THE ESSENTIAL IN-VIVO FUNCTION OF THE HERPES-SIMPLEX VIRUS UL42 PROTEIN CORRELATES WITH ITS ABILITY TO STIMULATE THE VIRAL-DNA POLYMERASE IN-VITRO

The product of the UL42 gene of herpes simplex Virus type 1. (HSV-1) i s an essential protein required for Viral DNA synthesis in both transi ent origin of replication-dependent DNA replication assays and in viru s-infected cells. In vitro, UL42 has been shown to form a heterodimeri c complex with the 140-kDa protein product of the viral DNA polymerase (pol) gene. Although the pol gene possesses catalytic activity in vit ro in the absence of UL42, UL42 stimulates pot activity presumably by increasing its processivity. In order to investigate whether the essen tial in vivo function for UL42 is related to its ability to associate with and modify pol activity, we have examined the ability of a UL42 n ull mutant, Cgal Delta 42, to induce pol activity in nonpermissive Ver o cells or permissive V9 cells. No detectable high salt-resistant pol activity was observed in Vero cells, although substantial activity was induced in V9 cells. Use of temperature-sensitive and host range muta nts with defects in other genes revealed that failure to induce pol ac tivity was due to neither direct nor indirect effects caused by lack o f viral DNA synthesis. Furthermore, pol protein accumulated in Cgal De lta 42 virus-infected nonpermissive cells with similar kinetics and to approximately the same level as in cells infected with wild-type viru s. These results suggest a direct dependence on UL42 for pol activity. We also examined whether the same domains of UL42 affected the abilit y of the protein to stimulate pol activity in vitro and to complement the replication of Cgal Delta 42. The excellent correlation between th e activities of the mutant UL42 proteins in the in vitro pol stimulati on assays and in the in vivo transient complementation assay, indicate s that the predominant in vivo role of UL42 is to provide pol accessor y function, although additional essential functions for UL42 cannot be ruled out. (C) 1994 Academic Press, Inc.