Retrovirus Insertion into Herpesvirus: Characterization of a Marek’s Disease Virus Harboring a Solo LTR

We recently reported the phenomenon of retroviral insertion into a herpesvirus (Isfort et al., (1992) Proc. Natl. Acad. Sci. USA 89, 991-995). We have now isolated Marek’s disease virus (MDV) clones that carry retroviral inserts. Retroviral LTR insertion has the potential to activate or inactivate herpesvirus genes leading to alterations of the biological properties of the herpesviruses. The structure and the expression pattern of a virus clone carrying a solitary LTR insertion was characterized in detail. The LTR insertion is accompanied by deletions in the herpesvirus genome at each of the junctions of the short unique region (U_s) and its inverted repeats (R_s). This results in a significantly truncated R_s region. Despite these structural rearrangements, the virus is replication-competent and demonstrates an enhanced growth rate in vitro. While the MDV homologs of herpes simplex U_s genes are retained in this virus, at least two open reading frames are deleted and one transcript observed in wild-type virus is disrupted. In addition to defining several regions of MDV that are nonessential for in vitro growth, this clone illustrates one way in which retroviral information can be stably transmitted by a herpesvirus.