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) is an essential protein required for viral DNA synthesis in both transient origin of replication-dependent DNA replication assays and in virus-infected cells. In vitro, UL42 has been shown to form a heterodimeric complex with the 140-kDa protein product of the viral DNA polymerase (pol) gene. Although the pol gene possesses catalytic activity in vitro in the absence of UL42, UL42 stimulates pol activity presumably by increasing its processivity. In order to investigate whether the essential 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 null mutant, Cgal delta 42, to induce pol activity in nonpermissive Vero 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 mutants with defects in other genes revealed that failure to induce pol activity was due to neither direct nor indirect effects caused by lack of viral DNA synthesis. Furthermore, pol protein accumulated in Cgal delta 42 virus-infected nonpermissive cells with similar kinetics and to approximately the same level as in cells infected with wild-type virus. These results suggest a direct dependence on UL42 for pol activity. We also examined whether the same domains of UL42 affected the ability of the protein to stimulate pol activity in vitro and to complement the replication of Cgal delta 42. The excellent correlation between the activities of the mutant UL42 proteins in the in vitro pol stimulation assays and in the in vivo transient complementation assay indicates that the predominant in vivo role of UL42 is to provide pol accessory function, although additional essential functions for UL42 cannot be ruled out.