DISTRIBUTION OF TARGETS FOR AVIAN RETROVIRUS DNA INTEGRATION IN-VIVO

The targeting of DNA integration in retrovirus infected cells is a cen tral yet very poorly understood aspect of the biology of the virus. To investigate this problem, we have assessed the use of specific sites for integration targets of avian leukosis virus (ALV) DNA within defin ed regions of turkey embryo fibroblast (TEF) cellular DNA. For this pu rpose, we developed an assay of sufficient sensitivity and specificity to allow detection and location of single integration events in a pop ulation of 5 million cells. Targets selected for study were either reg ions cloned by virtue of a previous integration event or clones chosen at random from cellular DNA. By use of this approach, we found that a ll genomic regions tested contained integration targets, with a freque ncy that varied from similar to 0.2 to 4 times that expected for rando m integration. Within regions, the frequency of use of specific sites varied considerably, with some sites used up to 280 times random frequ ency. When one region was introduced into cells at moderately high cop y number by transfection, it provided integration targets in a pattern very much like that seen with the same sequence in vitro. On the basi s of our sampling, we conclude that most or all regions of the TEF gen ome are accessible to ALV retroviral integration. As with integration in vitro, integration specificity seems to be determined largely by lo cal structural features rather than accessibility of specific regions.