THE PK DOMAIN OF THE LARGE SUBUNIT OF HERPES-SIMPLEX VIRUS TYPE-2 RIBONUCLEOTIDE REDUCTASE (ICP10) IS REQUIRED FOR IMMEDIATE-EARLY GENE-EXPRESSION AND VIRUS GROWTH

The large subunit of herpes simplex virus (HSV) ribonucleotide reducta se (RR), RR1, contains a unique amino-terminal domain which has serine /threonine protein kinase (PK) activity. To examine the role of the PK activity in virus replication, we studied an HSV type 2 (HSV-2) mutan t with a deletion in the RR1 PK domain (ICP10 Delta PK). ICP10 Delta P K expressed a 95-kDa RR1 protein (p95) which was PK negative but retai ned the ability to complex with the small RR subunit, RR2, Its RR acti vity was similar to that of HSV-2. In dividing cells, onset of virus g rowth was delayed, with replication initiating at 10 to 15 h postinfec tion, depending on the multiplicity of infection. In addition to the d elayed growth onset, virus replication was significantly impaired (1,0 00-fold lower titers) in nondividing cells, and plaque forming ability was severely compromised. The RR1 protein expressed by a revertant vi rus [HSV-2(R)] was structurally and functionally similar to the wild-t ype protein, and the virus had wild-type growth and plaque-forming pro perties. The growth of the ICP10 Delta PK virus and its plaque-forming potential were restored to wild-type levels in cells that constitutiv ely express ICP10. Immediate-early (IE) genes for ICP4, ICP27, and ICP 22 were not expressed in Vero cells infected with ICP10 Delta PK early in infection or in the presence of cycloheximide, and the levels of I CP0 and p95 were significantly (three- to sevenfold) lower than those in HSV-2- or HSV-2(R)-infected cells. IE gene expression was similar t o that of the wild-type virus in cells that constitutively express ICP 10. The data indicate that ICP10 PK is required for early expression o f the viral regulatory IE genes and, consequently, for timely initiati on of the protein cascade and HSV-2 growth in cultured cells.