An activity specified by the XXXX line U2OS can substitute functionally for ICP0, a major regulatory protein of herpes simplex virus type 1.

@entity1 @entity778 @entity421 @entity281
Among the five immediate-early regulatory proteins of @entity778 , only ICP0 is capable of activating all kinetic classes of viral genes. Consistent with its broad transactivating activity, ICP0 plays a major role in enhancing the reactivation of HSV from latency both in vivo and in vitro. Although not essential for viral replication, ICP0 confers a significant growth advantage on the virus, especially at low multiplicities of @entity281 . In this report we describe the expression of a novel activity by the @entity421 cell line U2OS that can substitute functionally for ICP0. Compared with Vero cells, both @entity1 cells and cells of the ICP0-expressing line 0-28 significantly enhanced the plating efficiency of an ICP0 null mutant, 7134. In contrast, the plating efficiencies of the wild-type virus in all three cell types were similar. Single-step growth experiments demonstrated that the yield of 7134 in @entity1 cells was severalfold higher than that in 0-28 cells and about 100-fold higher than that in Vero cells. In order to identify the viral genes whose expression is enhanced by the activity in @entity1 cells, levels of expression of selected viral proteins in extracts of Vero and @entity1 cells were compared by Western blot (immunoblot) analysis following low-multiplicity @entity281 . At a multiplicity of 0.1 PFU per cell, the levels of expression of the immediate-early protein ICP4 and the early protein gD in 7134-infected @entity1 cells were significantly higher than those in 7134-infected Vero cells. When infections were carried out at a multiplicity of 1 PFU per cell, however, no major differences in the levels of expression of these proteins in @entity1 and Vero cells were observed. Cycloheximide reversal experiments demonstrated that the cellular activity expressed in @entity1 cells that promotes high-level expression of ICP4 is not synthesized de novo but appears to exist as a preformed protein(s). To confirm this observation and to determine whether, like immediate-early genes, early, delayed-early, and late viral genes are also responsive to the cellular activity, transient-expression assays were performed. The results of these tests demonstrated that basal levels of expression from immediate-early, early, and delayed-early promoters, but not that from a late promoter, were significantly higher in @entity1 cells than in Vero cells and that this enhancement occurred in the absence of viral proteins.(ABSTRACT TRUNCATED AT 400 WORDS)
