The 5' and 3' limits of transcription in the pseudorabies virus latency associated transcription unit

While latent in sensory neurons of infected pigs, pseudorabies virus expresses transcripts from a limited genomic area. These RNAs are transcribed from the strand opposite to that which encodes the pseudorabies immediate-early protein. Using a combination of in situ nucleic acid hybridization performed on latently infected pig trigeminal ganglia and DNA sequencing, 5' and 3' limits of transcription for the pseudorabies LAT transcription unit have been defined. The 5' limit of transcription has been localized to a NarI-BamHI subfragment of the BamHI-6 fragment. Several promoter elements in the correct orientation for the transcript are present including consensus TATA and CAAT boxes and an SP1 site. The 3' limit of transcription has been localized to a HindIII-KpnI subfragment of the BamHI-5 fragment which contains a consensus polyadenylation signal and two termination codones in the correct orientation. From these results we conclude that the region of pseudorabies virus DNA which is active during latency can be no longer than 12.6 kb and completely overlaps the gene encoding the pseudorabies immediate-early protein.