Definition of the minimal cis-acting sequences necessary for genome maturation of the herpesvirus murine cytomegalovirus

Herpesvirus DNA replication proceeds via concatemeric replicative intermediates that are comprised of head-to-tail-linked genomes. Genome maturation is carried out by the terminase, a protein complex that mediates both insertion of concatemer DNA into capsids and its subsequent cleavage to release genomes within these capsids. This cleavage is sequence specific, but the governing cis-acting DNA sequences are only partially characterized. Two highly conserved motifs called pac1 and pac2 lie near the ends of herpesvirus genomes and are known to be critical for genome maturation. However, the potential importance of other sequences has not been fully investigated. We have undertaken to define all of the sequences necessary for efficient genome maturation for a herpesvirus by inserting ectopic cleavage sites into the murine cytomegalovirus genome and assessing their ability to mediate genome maturation. A combination of deletion and substitution mutations revealed that the minimal cleavage site is large (~180 bp) and complex. Sequences distal of pac1 (relative to the point of cleavage) were dispensable, suggesting that pac1 may be the sole cis-acting element on this side of the cleavage site. In contrast, a region distal to pac2 up to 150 bp from the point of cleavage was essential. Scanning substitutions revealed that the pac2 side of the cleavage site is complex and may contain multiple cis-acting sequence elements in addition to pac2. These results should facilitate the identification of trans-acting factors that bind to these elements and the elucidation of their functions. Such information will be critical for understanding the molecular basis of this complex process.