STAT1 binds to the herpes simplex virus type 1 latency-associated transcript promoter

The authors hypothesized that environmental stimuli induce cytokines that act through an intracellular cascade, which includes signal transducers and activators of transcription (STATs), to change herpes simplex virus (HSV) gene expression, thereby inducing viral reactivation. The HSV type 1 (HSV-1) latency-associated transcript (LAT) gene regulates viral reactivation within neurons via an unknown mechanism. HSV-1 deletion mutants that are missing key portions of the LAT gene, particularly the 3' region of the LAT promoter, do not reactivate normally in vivo. The authors hypothesized that STAT transcription factors may bind in this region to regulate viral reactivation. Electrophoretic mobility shift assay (EMSA) experiments were performed by incubating mouse trigeminal ganglion (TG) nuclear extracts with each of three overlapping sequences representing the 3' region of the HSV-1 LAT promoter (referred to as oligos L1, L2, and L3). The ganglionic nuclear extracts bound specifically to oligos L1 and L3, but not L2. Oligos L1 and L3 contain predicted STAT binding sequences whereas L2 does not. Specific binding to oligo L3 (including the TATA box sequence) was supershifted by incubating with anti-STAT1 antibodies, but not by incubating with anti-STAT3 or anti-STAT5a antibodies. Specific L3 binding was reduced by competing with excess unlabeled STAT1 consensus sequences. These results indicate that STAT1, probably as part of a complex, is capable of binding to the LAT promoter on or near the TATA box. Further studies are required to determine if STAT1 is required for LAT expression in vivo. This work supports the hypothesis that interferons act through STAT1 to regulate the expression of HSV-1 LAT.