Machinery to support genome segment inversion exists in a herpesvirus which does not naturally contain invertible elements

In many herpesviruses, genome segments flanked by inverted repeats invert during DNA replication. It is not known whether this inversion is a consequence of an inherently recombinagenic replicative mechanism common to all herpesviruses or whether the replication enzymes of viruses with invertible segments have specifically evolved additional enzymatic activities to drive inversion. By artificially inserting a fusion of terminal sequences into the genome of a virus which normally lacks invertible elements (murine cytomegalovirus), we created a genome composed of long and short segments flanked by 1,359- and 543-bp inverted repeats. Analysis of genomic DNA from this virus revealed that inversion of both segments generates equimolar amounts of four isomers during the viral propagation necessary to produce DNA for analysis from a single viral particle. We conclude that a herpesvirus which naturally lacks invertible elements is able to support efficient segment inversion. Thus, the potential to invert is probably inherent in the replication machinery of all herpesviruses, irrespective of genome structure, and therefore genomes with invertible elements could have evolved simply by acquisition of inverted repeats and without concomitant evolution of enzymatic activities to mediate inversion. Furthermore, the recombinagenicity of herpesvirus DNA replication must have some importance independent of genome segment inversion.