The Herpes Simplex Virus Type 1 Latency-Associated Transcript (LAT) Enhancer/ rcr Is Hyperacetylated during Latency Independently of LAT Transcription

Abstract

During herpes simplex virus type 1 (HSV-1) latency, only one region of the viral genome is actively transcribed: the region encoding the latency-associated transcript (LAT). A previous study demonstrated that during latency the LAT promoter is hyperacetylated at histone H3 (K9, K14) relative to lytic genes examined. In the present study, we examine the acetylation profile of regions downstream of the LAT promoter during a latent infection of murine dorsal root ganglia. These analyses revealed the following: (i) the region of the genome containing the 5 exon of the LAT primary transcript was at least as enriched in acetylated H3 as the LAT promoter, and (ii) the region of hyperacetylation does not extend to the ICP0 promoter. In order to assess the contribution of LAT transcription to the acetylation of the 5 exon region, the acetylation profile of KOS/29, a recombinant with a deletion of the LAT promoter, was examined. The region containing the 5 exon of KOS/29 was hyperacetylated relative to lytic gene regions in the absence of detectable LAT transcription. These results indicate that the region containing the 5 exon of LAT, known to contain enhancer activities and to be critical for induced reactivation (rcr), exists in a chromatin structure during latency that is distinct from other lytic gene regions. This result suggests a role for the 5 exon LAT enhancer region as a cis-acting regulator of transcription that maintains a transcriptionally permissive chromatin domain in the HSV-1 latent episome.Herpes simplex virus type 1 (HSV-1) establishes latent infections in sensory neurons as a circular episome associated with histones (8,28,37). Active transcription occurs from only one region of this episome: the region encoding the latencyassociated transcript (LAT) (40, 42). The LAT region carries an 8.3-kb polyadenylated RNA that is spliced to yield a 2.0-kb stable intron that accumulates abundantly in a subset of the sensory neurons (10,27,41). This 2.0-kb intron can be alternatively spliced in some neurons to yield a 1.5-kb intron (38,48). While the LAT region has not been shown to encode any proteins, this region has been implicated in a number of pathogenic functions, including neuronal survival and antiapoptosis (32, 45), virulence (34, 45), suppression of latent transcription (5), establishment of latency (35,46), and reactivation from latency (14,22). Whether this region mediates these different functions through one or more distinct genetic elements remains to be determined, but evidence for the existence of multiple promoters attests to the transcriptional complexity of this region (9, 13, 31, 33). A striking feature of HSV-1 latency is the general suppression of lytic transcription, and this suppression seems to correlate with the association of certain histones with specific tail modifications (20).Cellular chromatin is known to be separated into regions that range from permissive to nonpermissive for polymerase II-mediated transcription, with pericentric heterochromatin being the most nonpermissive and ...

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