The defective interfering particles of equine herpesvirus 1 encode an ICP22/ICP27 hybrid protein that alters viral gene regulation

The genomes of equine herpesvirus 1 (EHV-1) defective interfering (DI) particles that mediate persistent infection were shown to encode a unique hybrid open reading frame composed of sequences that encode the 196 N-terminal amino acids of ICP22 linked in-frame to the C-terminal 68 amino acids of ICP27. Previous studies demonstrated that this hybrid gene, designated as ICP22/ICP27, was expressed abundantly at both the mRNA and the protein levels in DI particle-enriched infections, but not in standard EHV-1 infection (Chen et al., 1996 J. Virol. 70, 313–320). Since the ICP22/ICP27 hybrid protein contains portions of two EHV-1 early regulatory proteins, its effect on EHV-1 gene regulation was investigated. In EHV-1-infected cells, the ICP22/ICP27 hybrid protein expressed from plasmid vectors significantly reduced expression of a reporter gene under the control of the EHV-1 immediate-early (IE) gene promoter and early gene promoter, such as the viral ICP27 gene. In uninfected cells, the ICP22/ICP27 hybrid protein moderately down-regulated the IE and ICP22 promoters, up-regulated late gene promoters such as IR5, and altered the regulatory function of the IE and ICP22 proteins in co-transfected cells. These results demonstrated that DI particles might alter viral gene regulation by expression of a unique hybrid gene encoded on the DI particle genome.