The molecular structure of the genome of equine herpesvirus type 1 (EHV-1) was determined by restriction endonuclease mapping studies. Primary restriction enzyme digestion of purified EHV-1 DNA, either unlabeled, 32P04 labeled, or [3H]TdR labeled, gave the following cleavage patterns: EcoRI yielded 17 fragments of 23.4 to 1.3 megadaltons (Mdalton); BglII, 16 fragments of 24.5 to 1.0 Mdalton; XbaI, 15 major fragments of 18.6 to 1.7 Mdalton; and BamHI,17 fragments of 13.7 to 2.8 Mdalton. Several fragments were present in 0.5 M amounts while all others were 1.0 M no 0.25 M fragments were detected. Secondary restriction enzyme digestion of these isolated fragments with various enzymes, analysis of terminal fragments using both the methods of λ 5′ exonuclease digestion and end labeling with polynucleotide kinase, and blot hybridization experiments with 32P-labeled BamHI fragments indicated that this herpesvirus genome is a 92-Mdalton linear, double-stranded DNA molecule and is comprised of two segments designated as L (Long) and S (Short) which are 71.6 and 20.4 Mdalton, respectively. The 0.5 M fragments are located at the ends of the S region, an arrangement which allows the S region to invert relative to the L region; thus, two structural arrangements (isomers) of the genome exist. In addition, areas of heterogeneity were detected at the L terminus, within the S segment, and at a split variable locus in the L region.
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