The EHV-1 UL4 protein that tempers viral gene expression interacts with cellular transcription factors

•The UL4 gene is conserved in the genome of DI particles of EHV-1.•The UL4 gene is not essential for EHV-1 lytic replication.•The UL4 protein binds to cellular transcription factors TBP and Pol II.•Late viral gene expression is enhanced in UL4 null virus infection.•Viral DNA synthesis is not retarded in cells infected with the UL4-null virus.

The UL4 gene is conserved within the genome of defective interfering particles of equine herpesvirus type 1 (EHV-1) that mediate persistent infection. Here, we show that the UL4 protein inhibits EHV-1 reporter gene expression by decreasing the level of transcribed mRNA. The UL4 protein did not bind any gene class of EHV-1 promoters in electromobility or chromatin immunoprecipitation assays, but directly interacted with the TATA box-binding protein (TBP) and the carboxy-terminal domain of RNA polymerase II both in vitro (GST-pulldown assays) and in infected cells (coimmunoprecipitation analyses). Microarray analyses of the expression of the 78 EHV-1 genes revealed that viral late genes important for virion assembly displayed enhanced expression in cells infected with UL4-null virus as compared to wild-type or UL4-restored EHV-1. Quantitative PCR analyses showed that viral DNA replication was not retarded in cells infected with the UL4-null virus as compared to wild-type EHV-1.