(A) Sequence details of 3.2 kb band fromFigure 5Bshowing reconstitution of incomplete DR by fusion of DRLwith a part of DRR. incomplete reconstitution of circular viral genomes made up of single viral direct repeat (DR). Correspondingly, short t-circles containing parts of the HHV-6 DR were detected in cells from individuals with genetically inherited ciHHV-6. Furthermore, telomere shortening induced in the absence ofChlamydiainfection also caused circularization of ciHHV-6, supporting a Pepstatin A t-circle based mechanism for ciHHV-6 reactivation. == Author Summary == Human herpesviruses (HHVs) can reside in a lifelong non-infectious state displaying limited activity in their host and guarded from immune responses. One possible way by which HHV-6 achieves this state is usually by integrating into the telomeric ends of human chromosomes, which are highly repetitive sequences that safeguard the Pepstatin A ends of chromosomes from damage. Numerous stress conditions can reactivate latent HHV-6 thus increasing the severity of multiple human disorders. Recently, we have identifiedChlamydiainfection as a natural cause of latent HHV-6 reactivation. Here, we have sought to elucidate the molecular mechanism of HHV-6 reactivation. HHV-6 efficiently utilizes the well-organized telomere maintenance machinery of the host cell to exit from its inactive state and initiate replication to form new viral DNA. We provide experimental evidence that this shortening of telomeres, as a consequence of interference with telomere maintenance, triggers the release of the integrated computer virus from your chromosome. Our data provide a mechanistic basis to understand HHV-6 reactivation scenarios, which in light of the high prevalence of HHV-6 contamination and the possibility of chromosomal integration of other common viruses like HHV-7 have important medical effects for several million people worldwide. == Introduction == Human herpesvirus 6 (HHV-6) is usually a ubiquitous pathogen with >90% seroprevalence in healthy adults. Although the process of viral latency is not completely comprehended, in some cases it is achieved by the integration of the viral genome into telomeric regions of host cell chromosomes (ciHHV-6)[1], and then subsequently vertically transmitted through the germ collection[1][3]. Approximately 1% of the human population carries genetically inherited HHV-6. After becoming latent, HHV-6 persists in a dormant state with minimal viral transcription or translation in human host cells and without the production of infectious virions and any detectable clinical complications. However, under numerous physiological NNT1 conditions, latent HHV-6 is usually reactivated and forms infectious viral particles (for further details see reviews[4][6]). Reactivated HHV-6 has been associated with numerous human diseases[7][10]. The 160 kb linear double-stranded genome of both species of HHV-6 (HHV-6A and -6B) is usually flanked by two unique regions, ranging from 8 to 13 kb in length[11][13], called direct repeats (DR) at the left (DRL) and the right (DRR) ends of the genome, respectively. Multiple passages of HHV-6 in the laboratory have led to the shortening of DR due to the deletion of specific regions in DRLand DRR[11],[12]. Both the DR regions possess two well-defined stretches of telomeric repeats (T1 and T2). These repeat regions contain several copies of the sequence (TTAGGG)n, which are also found at the termini of linear eukaryotic chromosomes. The left end of each DR has short heterogeneous stretches of telomeric repeats (DRL-T1 and DRR-T1) whereas the right end of both the DRs has a single long stretch of homogeneous telomeric repeats (DRL-T2 and DRR-T2). Several other herpesviruses including Marek’s disease computer virus (MDV) and HHV-7 have a similar genome organization made up of multiple stretches of telomeric repeats at both ends. Although chromosomal integration of HHV-7 has not been identified so far, it has been suggested Pepstatin A that homologous recombination between viral telomeric repeats and human telomeres mediates the integration of HHV-6[14]and MDV[15]into the host cell genome. The presence of telomeric repeats within the viral genome may have dual functions, required for both the integration (to acquire latency) and its excision (to reactivate from latency)[14],[16]. However, the reactivation machinery and the exact mechanism for HHV-6 reactivation are currently unknown. We have previously shown that replication of the ciHHV-6 genome is usually efficiently reactivated in blood cells of patients that are infected withC. trachomatiswithout the formation of viral particles[17]. In this study, we have usedChlamydiainfection as a model to understand the mechanism of ciHHV-6 reactivation. Our results provide strong evidence for the presence of a t-circle based mechanism for the circularization of the integrated viral genome, which is usually possibly independent of the viral infectious cycle. == Results == == A model for ciHHV-6 reactivation == The reproducible and strong reactivation of ciHHV-6 replication byChlamydiasuggests that chlamydial contamination triggers the exit of the integrated computer virus from the host genome and the subsequent formation of circular viral DNA, which functions as a template for rolling circle replication. Since previous work has exhibited that.