ALTERATIONS IN THE EQUINE HERPESVIRUS TYPE-1 (EHV-1) STRAIN RACH DURING ATTENUATION

The equine herpesvirus type-1 modified live-vaccine strain RacH (256th passage on porcine embryonic kidney cells) was investigated by restri ction-enzyme analysis and compared to representative plaque isolates o f the 12th passage (RacL11, RacL22) and 185th passage (RacM24, RacM36) . The restriction patterns of all Rac plaque isolates differed compare d with reference strain Ab4. The left U-L terminus was shortened by 0. 1 kbp and a missing BamHI, site led to the fusion of the f and t fragm ents. In some Rac derivatives, losses of restriction sites without del etions were observed: 1. One BamHI site located in the ribosyl reducta se gene was missing in RacH, RacM24, RacM36, and RacL22; and 2. An Sai l site mapping to the gp14 (gB) gene was absent in RacM24, RacM36 and RacH. An identical deletion of 0.85 kbp in size was found in both copi es of the inverted repeat (IR) regions of RacH. The deletion was prese nt only in the terminal IR of the medium-passage derivative RacM36. By contrast, in the genomes of the apathogenic RacM24, as well as the pa thogenic plaque isolates RacL11 and RacL22, no deletions in the IRs we re detectable. Nucleotide-sequence and Northern-blot analyses revealed chat the deletions led to the elimination of one or both copies of th e gene 67 (IR6) open-reading frame in RacM36 and RacH and affected the gene 68 (EUS1) in RacH.