Site-specific cleavage/packaging of herpes simplex virus DNA and the selective maturation of nucleocapsids containing full-length viral DNA

Abstract

Defective genomes present in serially passaged herpes simplex virus (HSV) stocks have been shown to consist of tandemly arranged repeat units containing limited sets of the standard virus DNA sequences. Invariably, the HSV defective genomes terminate with the right (S component) terminus of HSV DNA. Because the oligomeric forms can arise from a single repeat unit, it has been concluded that the defective genomes arise by a rolling circle mechanism of replication. We now report on our studies of defective genomes packaged in viral capsids accumulating in the nuclei and in mature virions (enveloped capsids) translocated into the cytoplasm of cells infected with serially passaged virus. These studies have revealed that, upon electrophoresis in agarose gels, the defective genomes prepared from cytoplasmic virions comigrated with nondefective standard virus DNA (M(r) 100 x 10(6)). In contrast, DNA prepared from capsids accumulating in nuclei consisted of both full-length defective virus DNA molecules and smaller DNA molecules of discrete sizes, ranging in M(r) from 5.5 to 100 x 10(6). These smaller DNA species were shown to consist of different integral numbers (from 1 to approximately 18) of defective genome repeat units and to terminate with sequences corresponding to the right terminal sequences of HSV DNA. We conclude on the basis of these studies that (i) sequences from the right end of standard virus DNA contain a recognition signal for the cleavage and packaging of concatemeric viral DNA, (ii) the sequence-specific cleavage is either a prerequisite for or occurs during the entry of viral DNA into capsid structures, and (iii) DNA molecules significantly shorter than full-length standard viral DNA can become encapsidated within nuclear capsids provided they contain the cleavage/packaging signal. However, capsids containing DNA molecules significantly shorter than standard virus DNA are not translocated into the cytoplasm.