Virus particles produced by the herpes simplex virus type 1 alkaline nuclease null mutant ambUL12 contain abnormal genomes

Open reading frame UL12 of herpes simplex virus type 1 (HSV-1) encodes an alkaline nuclease that has previously been implicated in processing the complex, branched, viral DNA replication intermediates and allowing egress of DNA-containing capsids from the nucleus. This report describes experiments using the HSV-1 UL12 null mutant ambUL12, which aim to explain the approximately 200- to 1000-fold decrease in the yield of infectious virus, compared with wild-type (wt) HSV-1, from non-complementing cells. A detailed examination revealed that both DNA replication and encapsidation were affected in ambUL12-infected cells, resulting in an approximately 15- to 20-fold reduction in the amount of packaged DNA. In contrast to previous reports, the absence of UL12 function did not greatly impair capsid release into the cytoplasm, and virus particles were readily detected in the supernatant medium from ambUL12-infected cells. The released virus, however, exhibited much higher particle/p.f.u. ratios than wt HSV-1, and this made a further important contribution to the overall reduction in yield. Gel analyses of packaged ambUL12 and wt DNAs revealed the presence of structural abnormalities. The DNA obtained from extracellular ambUL12 virions was non-infectious in transfection assays, and both ambUL12 DNA and virus particles exerted a dominant inhibitory effect on the growth of wt virus. These results suggest that ambUL12 virions produced in non-complementing cells have a greatly reduced ability to initiate new cycles of infection, and that this defect results from the encapsidation of abnormal genomes.