Sp1 binds to promoter sequences and activates herpes simplex virus 'immediate-early' gene transcription in vitro

During a herpes simplex virus (HSV-1) lytic infection, three classes of viral genes are transcribed in a temporally regulated manner. The HSV-1 'immediate-early' (IE) promoter sequences contain multiple copies of a hexanucleotide sequence, GGGCGG, known as a GC box, and one or more copies of an 11-base pair (bp) conserved A + T-rich element, designated TAATGARAT. The TAATGARAT elements are thought to mediate the trans-activation of IE RNA synthesis by a virion-associated protein(s), and the flanking G + C-rich sequences appear both to potentiate this induction and to direct IE promoter activity in vivo. The similarity of the herpesvirus GC box repeats to those of the simian virus 40 (SV40) early promoter prompted the in vitro analysis of HSV IE transcription reported here. We show that the mammalian gene-specific transcription factor Sp1 binds to eight distinct regions of the HSV short terminal repeat and stimulates transcription 25-fold from the divergent IE-3 (ICP-4) and IE-4/5 promoters.