The linear virion Epstein-Barr virus (EBV)
DNA is terminated at both ends by a variable number of direct, tandemly arranged terminal repeats (TRs) which are approximately 500 bp in size The number of TRs at each terminus can vary. After
infection of host cells, the EBV
DNA circularizes via the TRs by an unknown mechanism, and replication of the
viral DNA during the lytic phase of the EBV life cycle leads to large
DNA concatemers which need to be cleaved into virion
DNA units, eventually. This cleavage event occurs at an unknown locus within the TRs of EBV, which are the cis-acting elements essential for cleavage of the concatemers and encapsidation of the virion
DNA. To investigate the mechanism of
DNA processing during genome circularization and cleavage of concatemeric
DNA, the genomic termini of EBV were cloned, sequenced, and analyzed by direct labeling of the virion
DNA. Both termini ended with identical 11-bp elements; the right end has acquired an additional 9-bp stretch that seemed to originate from the leftmost unique sequences. The left terminus is blunt, whereas the right terminus appears to have a 3' single-base extension. In a transient packaging assay, a single terminal repeat was found to be sufficient for encapsidation of plasmid
DNA, and mutagenesis of the TR
element defined a region of 159 bp, including the 11-bp
element, which is essential for packaging. These results indicate that the genomic termini of EBV are not generated by a simple cut of a hypothetical
terminase. The mechanism for cleavage of concatemers seems to involve recombination events.