Herpes simplex virus latency-associated transcript encodes a protein which greatly enhances virus growth, can compensate for deficiencies in immediate-early gene expression, and is likely to function during reactivation from virus latency.

Herpes simplex virus types 1 and 2 (HSV1 and HSV2) enter and reactivate from latency in sensory neurons, although the events governing these processes are little understood. During latency, only the latency-associated transcripts (LATs) are produced. However, although the LAT RNAs were described approximately 10 years ago, their function remains ambiguous. Mutations affecting the LATs have minimal effects other than a small reduction in establishment of and reactivation from latency in some cases. Mutations in putative LAT-contained open reading frames (ORFs) have so far shown no effect. The LATs consist of a large species from which smaller (approximately 2 kb), nuclear, nonlinear LATs which are abundant during latency are spliced. Thus, translation of ORFs in these smaller LATs would not usually be expected to be possible, and if expressed at all, their expression might be tightly regulated. Here we show that deregulated expression of the largest HSV1 2-kb LAT-contained ORF in various cells of neuronal and nonneuronal origin greatly enhances virus growth in a manner specific to HSV1-the HSV1 LAT ORF has no effect on the growth of HSV2. Similar results of enhanced growth were found when the HSV1 LAT ORF was constitutively expressed from within the HSV1 genome. The mechanism of LAT ORF action was strongly suggested to be by substituting for deficiencies in immediate-early (IE) gene expression (particularly ICP0), because deregulated LAT ORF expression, as well as enhancing wild-type virus growth, was also found to allow efficient growth of viruses with mutations in ICP0 or VMW65. Such viruses otherwise exhibit considerable growth defects. IE gene expression deficiencies are often the block to productive infection in nonpermissive cells and are also evident during latency. These results, which we show to be protein- rather than RNA-mediated effects, strongly suggest a function of the tightly regulated expression of a LAT ORF-encoded protein in the reactivation from HSV latency.