Promoter-independent activation of heterologous virus gene expression by the herpes simplex virus immediate-early protein ICP27.

The herpes simplex virus (HSV) immediate-early protein ICP27 has been postulated to play an auxillary role in HSV. gene expression, augmenting or inhibiting the activation of different viral promoters by the other immediate-early proteins ICPO and ICP4. Here we show that ICP27 alone can up-regulate gene expression of a retroviral vector containing Moloney murine leukemia virus (MoMuLV) regulatory sequences. This is the first report of an effect of ICP27 on gene expression driven by heterologous virus regulatory sequences. The effect does not involve the region of ICP27 which inhibits gene activation of HSV early gene promoters by ICPO and ICP4, but rather is dependent on the same region of ICP27 as is required to augment the activation of HSV late gene promoters by ICPO and ICP4. This indicates that the two activation effects are likely to operate via the same mechanism. Activation by ICP27 is dependent on the 3' LTR and adjacent region of MoMuLV but is independent of the promoter in the 5' LTR which can be replaced by a heterologous promoter such as that of SV40 without affecting activation by ICP27. The significance of this effect for an understanding of the mechanism of action of ICP27 and its role in regulating the gene expression of HSV and potentially of other viruses is discussed.