INTEGRATION OF BOVINE LEUKEMIA-VIRUS DNA IN THE BOVINE GENOME

DNA preparations from circulating leukocytes, lymph node tumors, and spleens of three bovine leukemia virus-infected cattle were fractionated by Cs2SO4/3,6-bis(acetatomercurimethyl)dioxane density gradient centrifugation. Bovine leukemia virus proviral sequences were found in large GC-rich fragments having a buoyant density in CsCl close to 1.708 g/cm3. Provirus integration, therefore, does not take place at random locations in the host genome, but in a specific class of DNA segments. Hybridization of cDNA synthesized on viral RNA to EcoRI and Xba I restriction fragments of the DNA from infected cells showed that: (i) only one copy of proviral DNA is integrated per haploid genome; (ii) different restriction patterns were found in the proviral DNAs present in the genomes of different animals, providing evidence for the existence of several viral strains or mutants; and (iii) different integration sites for the proviral DNA were found in the genome of different animals and of different cells in the same animal. The latter finding strongly suggests a polyclonal origin of bovine leukemia virus-infected cells.