The herpes simplex virus type 1 2.0-kilobase latency-associated transcript is a stable intron which branches at a guanosine

We have used a minigene construct of the herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene to analyze its transcripts in transient transfection assays. A 2.8-kb fragment of the approximately 8.5-kb LAT gene encompassing the 2.0-Wb LAT,vas cloned into a eukaryotic expression vector downstream of the cytomegalovirus immediate-early gene promoter. Northern hybridization of RNA isolated from transfected COS-I cells identified three LAT-specific transcripts, 3.4, 2.0, and 1.4 kb in size. Mapping of these transcripts by Northern hybridization indicated that the 1.4- and 2.0-kb RNAs are nonoverlapping, while the 3.4-kb RNA overlaps both smaller RNAs. Reverse transcription-PCR (RT-PCR) and partial sequencing of the 1.4-kb RNA revealed that this RNA is the spliced exons of the 3.4-kb primary transcript. The 2.0-kb LAT appears to be an intron accumulating after splicing of the minor LAT (mLAT) pre-mRNA. The splice donor and acceptor sites for the 2.0-kb LAT identified in transfected and HSV-l-infected cells are identical. Mapping of the branch point of this intron by RT-PCR in transfected and HSV-l-infected cells, as well as in latently infected murine trigemial ganglia, shows that it is a guanosine. This branch site does not bear homology to consensus mammalian branch site sequences. These data provide evidence that the 2.0-kb LAT is an intron of the mLAT pre-mRNA with a unique branch point.