Exon 3 of the human cytomegalovirus major immediate-early region is required for efficient viral gene expression and for cellular cyclin modulation.

The human cytomegalovirus (HCMV) major immediate-early (IE) proteins share an 85-amino-acid N-terminal domain specified by exons 2 and 3 of the major IE region, UL122-123. We have constructed IE Delta30-77, a recombinant virus that lacks the majority of IE exon 3 and consequently expresses smaller forms of both IE1 72- and IE2 86-kDa proteins. The mutant virus is viable but growth impaired at both high and low multiplicities of infection and exhibits a kinetic defect that is not rescued by growth in fibroblasts expressing IE1 72-kDa protein. The kinetics of mutant IE2 protein accumulation in IE Delta30-77 virus-infected cells are approximately normal compared to wild-type virus-infected cells, but the IE Delta30-77 virus is delayed in expression of early viral genes, including UL112-113 and UL44, and does not sustain expression of mutant IE1 protein as the infection progresses. Additionally, cells infected with IE Delta30-77 exhibit altered expression of cellular proteins compared to wild-type HCMV-infected cells. PML is not dispersed but is retained at ND10 sites following infection with IE Delta30-77 mutant virus. While the deletion mutant retains the ability to mediate the stabilization of cyclin B1, cdc6, and geminin in infected cells, its capacity to upregulate the expression of cyclin E has been reduced. These data indicate that the activity of one or both of the HCMV major IE proteins is required in vivo for the modulation of cell cycle proteins observed in cells infected with wild-type HCMV.