Enhancer and Long-Term Expression Functions of Herpes Simplex Virus Type 1 Latency-Associated Promoter Are both Located in the Same Region

During herpes simplex virus type 1 (HSV-1) latent infection in vivo, the latency-associated promoter (LAP) is the only promoter to remain highly active long term. In a previous attempt to characterize LAP activity in vitro and in a mouse model, we showed that a 1.5-kb fragment called the long-term expression element (LTE), located immediately downstream from the transcriptional start site of LAP, was able to (i) increase gene expression in an orientation-independent manner, regardless of the cell type or the promoter used in vitro (enhancer activity) and (ii) keep LAP active during latency in vivo (long-term expression activity) (H. Berthomme, J. Lokensgard, L. Yang, T. Margolis, and L. T. Feldman, J. Virol. 74:3613–3622, 2000). To determine if these two functions could be separated genetically, we conducted a mutational analysis on the LTE and analyzed the effect on the LAP-LTE properties in both transient expression in cell culture and mouse dorsal root ganglia lytic and latent infection. In this report, we show that the first half of the LTE sequence, corresponding to the region previously described as LAP2 or exon1, encodes the enhancer function. This same region is also required to keep the LAP active during latency. These results exclude the intron region as containing any significant enhancer activity or any ability to keep the LAP active during latency. The results also show that these two functions have not been separated, leaving open the possibility that there is no long-term expression function per se but that the enhancer itself may function to keep the LAP active during latency by raising the level of expression to a detectable one. Further mutational analysis will be required to determine if these two potential functions continue to cosegregate.