Tandem repeat defective DNA from the L segment of the HSV genome

Summary

Restriction enzyme analysis of the DNA from a collection of defective populations of herpes simplex virus generated by serial undiluted passage in HEp-2 or Vero cells indicates that the amount and sequence complexity of the accumulated tandem repeat DNA species varies considerably in different strains and passaging series. One particular reiterated DNA species accounting for between 50% and 90% of the viral DNA of the 14th to 20th passage of HSV-1(MP) virus has been studied in some detail. It differs from previously well characterized species by not having a higher buoyant density and (G + C)- content than the complete parent qenomes and being derived from the centre of the L-segment of the genome rather than the right-hand end of the S-segment. The repeat unit of this new defective DNA has a molecular size of 5.2 × 10⁶ daltons or approximately 7800 base pairs and has homology with sequences from coordinates 0.385 to 0.437 in the standard HSV-1 physical map. This sequence overlaps with part or all of the DNA polymerase gene and we presume that it also possesses an origin for the initiation of DNA synthesis. Each repeat unit also retains approximately 200 base pairs from near the right-hand terminus of the parental S segment that are apparently necessary for site-specific cleavage events during the packaging of mature virion DNA. An exactly integral number of copies of adjacent tandem repeat units totalling close to 100 × 10⁶ daltons are encapsidated to form the completed defective DNA molecules from a pool of much larger replicating molecules in the nucleus.