The spontaneous reactivation function of the herpes simplex virus type 1 LAT gene resides completely within the first 1.5 kilobases of the 8.3-kilobase primary transcript

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is essential for efficient spontaneous reactivation of HSV-1 from latency. We report here that although the LAT gene is 8.3 kb in length, the first 1.5 kb of the LAT gene alone is sufficient for wild-type levels of spontaneous reactivation. We began with a LAT deletion mutant of HSV-1 strain McKrae in which the LAT promoter and the first 1.6 kb of the 5′ end of the LAT gene had been deleted from both copies of LAT (one in each viral long repeat). As we previously reported, this mutant (dLAT2903) was significantly impaired for spontaneous reactivation (G. C. Perng, E. C. Dunkel, P. A. Geary, S. M. Slanina, H. Ghiasi, R. Kaiwar, A. B. Nesburn, and S. L. Wechsler, J. Virol. 68:8045-8055, 1994). We then inserted the LAT promoter and the first 1.5 kb of the LAT gene into a location in the unique long region of dLAT2903 far removed from the normal location of LAT in the long repeats. This resulted in a virus (LAT15a) whose capacity for transcribing LAT RNA was limited to the first 1.5 kb of the 8.3-kb LAT primary transcript. Rabbits were ocularly infected with this mutant, and spontaneous reactivation was measured in comparison to those of the original LAT-negative mutant and its marker-rescued (wild-type) virus, dLAT2903R. LAT15a had an in vivo spontaneous reactivation rate of 12%, compared with a rate of 11% for the marker-rescued virus and 0% for the LAT-negative virus. Southern analysis confirmed that the spontaneously-reactivated LAT15a virus retained the original deletions in both copies of LAT and the 1.5-kb LAT insertion in the unique long region. Thus, insertion of the first 1.5 kb of LAT (and its promoter) at a site distant from the normal LAT location appeared to completely restore in vivo spontaneous reactivation to wild-type levels, despite the remaining inability of the original LAT genes to transcribe any LAT RNA. The function of LAT involved in efficient spontaneous reactivation therefore appeared to map completely within the first 1.5 kb of the LAT gene.