The open reading frames 1, 2, 71, and 75 are nonessential for the replication of equine herpesvirus type 1 in vitro.

Equine herpesvirus type 1 (EHV-1) strain Ab4 has five genes, 1, 2, 67, 71, and 75, which have no homologues in any of the herpesviruses sequenced to date; i.e., they are unique to EHV-1. The functions of these unique genes are not known. To study their role in the virus life cycle, we have constructed four independent mutants in which the majority of the coding sequences of genes 1, 2, 71, and 75 has been deleted and replaced by the Escherichia coli lacZ gene. Mutant ED1 has a deletion of 508 bp within the 608-bp gene 1 open reading frame (ORF); mutant ED2 has a 400-bp deletion within the 617-bp gene 2 ORF; mutant ED71 has a 1811-bp deletion within the 2393-bp gene 71 ORF, whereas mutant ED75 lacks 189 bp of the 392-bp gene 75 ORF. Mutants ED1, ED2, and ED75 display growth characteristics indistinguishable from those of wild-type virus in cell culture. The growth in vitro of ED71 is marginally impaired compared to that of wild-type virus. Our results demonstrate that genes 1, 2, 71, and 75 are not essential for EHV-1 growth in tissue culture. The isolation of a deletion mutant in gene 67 was unsuccessful, indicating that gene 67 is probably essential for virus growth in tissue culture.