Leukemogenesis by bovine leukemia virus: proviral DNA integration and lack of RNA expression of viral long terminal repeat and 3' proximate cellular sequences.

The DNA from 17 lymphoid tumors induced by bovine leukemia virus (BLV) was digested with the restriction endonuclease EcoRI. Filter hybridization analysis using radioactive probes specific for the BLV genome showed that all tumors contained at least one or a portion of one provirus. Digestion of these proviruses with Sac I demonstrated that deletions occurred in about 25% of the cases and involved sequences located in the 5' half of the provirus. No sequence homology was observed between the cloned proximate cellular sequences flanking two different proviruses at their 3' end and the corresponding sequences in 16 other tumor DNAs, thus showing that a wide range of genomic sites could accommodate BLV proviruses. Transcription of viral DNA including long terminal repeated sequences was not detected, strongly suggesting that viral gene expression is not required for maintenance of the tumor state. No expression of 3'-proximate cellular sequences was observed, indicating that no proximate downstream promotion took place in the cases examined.