AN EARLY GENE MAPS WITHIN AND IS 3' COTERMINAL WITH THE IMMEDIATE-EARLY GENE OF EQUINE HERPESVIRUS-1

The immediate-early (IE) gene (IR1 gene) of equine herpesvirus 1 (EHV-1) encodes a single, spliced 6.0-kb mRNA during cytolytic infection. However, under early (in the presence of phosphonoacetic acid) and late (8 h postinfection; no metabolic inhibitors) conditions, in addition to the 6.0-kb IE mRNA, a 4.4-kb early (E) mRNA is transcribed from the IE gene region beginning at approximately 4 h postinfection. To map and characterize the 4.4-kb E mRNA and the protein product of this early gene (IR2 gene), Northern (RNA) blot hybridization, S1 nuclease, primer extension, and in vitro transcription and translation analyses were used. The data from RNA mapping analyses revealed that the 4.4-kb E IR2 mRNA (i) maps at nucleotides 4481 to 635 within each of the inverted repeats of the short region and thus is encoded by sequences that lie entirely within the IE gene, (ii) is transcribed in the same direction as the IE mRNA, initiating at nucleotide 4481, which lies 25 bp downstream of a putative TATA-like sequence and 1,548 bp downstream of the transcription initiation site of the IE mRNA, and (iii) is 3' coterminal with the IE mRNA which terminates at nucleotide 635 of the inverted repeats. The IR2 open reading frame was inserted into the transcription expression vector pGEM-3Z, and the RNA transcribed from this construct (pGEM44) was shown to be a 4.2-kb transcript that contained all IR2 sequences. In vitro translation of the 4.2-kb RNA yielded a major protein of approximately 130 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. This protein corresponds to the predicted IR2 product of 1,165 amino acids that would be in frame with the major IE polypeptide (IE1 = 200 kDa; 1,487 amino acids) and thus would be a 5'-truncated form of the IE1 polypeptide. The presence and potential role of the IR2 gene embedded within the IR1 gene increase the complexity of the regulation of the IE gene region during various stages of a productive infection.