The interaction of ICP4 with cell/infected-cell factors and its state of phosphorylation modulate differential recognition of leader sequences in herpes simplex virus DNA.

Regulation of herpes simplex virus (HSV) gene expression requires the synthesis of functional ICP4, a phosphoprotein that binds to several specific sites in virus DNA and acts in trans either to activate or to repress transcription of the three major kinetic classes of virus genes. Binding of ICP4 to specific sites in alpha genes (which are the first to be transcribed) causes repression of alpha-gene expression. ICP4 also indirectly participates in the formation of DNA-protein complexes with sequences present in the promoter/regulatory and leader regions of the beta and gamma genes that are sequentially activated later in infection. Here we demonstrate that the extent of phosphorylation of ICP4 contributes to its ability to participate differentially in complex formation with cis-acting elements present in beta and gamma genes. Dephosphorylated ICP4 retains its binding properties for the high affinity sites present in alpha promoters, whereas only phosphorylated forms of the protein are able to participate in complex formation with model beta and gamma sequences. These studies also reveal a requirement for cell and infected-cell factors to recognize the beta and gamma sequences. Our data suggest that the state of phosphorylation and concentration of ICP4 within the nucleus of infected cells determine the extent to which ICP4 interacts with these other factors.