Replication of the murine cytomegalovirus genome: structure and role of the termini in the generation and cleavage of concatenates.

Following infection with murine cytomegalovirus (MCMV), the termini of the linear double-stranded DNA genome fuse to form circular or concatemeric forms which serve as replicative intermediates. To investigate the mechanisms involved in the generation and cleavage of the intracellular concatenates, we have used restriction endonuclease mapping and nucleotide sequence analyses to characterize the structure of the virion DNA termini and intracellular end-to-end fusion fragment. Four each of the cloned EcoRI X and EcoRI c terminal fragments were sequenced. All of the EcoRI X clones and three of the EcoRI c clones contained a 30-base-pair (bp) sequence which was directly repeated at the ends of the MCMV genome. The terminal sequence of the fourth EcoRI c clone began directly after the 30-bp direct repeat. The four EcoRI X clones also had minor length heterogeneity, differing in the number of GC bp at the terminus. Five fusion fragments were sequenced. Three of the fusion fragments contained both direct repeats, while two fusion fragments lacked one entire direct repeat. Direct analyses of the virion DNA and intracellular fusion fragments revealed that the clones accurately reflected the naturally occurring populations and that the relative proportion of fusion fragments containing only one direct repeat increased as the infection progressed. The data suggest that fusion of the termini arises by end-to-end ligation. We also show that adjacent to the MCMV termini are sequences highly conserved among the herpesviruses, and we discuss their potential role in the maturation of the viral genome.