Low infectivity of HSV-1 DNA caused by defective-interfering genomes.

The infectivity of herpes simplex virus, type 1, strain ANG progeny DNA from standard virus infections and of progeny DNA from infections involving defective-interfering virus particles (DI DNA) was compared in transfection assays. No difference in infectivity of virus DNA isolated either from infected cells or from progeny virus was found for a given type of infection. However, the values for two types of infection differed markedly, with DI progeny DNA being less infectious by more than 2 log10. The low infectivity was mainly due to the presence of interfering DNA molecules in DI progeny DNA, regardless of whether intracellular DNA or DNA extracted from mature virions was analysed. The interfering capacity of DI progeny DNA did not depend on the integrity of the genomes. The physical proximity provided by simultaneous precipitation of infectious and of interfering DNA is an important factor influencing the degree to which DI DNA interferes. Interference by DI DNA in the transfection assay can be partly reversed by the addition of XbaI fragments of standard DNA; in control experiments this fragmented DNA was shown to lead to a reduction rather than to an enhancement of the infectivity of standard virus DNA.