Recombination in Herpes Simplex Virus: Mapping of Mutations and Analysis

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) contain linear double-stranded DNA genomes with molecular weights of about 100 x l0 s (Becker et al. 1968; Kieff et al. 1971). Electron microscopic examination of self-annealed intact single strands demonstrated that virion DNA molecules were com-posed of unique long 01L) and unique short (Us) regions, each flanked by inverted repeated sequences. The sequences flanking UL are designated TRL (termi-nal repeat) and IRL (internal repeat). Those flanking Us are TRs and IRs (Sheldrick and Berthelot 1975). The HSV genome also exhibits a true terminal redun-dancy of about 0.5 x 10 6 daltons (Grafstrom et al. 1975a,b) which is commonly referred to as sequence a and is shared by the repeats flanking both UL and Us. The remaining unshared sequences present in these repeat regions are termed sequence b and sequence c, as summarized in Figure 1. Sheldrick and Berthelot (1975) further predicted that inversions of the L and S segments could generate four possible genome ar-rangements as indicated in Figure 1. Subsequent work by others has shown that these four arrangements are present within the population of virion DNA molecules in approximately equimolar amounts (Hay