THE ICP0 PROTEIN OF EQUINE HERPESVIRUS-1 IS AN EARLY PROTEIN THAT INDEPENDENTLY TRANSACTIVATES EXPRESSION OF ALL CLASSES OF VIRAL PROMOTERS

To assess the role of the equine herpesvirus type 1 (EHV-1) ICP0 prote in (EICP0) in gene regulation, a variety of molecular studies on the E ICP0 gene and gene products of both the attenuated cell culture-adapte d Kentucky A (KyA) strain and the Ab4p strain were conducted. These in vestigations revealed that (i) the ICP0 open reading frame (ORF) of th e KyA virus strain is 1,257 bp in size and would encode a protein of 4 19 amino acids, and in comparison to the ICP0 gene (ORF63) of the Ab4p strain of 1,596 bp (E. A. Telford, M. S. Watson, K. McBride, and A. J . Davison, Virology 189:304-316, 1992), it lass an internal in-frame d eletion of 339 bp; (ii) one early transcript of 1.4 kb predicted to en code the EICP0 protein and a late transcript of 1.8 kb are detected in Northern blot analysts using probes containing the EICP0 ORF; (iii) t he Kya EICP0 protein (50 kDa) and the Ab4p EICP0 protein (80 kDa) are expressed as several species of early proteins that are first detected at 3 to 4 h postinfection by Western blot analyses of infected-cell p olypeptides, using an antiserum generated to a TrpE fusion protein tha t. harbors amino acids 46 to 153 of the EICP0 protein: and (iv) the EI CP0 protein of both EHV-1 strains is a potent transactivator of EHV-1 genes. Transient expression assays using a simian virus 40 expression construct of the EICP0 protein of the KyA strain showed that the EICP0 protein independently transactivated chloramphenicol acetyltransferas e reporter constructs under the control of the immediate early promote r (3.9-fold), the early thymidine kinase promoter (95-fold), the late (gamma 1) IR5 promoter (85-fold), and the late (gamma 2 glycoprotein K promoter (21-fold), The finding that the EICP0 protein of the KyA vir us can function as an activator of gene expression indicates that amin o acids corresponding to residues 319 to 431 of the Ab4p EICP0 protein are not essential for EICP0 transactivation of EHV-1 promoters.