Replication of Mus dunni endogenous retrovirus depends on promoter activation followed by enhancer multimerization

Mus dunni endogenous virus (MDEV) is an apparently intact retrovirus that normally lies transcriptionally silent in cultured M. dunni cells, but the provirus can be activated by treatment of the cells with hydrocortisone or 5-iodo-2'-deoxyuridine. Sequence analysis of a molecular clone of the replicating virus revealed a simple retrovirus with a chimeric VL30/GALV-like structure. Interestingly, in the region of the long terminal repeat (LTR) that typically contains the retroviral transcription enhancers, we found over six 80-bp repeats with only a single mismatch, indicating that acquisition of the repeats was a recent event. Here we provide evidence for the following model of MDEV activation and replication. The MDEV provirus in M. dunni cells has a chimeric structure similar to that of the molecular clone but has only 1.15 copies of the 80-bp repeat sequence found in the molecular clone. Activating chemicals directly stimulate transcription from the LTR, allowing a low level of virus replication. Copying errors made during reverse transcription allow multimerization of the 80-bp enhancer region, resulting in viruses with higher transcriptional rates and improved fitness, but increased enhancer copy number is likely balanced by the natural instability of retroviral repeats and constraints imposed by virion packaging limits. The resultant population of replicating MDEV is widely heterogeneous, having from 2.15 to 13.15 enhancer repeats in the LTR. These results reveal a novel mechanism for regulation of transcription and replication of an endogenous retrovirus, in terms of both activation of the virus by the steroid hydrocortisone and the large number and variation in enhancer repeats observed.