeLV. The endogenous FeLV sequences isolated are .apprx. 4 kbp in length and are significantly shorter than the cloned infectious FeLV isolates, which are 8.5-8.7 kbp in length. The endogenous elements have 3.3-3.6 kbp deletions in the gag-pol region and .apprx. 0.7-1.0 kbp deletions in the env region. These deletions would render them incapable of encoding an infectious virus and thus may be related to the non-inducibility of FeLV from uninfected cat cells and the subgenomic expression of these endogenous sequences in placental tissue. It appears there that is conservation in the ordering of restriction sites previously reported in the proviruses of the infectious FeLV in sequences corresponding to the pol and env boundary as well as the region spanning the env gene of the endogenous clones; a greater divergence occurs among restriction sites mapped to the gag and part of the pol regions of the infectious FeLV. Such deleted, FeLV-related subsets of DNA sequences could have originated either by germ-line integration of a complete ecotropic virus followed by deletion, or by integration of a preexisting, defective, deleted variant of the infectious virus.