Equine herpesvirus type 1 defective-interfering (DI) particle DNA structure: the central region of the inverted repeat is deleted from DI DNA.

The inverted repeat (IRs) component of the genome of equine herpesvirus type 1 (EHV-1) is an important region of structure and function. It is a major constituent of the DNA of EHV-1 defective-interfering (DI) particles which have been shown to mediate the coestablishment of oncogenic transformation and persistent infection of hamster embryo cells. In addition, the IRs encodes the single EHV-1 immediate early gene and the 31.5K very early protein. DNA sequences encompassing EHV-1 internal IRs and the joint between the long (L) and short (S) regions were subcloned into the plasmid vectors pBR322 and pUC12. A total of 22 subclones were derived, including six Sa/l subclones in pBR322 and 12 SmaI subclones in pUC12. Individual subclones were employed in Southern blot hybridizations to define subclone homology to repeated, unique, or heterogeneous (het) DNA sequences within the EHV-1 genome. These studies revealed that the EHV-1 het region is contained entirely within the unique long region of the viral genome and is separated from the L/S junction by approximately 1.8 MDa of completely unique DNA sequences. Furthermore, these IRs subclones were employed in blot hybridizations to analyze the integrity of IRs DNA sequences within the cloned DNA of EHV-1 DI particles. These analyses demonstrated that IRs DNA sequences present in DI DNA were extensively rearranged and contained major deletions (0.80-0.83 map units) which removed a large portion of the single EHV-1 immediate early gene (0.78-0.83 and 0.95-1.00 map units) located in the IRs. Thus, these data and those previous studies (R. P. Baumann et al., 1984, J. Virol. 50, 13-21; R. P. Baumann, J. Staczek, and D. J. O-Callaghan, 1986, Virology 153, 188-200) indicate that the major subunits of the DI DNA molecule are comprised of selected sequences from the IRs component and a highly conserved short sequence located at the terminus of the L region of the standard viral genome.