Herpes simplex virus type 1 DNA replication is specifically required for high-frequency homologous recombination between repeated sequences

Using an assay for recombination that measures deletion of a β- galactosidase gene positioned between two directly repeated 350-bp sequences in plasmids transiently maintained in COS cells, we have found that replication from a simian virus 40 origin produces a high frequency of nonhomologous recombination. In contrast, plasmids replicating from a herpesvirus origin (ori(s)) in COS cells superinfected with herpes simplex virus type 1 (HSV-1) show high levels of homologous recombination between the repeats and an enhanced recombinogenicity of the HSV-1 a sequence that is not seen during simian virus 40 replication. When the same assay was used to study recombination between 120- to 150-bp repeats in uninfected Veto cells, the level of recombination was extremely low or undetectable (<0.03%), consistent with the fact that these repeats are smaller than the minimal efficient processing sequence for homologous recombination in mammalian cells. Recombination between these short repeats was easily measurable (0.5 to 0.8%) following HSV-1 infection, suggesting that there is an alteration of the recombination machinery. The frequency of recombination between repeats of the U(c)-DR1 region, previously identified as the only segment of the HSV- 1 a sequence indispensable for enhanced a-sequence recombination, was not significantly higher than that measured for other short sequences.