Terminally repeated sequences on a herpesvirus genome are deleted following circularization but are reconstituted by duplication during cleavage and packaging of concatemeric DNA.

The mechanisms underlying cleavage of herpesvirus genomes from replicative concatemers are unknown. Evidence from herpes simplex virus type 1 suggests that cleavage occurs by a nonduplicative process; however, additional evidence suggests that terminal repeats may also be duplicated during the cleavage process. This issue has been difficult to resolve due to the variable numbers of reiterated terminal repeats that the herpes simplex virus type 1 genome can contain. Guinea pig cytomegalovirus is a herpesvirus with a simple terminal repeat arrangement that defines two genome types. Type II genomes have a single copy of a 1-kb terminal repeat at both their left and right termini, whereas type I genomes have only one copy at their left termini and lack the repeat at their right termini. In a previous study, we constructed a recombinant guinea pig cytomegalovirus in which certain cis elements were disrupted such that only type II genomes were produced. Here we show that double repeats that are formed by circularization of infecting genomes are rapidly converted to single repeats, such that the junctions between genomes within replicative concatemers formed late in infection almost exclusively contain single copies of the terminal repeat. Therefore, for the recombinant virus, each cleavage event begins with a single repeat within a concatemer yet produces two repeats, one at each of the resulting termini, demonstrating that terminal repeat duplication occurs in conjunction with cleavage. For wild-type guinea pig cytomegalovirus, the formation of type I genomes further suggests that cleavage can also occur by a nonduplicative process and that duplicative and nonduplicative cleavage can occur concurrently. Other herpesviruses having terminal repeats, such as the herpes simplex viruses and human cytomegalovirus, may also utilize repeat duplication and deletion; however, the biological importance of these events remains unknown.