Relationship between retroviral DNA integration and gene expression

Although retroviruses can integrate their DNA into a large number of sites in the host genome, factors controlling the specificity of integration rema in controversial and poorly understood. To assess the effects of transcript ional activity on integration in vivo, we created quail cell clones contain ing a construct with a minigene cassette, whose expression is controlled by the papilloma virus E2 protein. From these clones we derived transcription ally active subclones expressing the wild-type E2 protein and transcription ally silent subclones expressing a mutant E2 protein that binds its target DNA but is unable to activate transcription. By infecting both clones and s ubclones with avian leukosis virus and using a PCR-based assay to determine viral DNA integration patterns, we were able to assess the effects of both protein binding and transcriptional activity on retroviral DNA integration . Contrary to the hypothesis that transcriptional activity enhances integra tion, we found an overall decrease in integration into our gene cassette in subclones expressing the wild-type E2 protein. We also found a decrease in integration into our gene cassette in subclones expressing the mutant E2 p rotein, but only into the protein binding region. Based on these findings, we propose that transcriptionally active DNA is not a preferred target for retroviral integration and that transcriptional activity may in fact be cor related with a decrease in integration.