Transcriptional suppression of IE180 and TK promoters by the EP0 of pseudorabies virus strains Ea and Fa

In our transient expression assays, the IE180 and TK promoters were markedly suppressed by the EP0 of the pseudorabies virus strains Ea and Fa (EP0/Ea, EP0/Fa). This is in contrast with the transactivating activity of EP0 from strain YS-81 (EP0/YS-81) in previous studies. Amino acid sequence alignment revealed several mutations in both EP0/Ea and EP0/Fa compared with EP0/YS-81. Most remarkable is a two-amino acid substitution next to the RING finger domain which was considered to be important for the transactivating activity of EP0 in previous studies. To address the effect of the two-amino acid substitution on the function of EP0, reverse mutants of EP0/Ea and EP0/Fa were generated. The subsequent expression assays indicated that this substitution was at least in part responsible for the effect on the regulatory activity of EP0. Our data suggested that EP0 may regulate the expression of the same genes in different pseudorabies virus strains differently.