A recombinant HIV provirus is synergistically activated by the HIV Tat protein and the HSV IE1 protein but not by the HSV IE3 protein:

To study the effects of regulatory proteins encoded by herpes viruses on the HIV long terminal repeat (LTR) in the presence or absence of HIV-encoded regulatory products, we prepared a proviral construct containing 5' and 3' HIV LTR, but lacking the coding sequences of any HIV proteins. This construct allowed the effects of herpesvirus regulatory proteins on the HIV LTR to be assessed in a construct similar to the HIV provirus whilst also allowing their interactions with HIV-encoded regulatory proteins to be studied. In this system, the herpes simplex virus (HSV) protein IE1 (ICPO) but not the IE3 (ICP4) protein can activate the HIV LTR, whereas both proteins are active on a single plasmid-borne HIV LTR. Although the activation of the LTR by IE1 is strongly stimulated by the HIV Tat protein, it can also be observed in the absence of Tat, indicating that HSV infection via IE1 has the potential to activate an entirely silent, latent HIV provirus.