Transcription of the herpes simplex virus genome during productive and latent infection

Functional modules important for activated expression of early genes of herpes simplex virus type 1 are clustered upstream of the TATA box

Functional analysis of two promoters controlling early herpes simplex virus type 1 (HSV-1) transcripts encoding the UL37 and UL50 (dUTPase) proteins are described in this report. Transcripts expressed under the control of these promoters were found to be expressed early regardless of the position of the transcription unit within the viral genome. Despite this, wt dUTPase mRNA was 6-10 times more abundant than the UL37 transcript both in wt and recombinant viruses. This same difference in transcript abundance was seen when a reporter gene (beta-galactosidase) was controlled by the two promoters in recombinant viruses in the heterologous glycoprotein C (gC) locus. Thus, both the kinetics and relative abundance of UL50 and UL37 transcripts are a direct function of their respective promoter regulatory elements. Characterization of mutated UL37 and UL50 promoters in recombinant viruses showed that the functional modules important for expression from these promoters are concentrated upstream of the transcription start site; however the extent and composition of these modules in terms of the cis-acting elements they contain was different for each. For the UL37 promoter, both a HiNF-P factor binding site (-53 to -58 bp) and the TATA homology (-22 to -27) were required for any detectable expression, while an Sp1 binding site at -123 augmented this but was not absolutely required. In contrast, the only functional elements crucial for expression from the UL50 promoter were the TATA box (-25 to -31) and an Sp1 binding site at -117 bp relative to the cap site. Despite differences in detail, when the functional architecture of these two early promoters were compared to the extensively characterized HSV-1 thymidine kinase (UL23) promoter, class-specific similarities are clearly apparent.

Experimental investigation of herpes simplex virus latency

The clinical manifestations of herpes simplex virus infection generally involve a mild and localized primary infection followed by asymptomatic (latent) infection interrupted sporadically by periods of recrudescence (reactivation) where virus replication and associated cytopathologic findings are manifest at the site of initial infection. During the latent phase of infection, viral genomes, but not infectious virus itself, can be detected in sensory and autonomic neurons. The process of latent infection and reactivation has been subject to continuing investigation in animal models and, more recently, in cultured cells. The initiation and maintenance of latent infection in neurons are apparently passive phenomena in that no virus gene products need be expressed or are required. Despite this, a single latency-associated transcript (LAT) encoded by DNA encompassing about 6% of the viral genome is expressed during latent infection in a minority of neurons containing viral DNA. This transcript is spliced, and the intron derived from this splicing is stably maintained in the nucleus of neurons expressing it. Reactivation, which can be induced by stress and assayed in several animal models, is facilitated by the expression of LAT. Although the mechanism of action of LAT-mediated facilitation of reactivation is not clear, all available evidence argues against its involving the expression of a protein. Rather, the most consistent models of action involve LAT expression playing a cis-acting role in a very early stage of the reactivation process.

Identification of a promoter-specific transactivation domain in the herpes simplex virus regulatory protein ICP4

ICP4 is expressed during the immediate-early phase of infection by herpes simplex virus (HSV) and activates transcription of viral genes during subsequent phases of productive infection. Several members of the alpha-herpesvirus family encode regulatory proteins that have extensive homology with ICP4 and exhibit a transactivation domain (TAD) at the N terminus. The portions of ICP4 required for nuclear localization, DNA binding, and dimerization have been defined, but a domain that is specifically required for transactivation has not been identified. We have defined a promoter-specific ICP4 TAD by analysis of the activity of GAL4-ICP4 fusion proteins cotransfected into HeLa cells with a luciferase reporter gene linked to a promoter with five GAL4 binding sites. The transactivation activity of GAL4-ICP4 hybrids is located entirely within the first 139 residues of ICP4 and is significantly less potent than the activity of GAL4-TAD hybrids derived from ICP4 homologs. ICP4 residues 97 to 109 are a critical component of this N-terminal TAD. Transient transfection assays performed with nonfusion forms of ICP4 and luciferase genes linked to the HSV glycoprotein D (gD) or thymidine kinase (tk) promoter revealed that ICP4 residues 97 to 109 are required for induction of the gD promoter but are not required for induction of the tk promoter. Comparative experiments with ICP4 homologs revealed that the pseudorabies virus TAD is a potent activator of the gD promoter and a weak activator of the tk promoter. Complementation assays revealed that loss of ICP4 residues 97 to 109 reduced the yield of virus from infected cells nearly 500-fold compared to wild-type ICP4. We conclude that ICP4 residues 97 to 109 are a core component of a promoter-specific transactivation domain that is required for efficient replication of herpes simplex virus.

Inhibition of topoisomerase II by ICRF-193 prevents efficient replication of herpes simplex virus type 1

Cellular topoisomerase II is specifically inactivated by the drug ICRF-193. This compound turns topoisomerase II into a closed clamp that is unable to cleave DNA. We have investigated the effects of this inhibitor on the replication of herpes simplex virus type 1. We show that ICRF-193 at low multiplicities of infection dramatically inhibits viral DNA synthesis and the production of infectious virus. The inhibition is less efficient at high multiplicities of infection. In addition, inhibition of viral DNA synthesis was observed only when ICRF-193 was present during the first 4 h of the infectious cycle. The transient replication of plasmids containing a herpes simplex virus type 1 origin of DNA replication, oriS, was affected by ICRF-193 in the same way. In contrast, neither cellular DNA synthesis nor replication of plasmids containing a simian virus 40 origin of DNA replication was inhibited. The observed effect on herpes simplex virus DNA replication was not caused by a decreased transcription of replication genes inasmuch as the levels of UL8, UL9, UL29, and UL30 rmRNAs were unaffected by the drug. These results suggest that topoisomerase II plays a vital role during the replication of herpes simplex virus type 1 DNA. We speculate that topoisomerase II is involved in the decatenation of newly synthesized daughter molecules.

The herpes simplex virus type 1 VP5 promoter contains a cis-acting element near the cap site which interacts with a cellular protein

The promoter controlling the expression of the transcript encoding the major herpes simplex virus type 1 capsid protein (VP5, UL19) extends only 60 bases or so from a functional Sp1 site at --48 to include a cis-acting element 3' of the transcript start site. In the present communication, we report the generation of recombinant viruses bearing mutations between --6 and + 8 relative to the cap site in the VP5 promoter controlling expression of a reporter gene. Analysis of the effects of these mutations upon reporter gene expression along with the results of protein binding assays demonstrates that this cap transcription element functionally interacts with a cellular protein of a normal size of 40 kDa. Thus, like the strict late UL38 promoter characterized earlier, the late VP5 promoter has the essential properties of a cellular promoter.