Early Growth Response gene 1 (Egr-1) regulates HSV-1 ICP4 and ICP22 gene expression

The molecular mechanisms mediating herpes simplex virus type 1 (HSV-1) gene silencing during latent infection arenot clear. Five copies of early growth response gene 1 (Egr-1) binding elements were identified in the intron of HSV-1ICP22 (infected cell protein No. 22) gene, leading to the hypothesis that Egr-1 binds to the viral genome and regulates theviral gene expression. Transient co-transfection assays indicated that Egr-1 negatively regulated the transcription of bothfull-length and intron-removed ICP22 promoters. The same assays also revealed that Egr-1 repressed ICP4 (infected cellprotein No. 4) promoter activity in a dose-dependent manner but showed less inhibition when the intron was removed.Histone deacetylation was not involved in this regulation since histone deacetylase inhibitor trichostatin Adid not exhibitany effect on Egr-1-mediated repression. Chromatin immunoprecipitation assays showed that Egr-1 reduced the bindingof Spl to the promoters and that the co-repressor Nab2 (NGFI-A/EGR1-binding protein) was recruited to the proximityof ICP4 in the presence of Egr-1. These results suggested that the multifunctional transcription factor Egr-1 can repressHSV-1 immediate-early gene expression through the recruitment of co-repressor Nab2 and reduction of Sp1 occupancy,and thus may play a critical role in HSV-1 gene silencing during latency.