THE EQUINE HERPESVIRUS-1 (EHV-1) UL3 GENE, AN ICP27 HOMOLOG, IS NECESSARY FOR FULL ACTIVATION OF GENE-EXPRESSION DIRECTED BY AN EHV-1 LATE PROMOTER

We have previously reported that the equine herpesvirus 1 (EHV-1) XbaI G restriction fragment (nucleotides 1436 to 7943 relative to the left terminus of the EHV-1 genome [Kentucky A strain]) is required in combination with the EHV-1 immediate-early (IE) gene to achieve significant activation of two representative EHV-1 late promoter-chloramphenicol acetyltransferase (CAT) recombinants in transient expression assays. In this report, we demonstrate that the XbaI G-encoded UL3 gene (an ICP27 homolog) provides a trans-acting factor which acts (in combination with the EHV-1 IE gene product) to increase reporter gene expression directed by an EHV-1 late promoter-CAT recombinant plasmid. We show that cloned copies of UL3 can successfully substitute for the XbaI G fragment in CAT assays and that stop codon insertion within the UL3 open reading frame inhibits the ability of UL3 to activate reporter gene expression in trans.