Distinct and separate roles for herpesvirus-conserved UL97 kinase in cytomegalovirus DNA synthesis and encapsidation

The human cytomegalovirus UL97 kinase, an important target of antiviral the rapy, has an impact on at least two distinct phases of viral replication. C ompared with wild-type virus, the UL97 deletion mutant exhibits an early re plication defect that reduces DNA accumulation by 4- to 6-fold, as well as a late capsid maturation defect responsible for most of the observed 100- t o 1000-fold reduction in replication. Block-release experiments with the an tiviral 2-bromo-5,6-dichloro-1-(beta -D-ribofuranosyl)-benzimidazole reveal ed an important role for UL97 kinase in capsid assembly. Although cleavage of concatemeric DNA intermediates to unit-length genomes remained unaffecte d, progeny mutant virus maturation was delayed, with accumulation of progen y at significantly reduced levels compared with wild type after release of this block. Transmission electron microscopy confirmed the aberrant accumul ation of empty A-like capsids containing neither viral DNA nor an internal scaffold structure, consistent with a failure to stably package DNA in muta nt virus-infected cells. The function of UL97 in DNA synthesis as well as c apsid assembly suggests that protein phosphorylation mediated by this herpe svirus-conserved kinase increases the efficiency of these two distinct phas es of virus replication.