Delineation of regulatory domains of early (beta) and late (gamma 2) genes by construction of chimeric genes expressed in herpes simplex virus 1 genomes

Adaptation of transgene mRNA translation boosts the anticancer efficacy of oncolytic HSV1

BACKGROUND

Transgenes deliver therapeutic payloads to improve oncolytic virus immunotherapy. Transgenes encoded within oncolytic viruses are designed to be highly transcribed, but protein synthesis is often negatively affected by viral infection, compromising the amount of therapeutic protein expressed. Studying the oncolytic herpes simplex virus-1 (HSV1), we found standard transgene mRNAs to be suboptimally translated in infected cells.

METHODS

Using RNA-Seq reads, we determined the transcription start sites and 5'leaders of HSV1 genes and uncovered the US11 5'leader to confer superior activity in translation reporter assays. We then incorporated this 5'leader into GM-CSF expression cassette in oncolytic HSV1 and compared the translationally adapted oncolytic virus with the conventional, leaderless, virus in vitro and in mice.

RESULTS

Inclusion of the US11 5'leader in the GM-CSF transgene incorporated into HSV1 boosted translation in vitro and in vivo. Importantly, treatment with US11 5'leader-GM-CSF oncolytic HSV1 showed superior antitumor immune activity and improved survival in a syngeneic mouse model of colorectal cancer as compared with leaderless-GM-CSF HSV1.

CONCLUSIONS

Our study demonstrates the therapeutic value of identifying and integrating platform-specific cis-acting sequences that confer increased protein synthesis on transgene expression.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

Immediate Early Proteins of Herpes Simplex Virus Transiently Repress Viral Transcription before Subsequent Activation

HSV-1 transcription during productive replication is believed to comprise a series of activation steps leading to a specific sequence of gene expression. Here, we show that virion components and IE gene products ICP0, ICP4, and ICP22 first repress viral gene transcription to various degrees before subsequently activating specific gene subsets.

Human cytomegalovirus IE2 drives transcription initiation from a select subset of late infection viral promoters by host RNA polymerase II

Herpesvirus late promoters activate gene expression after viral DNA synthesis has begun. Alphaherpesviruses utilize a viral immediate-early protein to do this, whereas beta- and gammaherpesviruses primarily use a 6-member set of viral late-acting transcription factors (LTF) that are drawn to a TATT sequence in the late promoter. The betaherpesvirus, human cytomegalovirus (HCMV), produces three immediate-early 2 protein isoforms, IE2-86, IE2-60, IE2-40, late in infection, but whether they activate late viral promoters is unknown. Here, we quickly degrade the IE2 proteins in late infection using dTag methodology and analyze effects on transcription using customized PRO-Seq and computational methods combined with multiple validation methods. We discover that the IE2 proteins selectively drive RNA Pol II transcription initiation at a subset of viral early-late and late promoters common to different HCMV strains, but do not substantially affect Pol II transcription of the 9,942 expressed host genes. Most of the IE2-activated viral late infection promoters lack the TATT sequence bound by the HCMV UL87-encoded LTF. The HCMV TATT-binding protein is not mechanistically involved in late RNA expression from the IE2-activated TATT-less UL83 (pp65) promoter, as it is for the TATT-containing UL82 (pp71) promoter. While antecedent viral DNA synthesis is necessary for transcription from the late infection viral promoters, continued viral DNA synthesis is unnecessary. We conclude that in late infection the IE2 proteins target a distinct subset of HCMV early-late and late promoters for transcription initiation by RNA Pol II. Commencement of viral DNA replication renders the HCMV genome late promoters susceptible to late-acting viral transcription factors.

The herpesvirus subfamilies differ in the viral proteins used in generating the cascade of viral immediate-early, early, early-late, or late gene transcription. With the application of advanced technologies, we discovered that the betaherpesvirus, human cytomegalovirus, has evolved strategies analogous to those used by both alpha- and gammaherpesviruses to bring about RNA Pol II transcription from its late infection promoters. Like alphaherpesviruses, human cytomegalovirus purposes a pivotal immediate-early viral transcription factor to initiate transcription from early, early-late, and late viral promoters. However, the cytomegalovirus transcription factor only targets a select set of viral early-late and late promoters without appreciably affecting host promoters at late times. Most of these late infection viral promoters are structurally and mechanistically different from promoters activated by the 6-member viral transcription factor complex that is analogous to the transcription factor complex utilized by gammaherpesviruses. Human cytomegalovirus genome amplification must first take place, but need not continue, to enable the two different mechanisms of late viral promoter activation.

Genome replication affects transcription factor binding mediating the cascade of herpes simplex virus transcription

In herpes simplex virus type 1 (HSV-1) infection, the coupling of genome replication and transcription regulation has been known for many years; however, the underlying mechanism has not been elucidated. We performed a comprehensive transcriptomic assessment and factor-binding analysis for Pol II, TBP, TAF1, and Sp1 to assess the effect genome replication has on viral transcription initiation and elongation. The onset of genome replication resulted in the binding of TBP, TAF1, and Pol II to previously silent late promoters. The viral transcription factor, ICP4, was continuously needed in addition to DNA replication for activation of late gene transcription initiation. Furthermore, late promoters contain a motif that closely matches the consensus initiator element (Inr), which robustly bound TAF1 postreplication. Continued DNA replication resulted in reduced binding of Sp1, TBP, and Pol II to early promoters. Therefore, the initiation of early gene transcription is attenuated following DNA replication. Herein, we propose a model for how viral DNA replication results in the differential utilization of cellular factors that function in transcription initiation, leading to the delineation of kinetic class in HSV-productive infection.

The 3 facets of regulation of herpes simplex virus gene expression: A critical inquiry

On entry into the body herpes simplex viruses (HSV) replicate in a series of steps that involves derepression of viral DNA activated by VP16, a virion protein, and sequential transcription of viral genes in a cascade fashion. HSV also enters into neurons in which viral DNA maintained as heterochromatin and with few exceptions viral gene expression is silenced. A third face of the interaction of HSV with its host cells takes place at the moment when the silenced viral genome in neurons is abruptly derepressed. The available data do no reveal evidence that HSV encodes different regulatory programs for each facet of its interaction with its host cells. Rather the data point to significant gaps in our knowledge of the mechanisms by which each facet is initiated and the roles of the infected cells at each facet of the interaction of viral gene products with the host cell.

Identification of a divalent metal cation binding site in herpes simplex virus 1 (HSV-1) ICP8 required for HSV replication

Herpes simplex virus 1 (HSV-1) ICP8 is a single-stranded DNA-binding protein that is necessary for viral DNA replication and exhibits recombinase activity in vitro. Alignment of the HSV-1 ICP8 amino acid sequence with ICP8 homologs from other herpesviruses revealed conserved aspartic acid (D) and glutamic acid (E) residues. Amino acid residue D1087 was conserved in every ICP8 homolog analyzed, indicating that it is likely critical for ICP8 function. We took a genetic approach to investigate the functions of the conserved ICP8 D and E residues in HSV-1 replication. The E1086A D1087A mutant form of ICP8 failed to support the replication of an ICP8 mutant virus in a complementation assay. E1086A D1087A mutant ICP8 bound DNA, albeit with reduced affinity, demonstrating that the protein is not globally misfolded. This mutant form of ICP8 was also recognized by a conformation-specific antibody, further indicating that its overall structure was intact. A recombinant virus expressing E1086A D1087A mutant ICP8 was defective in viral replication, viral DNA synthesis, and late gene expression in Vero cells. A class of enzymes called DDE recombinases utilize conserved D and E residues to coordinate divalent metal cations in their active sites. We investigated whether the conserved D and E residues in ICP8 were also required for binding metal cations and found that the E1086A D1087A mutant form of ICP8 exhibited altered divalent metal binding in an in vitro iron-induced cleavage assay. These results identify a novel divalent metal cation-binding site in ICP8 that is required for ICP8 functions during viral replication.

ORF30 and ORF34 are essential for expression of late genes in murine gammaherpesvirus 68

A hallmark of productive infection by DNA viruses is the coupling of viral late gene expression to genome replication. Here we report the identification of open reading frame 30 (ORF30) and ORF34 as viral trans factors crucial for activating late gene transcription following viral DNA replication during lytic infection of murine gammaherpesvirus 68 (MHV-68). The mutant virus lacking either ORF30 or ORF34 underwent normal DNA replication but failed to express viral late gene transcripts, leading to nonproductive infection. In a reporter assay system, ORF30 and ORF34 were required for MHV-68 to activate the viral late gene promoters. Furthermore, studies using chromatin immunoprecipitation assays showed that the recruitment of RNA polymerase II to the viral late promoters during lytic infection was significantly reduced in the absence of ORF30 or ORF34. Together, the results suggest that ORF30 and ORF34 may play an important role in the assembly of the transcription initiation complex at the late gene promoters. Our discovery of the viral mutants that uncouple late gene transcription from DNA replication lays an important foundation to dissect the mechanism of this critical step of gene expression regulation.

Regulation of herpes simplex virus gene expression

Expression of the more than 80 individual genes of herpes simplex virus 1 (HSV-1) takes place in a tightly regulated sequential manner that was first described over 20 years ago. Investigations since that time have focused on understanding the mechanisms that regulate this orderly and efficient expression of viral genes. This review examines recent findings that have shed light on how this process is regulated during productive infection of the cell. Although the story is still not complete, several aspects of HSV gene expression are now clearer as a result of these findings. In particular, several new functions have recently been ascribed to some of the known viral regulatory proteins. The results indicate that the viral gene expression is regulated through transcriptional as well as post-transcriptional mechanisms. In addition, it has become increasingly clear that the virus has evolved specific functions to interact with the host cell in order to divert and redirect critical host functions for its own needs. Understanding the interactions of HSV and the host cell during infection will be essential for a complete understanding of how viral gene expression is regulated. Future challenges in the field will be to develop a complete understanding of the mechanisms that temporally regulate virus gene expression, and to identify and characterize the relevant interactions between the virus and the distinctive cell types normally infected by the virus.

On the control of late gene expression in Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8)

Herpesvirus late genes require viral DNA replication for maximal expression. Although late gene expression appears to require DNA replication in cis in alphaherpesviruses, studies in Epstein-Barr virus (EBV) suggest that this cis-requirement might not pertain to the gammaherpesviruses. Based on these findings, a system was created to investigate the elements required for the regulation of Kaposi's sarcoma-associated herpesvirus (KSHV; human herpesvirus-8) late gene expression. The transcript of a classic late gene encoding the viral assembly protein was characterized and reporter genes driven by the assembly protein promoter region were constructed. Unlike the EBV case, expression of a reporter gene under the control of the assembly protein promoter did not display authentic regulation when removed from the context of the viral genome. Although reporter expression rose in cells displaying lytic replication, this expression was not diminished by specific inhibitors of viral DNA synthesis. Minimal core promoters were similarly unable to reproduce late gene regulation. These results suggest that proper KSHV late gene expression is likely to be dependent upon virus lytic replication in cis and indicate that the regulation of KSHV late genes more closely resembles that observed in herpes simplex virus than that described for EBV.

Two leaky-late HSV-1 promoters differ significantly in structural architecture

The HSV-1 VP5 and VP16 transcripts are expressed with leaky-late (gamma1) kinetics and reach maximal levels after viral DNA replication. While the minimal VP5 promoter includes only an Sp1 site at -48, a TATA box at -30, and an initiator (Inr) element at the cap site, here we show that elements upstream of -48 can functionally compensate for the mutational loss of the critical Sp1 site at -48. To determine whether this is a general feature of leaky-late promoters, we have carried out a detailed analysis of the VP16 promoter in the context of the viral genome at the gC locus. Sequence analysis suggests a great deal of similarity between the two. Despite this, however, mutational analysis revealed that the 5' boundary of the VP16 promoter extends to ca. -90. This region includes an Sp1 binding site at -46, CAAT box homology at -77, and "E box" (CACGTG) at -85. Mutational and deletional analyses demonstrate that the proximal Sp1 site plays little or no role in promoter strength; despite this it can be shown to bind Sp1 protein using DNA mobility shift assays. Like the VP5 promoter, the VP16 promoter also requires an initiator element at the cap site. The VP16 Inr element differs in sequence from that of the VP5 promoter, and its deletion or mutation has a significantly smaller effect on promoter strength. The difference between these two Inr elements was confirmed by our finding that the VP16 initiator element binds to the 65-kDa YY1 transcription factor, and the VP5 Inr element competes poorly for the binding between the VP16 element and infected cell proteins in comparative bandshift assays. While the VP16 Inr sequence is identical to that of several murine TATA-less promoters, the VP16 Inr requires a TATA box for measurable activity.

The Epstein-Barr virus lytic program is controlled by the co-operative functions of two transactivators

The propagation of herpesviruses has long been viewed as a temporally regulated sequential process that results from the consecutive expression of specific viral transactivators. As a key step in this process, lytic viral DNA replication is considered as a checkpoint that controls the expression of the late structural viral genes. In a novel genetic approach, we show that both hypotheses do not hold true for the Epstein-Barr virus (EBV). The study of viral mutants of EBV in which the early genes BZLF1 and BRLF1 are deleted allowed a precise assignment of the function of these proteins. Both transactivators were absolutely essential for viral DNA replication. Both BZLF1 and BRLF1 were required for full expression of the EBV proteins expressed during the lytic program, although the respective influence of these molecules on the expression of various viral target genes varied greatly. In replication-defective viral mutants, neither early gene expression nor DNA replication was a prerequisite for late gene expression. This work shows that BRLF1 and BZLF1 harbor distinct but complementary functions that influence all stages of viral production.

Herpes simplex virus 1 late gene expression is preferentially inhibited during infection of the TAF250 mutant ts13 cell line

A key component of the polymerase II transcription machinery is the transcription factor TFIID, a complex that contains the TATA-box binding protein and several (10-12) associated factors designated as TAFs (TBP-associated factors). ts13 cells, which contain a temperature-sensitive mutant in TAF250, the largest subunit of TFIID, exhibit promoter-specific defects in gene expression at the nonpermissive temperature, suggesting that individual TAFs are required for transcription of specific subsets of eukaryotic genes. Herpes simplex virus 1 (HSV-1) does not replicate in ts13 cells at the nonpermissive temperature, but the point at which the replicative process is blocked is not known. We used the TAF250 defect in ts13 cells to investigate the role of TAF250 in the expression of HSV-1 genes of each temporal class. At a low m.o.i., expression of most immediate-early mRNAs is reduced at the nonpermissive temperature, and consequently, there is little expression of early genes and no viral DNA replication. In contrast, at high m.o.i., expression of immediate-early genes is unaffected by the TAF250 defect and is not dependent on de novo viral protein synthesis. Early genes and early proteins are produced under these conditions, and viral DNA replication ensues, albeit at somewhat reduced levels. In contrast, late gene expression and late protein synthesis are severely restricted, even in the presence of appreciable viral DNA replication. Thus the lack of late protein synthesis is responsible for the inability of HSV-1 to replicate in ts13 cells at the nonpermissive temperature. Further, it appears that late viral gene expression may be preferentially inhibited by the TAF250 mutation in ts13 cells.

Characterization of the initiator and downstream promoter elements of herpes simplex virus 1 late genes

Previously identified cis-acting regulatory elements of herpes simplex virus (HSV) 1 late promoters include a TATA element upstream from the start of transcription, an initiator-like element at the start of transcription, and sequences downstream from the start of transcription. To determine whether these elements are functionally equivalent to similar elements from other eukaryotic genes, model late promoters were constructed using well-characterized regulatory elements from non-HSV genes. These modular promoters were then inserted into the viral genome upstream from a lacZ marker gene. Results showed that a eukaryotic initiator element, along with a TATA element, can function as a late HSV promoter. Several initiator sequences from both viral and nonviral genes were functionally similar to the initiator-like element in HSV-1 late promoters; however, a random sequence of the same size and a similarly located sequence from the HSV-1 early thymidine kinase promoter could not substitute for the initiator element. These results indicate that eukaryotic initiator elements are functionally equivalent to HSV-1 late promoter initiator elements. In addition, the downstream element of the late glycoprotein C promoter was further analyzed by construction of a series of small deletions and insertions. The presence of the downstream glycoprotein C region in a promoter consisting of a strong TATA and initiator element increased mRNA expression by a modest amount; this effect appeared to be sequence specific and dependent on its exact alignment with the upstream elements of the promoter.

Characterization of the US8.5 protein of herpes simplex virus

In a previous study a novel gene designated as US8.5 was identified in the unique short component of the herpes simplex virus type 1 (HSV1) genome. Epitope tagging experiments suggested the existence of a protein encoded by this gene in HSV1 infected cells. To further analyze the US8.5 gene product and function, a rabbit polyclonal antiserum was raised against a recombinant beta-galactosidase-US8.5 fusion protein expressed in E. coli. This antiserum reacted specifically with a 19 kDa protein in HSV1(F) infected cells as shown by immunoblotting and immunoprecipitation experiments. In addition, using the same antiserum a 21 kDa protein was detected in lysates from cells infected with HSV2(G), which was most likely the HSV2 homolog of US8.5. Kinetic studies indicated that US8.5 is expressed as gamma 1 gene. In addition, the US8.5 protein was immunoprecipitated with the anti-US8.5 serum from 32P-labeled lysates of Vero cells infected with HSV1, demonstrating that the protein is phosphorylated. Immunofluorescence studies localized the US8.5 protein to the nucleoli of HSV1 infected cells.

Arginine-rich regions succeeding the nuclear localization region of the herpes simplex virus type 1 regulatory protein ICP27 are required for efficient nuclear localization and late gene expression

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is an essential regulatory protein that localizes to the nuclei of infected cells. The strong nuclear localization signal (NLS) of ICP27 was identified recently and shown to reside in the amino-terminal portion of the polypeptide from residues 110 to 137 (W.E. Mears, V. Lam, and S.A. Rice, J. Virol. 69:935-947, 1995). There are also two arginine-rich regions directly succeeding the NLS. The first of these arginine-rich sequences (residues 141 to 151), together with the NLS, has been shown by Mears et al. to form the nucleolar localization signal. Arginine-rich motifs are common in domains involved in nuclear localization and RNA binding. To analyze the role of the arginine-rich regions in ICP27, we constructed stably transformed cell lines containing ICP27 mutants with deletions of all or parts of the NLS and arginine-rich regions. We also constructed mutants in which these regions were replaced with heterologous NLSs or RNA-binding domains. Characterization of these mutants indicated that the arginine-rich regions were required but not sufficient for wild-type localization of ICP27. More importantly, the NLS and arginine-rich regions were also essential to the function of ICP27. Mutants lacking these sequences were defective in late gene expression during infection even when ICP27 was properly localized to the nucleus by substitution of the NLS from simian virus 40 large T antigen. Further, the defect in late gene expression could not be overcome by replacement with the highly basic RNA-binding domain of human immunodeficiency virus type 1 Tat. The deficiency in late gene expression was independent of ICP27's role in stimulating viral DNA replication. In addition, localization of the HSV-1 proteins ICP4, ICP0, and ICP8 was unaffected by ICP27 mutants in this region. These results suggest that the arginine-rich regions are required for efficient nuclear localization and for the regulatory activity of ICP27 involved in viral late gene expression.

Herpes simplex virus recombination vectors designed to allow insertion of modified promoters into transcriptionally "neutral" segments of the viral genome

The use of recombinant viruses has been essential in investigation of the biology of herpes simplex virus (HSV). In this communication we describe a number of viral recombination vectors that we have generated for use in promoter structure/function analysis within the context of the HSV-1 genome. We have utilized two regions of the HSV genome that contain genes nonessential for replication in cultured cells--the glycoprotein C (gC or UL44) locus in the UL of the genome and the area encompassing the promoter and 5' portion of the latency associated transcript (LAT) within the RL factual influence on promoters due to the site of insertion. Two different kinetic promoters were analyzed, those controlling expression of the gamma UL 38 and the beta dUTPase genes, in both loci. All constructs tested displayed reporter gene mRNA expression with expected kinetics, and we conclude that there are no neighboring cryptic promoter elements that could interfere with expression studies using the vectors described.

Transcriptional activation of the herpes simplex virus type 1 UL38 promoter conferred by the cis-acting downstream activation sequence is mediated by a cellular transcription factor

The herpes simplex virus (HSV) type 1 strict late (gamma) UL38 promoter contains three cis-acting transcriptional elements: a TATA box, a specific initiator element, and the downstream activation sequence (DAS). DAS is located between positions +20 and +33 within the 5' untranslated leader region and strongly influences transcript levels during productive infection. In this communication, we further characterize DAS and investigate its mechanism of action. DAS function has a strict spacing requirement, and DAS contains an essential 6-bp core element. A similarly positioned element from the gamma gC gene (UL44) has partial DAS function within the UL38 promoter context, and the promoter controlling expression of the gamma US11 transcript contains an identically located element with functional and sequence similarity to UL38 DAS. These data suggest that downstream elements are a common feature of many HSV gamma promoters. Results with recombinant viruses containing modifications of the TATA box or initiator element of the UL38 promoter suggest that DAS functions to increase transcription initiation and not the efficiency of transcription elongation. In vitro transcription assays using uninfected HeLa nuclear extracts show that, as in productive infection with recombinant viruses, the deletion of DAS from the UL38 promoter dramatically decreases RNA expression. Finally, electrophoretic mobility shift assays and UV cross-linking experiments show that DAS DNA forms a specific, stable complex with a cellular protein (the DAS-binding factor) of approximately 35 kDa. These data strongly suggest that the interaction of cellular DAS-binding factor with DAS is required for efficient expression of UL38 and other HSV late genes.

The herpes simplex virus type 1 major capsid protein (VP5-UL19) promoter contains two cis-acting elements influencing late expression

The herpes simplex virus type 1 (HSV-1) major capsid protein VP5 gene (UL19) is expressed with beta gamma (gamma 1 [leaky late]) kinetics. We have previously described the construction of recombinant HSV-1 in which the VP5 promoter was engineered to control the expression of the bacterial beta-galactosidase gene as a reporter (C.-J. Huang, S. A. Goodart, M. K. Rice, J. F. Guzowski, and E. K. Wagner, J. Virol. 67:5109-5116, 1993). Here we describe further mutational analysis in recombinant viruses. We have precisely defined the boundaries of the VP5 promoter and identified two regions important for both the level and the kinetics of expression. The 5' boundary was located at -48 relative to the initiation site of transcription by analyzing a series of nested deletions in the upstream sequence, and although a number of cis-acting sites influencing transient expression have been identified upstream of this point, these sites have no role in promoter activity during productive infection. Deletion of an Sp1-binding site located between -48 and the TATA box at -30 greatly reduced VP5 promoter activity late but not early after infection. A cis-acting element whose sequence resembles the human immunodeficiency virus type 1 initiator was located between -2 and +10 in the VP5 sequence by characterizing a series of deletions and site-directed block mutations downstream the TATA box. This element defines the 3' limit of the VP5 promoter, and like the upstream element, disruption of this element also inhibited promoter activity late in the productive cycle.

Binding sites for the herpes simplex virus immediate-early protein ICP4 impose an increased dependence on viral DNA replication on simple model promoters located in the viral genome

We examined the ability of binding sites for the herpes simplex virus immediate-early protein ICP4 to alter the regulation of closely linked promoters by placing strong ICP4 binding sites upstream or downstream of simple TATA promoters in the intact viral genome. We found that binding sites strongly reduced the levels of expression at early times postinfection and that this effect was partially overcome after the onset of viral DNA replication. These data confirm that DNA-bound ICP4 can inhibit the activity of a closely linked promoter and raise the possibility that ICP4 binding sites contribute to temporal regulation during infection.

Mutational analysis of sequences downstream of the TATA box of the herpes simplex virus type 1 major capsid protein (VP5/UL19) promoter

Transient expression assays with the herpes simplex virus type 1 (HSV-1) promoter/leader controlling the beta gamma (leaky-late) VP5 (UL19) mRNA encoding the major capsid protein showed that no more than 36 to 72 bases of VP5 leader are required for full-level expression. Constructs lacking the viral leader and the transcription initiation site expressed the reporter gene at about 20% of the maximum level. We confirmed this observation by using recombinant viruses in which VP5 promoter/leader deletions controlling the bacterial beta-galactosidase gene were inserted into the nonessential glycoprotein C (UL44) locus of the genome. Sequences within +36 are required for full-level expression, and removal of all leader sequences including the cap site resulted in a 10-fold decrease in reporter mRNA accumulation. The removal of the leader sequence had a measurable effect upon the kinetics of reporter mRNA accumulation, but insertion of the entire VP5 leader and cap site into a construct in which the reporter gene was controlled by the kinetically early (beta) dUTPase (UL50) promoter did not result in any significant change in the kinetics of dUTPase promoter expression. These results suggest that DNA sequences both 5' and 3' of the TATA box are important determinants of the beta gamma kinetics and levels of VP5 mRNA accumulation in the infected cell.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

Mutational analysis of the ICP4 binding sites in the 5' transcribed noncoding domains of the herpes simplex virus 1 UL 49.5 gamma 2 gene

A previous report (P. Mavromara-Nazos and B. Roizman, Proc. Natl. Acad. Sci. USA 86:4071-4075, 1989) demonstrated that substitution of sequences of the thymidine kinase (tk) gene, a beta gene, extending from -16 to +51 with sequences extending from -12 to +104 of the gamma 2 UL 49.5 gene in viral recombinant R3820 conferred upon the chimeric gene gamma 2 attributes in the context of the viral genome in a productive infection. The UL49.5 gene sequences extending from -179 to +104 contain four DNA binding sites for the major regulatory protein ICP4. Of these sites, two map between nucleotides +20 and +80 within the sequence which confers gamma 2 regulation upon the chimeric gene. To determine the role of these ICP4 binding sites in conferring the gamma 2 gene attributes, sequences comprising the two ICP4 binding sites were mutagenized and used to reconstruct the R3820 recombinant virus. In addition, a new recombinant virus (R8023) was constructed in which tk sequences extending from -240 to +51 were replaced with wild-type or mutated sequences contained between nucleotides -179 to +104 of the UL 49.5 gene. Vero cells infected with the recombinant viruses in the presence or absence of phosphonoacetate, a specific inhibitor of viral DNA synthesis, were then tested for accumulation of tk RNA by using an RNase protection assay. The results indicate that in the recombinant R3820, a mutation which destroyed one of the two UL49.5 ICP4 DNA binding sites significantly reduced the accumulation of tk RNA at both early and late times after infection. The effect of this mutation was less pronounced in cells infected with the R8023 virus, whose chimeric tk gene contains the two upstream UL49.5 ICP4 binding sites. None of the mutations affected the sensitivity of the chimeric genes to phosphonoacetate. The mutated site appears to be involved in the accumulation of RNA.

The unique sequence of the herpes simplex virus 1 L component contains an additional translated open reading frame designated UL49.5

We present evidence for the existence of an additional herpes simplex virus 1 gene designated UL49.5. The sequence, located between genes UL49 and UL50, predicts a hydrophobic protein with 91 amino acids. Attempts to delete UL49.5 were not successful. To demonstrate that UL49.5 is expressed, we made two recombinant viruses. First, we inserted in frame an oligonucleotide encoding a 15-amino-acid epitope known to react with a monoclonal antibody. This gene, consisting of the authentic promoter and chimeric coding domain, was inserted into the thymidine kinase gene of wild-type virus and in infected cells expressed a protein which reacted with the monoclonal antibody. The second recombinant virus contained a 5' UL49.5-thymidine kinase fusion gene. The protein expressed by this virus confirmed that the first methionine codon of UL49.5 served as the initiating codon. The predicted amino acid sequence of UL49.5 is consistent with the known properties of NC-7, a small capsid protein whose gene has not been previously mapped. A homolog of UL49.5 is present in the genome of varicella-zoster virus, located between homologs of UL49 and UL50.

Mutational analysis of two herpes simplex virus type 1 late promoters

To investigate the cis-acting sequence elements that are involved in the regulation of herpes simplex virus type 1 late gene expression, linker-scanning mutations were constructed in the promoters of the glycoprotein C and glycoprotein H genes. Each promoter mutation was inserted upstream of the Escherichia coli lacZ gene in a recombinant virus, and the relative activities of beta-galactosidase expressed from individual recombinant viruses were compared. This analysis identified three sequence elements in each promoter: a TATA element, an element that overlapped the start of transcription, and an element downstream from the start of transcription. Primer extension analysis confirmed these results and showed that mutations in either the TATA element or the initiation sequence could eliminate normal transcription initiation. Analysis of expression from hybrid promoters revealed that the TATA and the initiation elements were interchangeable, at least when correctly aligned, and that the initiation element plays a pivotal role in determining the actual site of transcription initiation.

Regulation of herpes simplex virus true late gene expression: sequences downstream from the US11 TATA box inhibit expression from an unreplicated template

The true late genes of herpes simplex virus type 1 (HSV-1) are expressed only after the onset of viral DNA replication. Previous studies demonstrated that late promoters lack elements upstream of the TATA box and suggested that only a subset of TATA elements can function in the context of true late promoters. We determined which structural features of true late promoters are responsible for the stringent requirement for viral DNA replication by inserting a series of simple model constructs into the HSV-1 genome in place of one of the two promoters of the UL24 gene. An oligonucleotide consisting of 19 nucleotides spanning the TATA box of the HSV-1 true late US11 gene drove barely detectable levels of expression; by contrast, the corresponding regions of the Adenovirus type 2 major late promoter and the HSV-1 true late glycoprotein C promoter were much more active. Transcripts driven from all of these minimal TATA box promoters accumulated without viral DNA replication. The activity of the US11 TATA box was stimulated by adding upstream Sp1-binding sites or placing the US11 or rabbit beta-globin cap/leader region (-11 to +39) downstream. The Sp1-TATA and TATA-beta-globin cap/leader constructs remained replication independent, while the TATA-US11 cap/leader promoter displayed true late regulation. These results demonstrate that sequences located within the US11 cap/leader region impose a strict requirement for viral DNA replication on a minimal TATA box promoter.

Upstream promoter elements of the herpes simplex virus type 1 glycoprotein H gene

To investigate the cis-acting sequence elements that are involved in the regulation of herpes simplex virus type 1 late-gene expression, recombinant viruses were constructed that express the Escherichia coli lacZ gene from the promoter of the glycoprotein H (gH) gene. Deletion experiments established an upstream boundary for the gH promoter of no more than 83 bp from the start of gH transcription and showed that the promoter sequences did not overlap with coding sequences of the upstream thymidine kinase (tk) gene. Sequences of the tk gene previously shown to be required for efficient processing of the tk transcript were essential for expression form the gH promoter and included a TATA-like element. In addition, the gH TATA element was specifically mutagenized to substitute the TATA elements of immediate-early, early, and other late viral promoters for the gH TATA element. The results indicated that the TATA element was an interchangeable component of herpes simplex virus type 1 promoters and did not regulate temporal expression.

Herpes simplex virus transactivator ICP4 operationally substitutes for the cellular transcription factor Sp1 for efficient expression of the viral thymidine kinase gene

The herpes simplex virus type 1 (HSV-1) ICP4 protein is a transcriptional activator of many eucaryotic RNA polymerase II promoters. The HSV-1 thymidine kinase gene (tk) promoter is induced by ICP4 and contains binding sites for the cellular transcription factors TFIID, Sp1, and CCAAT-binding proteins, each of which affects expression of the tk gene. In this study, the effects of mutations in these sites on the transcription of tk in the presence and absence of ICP4 were determined during viral infection. Only the TATA box was necessary for efficient expression in the presence of ICP4; however, ICP4 apparently can still induce tk transcription even when the TATA box is disrupted. Alteration of the Sp1 sites had a minor effect on ICP4-induced expression in comparison to a large effect in the absence of ICP4, indicating that ICP4 can operationally substitute for the function of the transcription factor Sp1. In addition, tk was still expressed with the kinetics of an early gene in the absence of binding sites for Sp1 and CCAAT-binding proteins.

Analysis of the herpes simplex virus type 1 promoter controlling the expression of UL38, a true late gene involved in capsid assembly

The cistrons encoding the herpes simplex virus type 1 (HSV-1) UL37 and UL38 genes are adjacent to one another but are transcribed from opposite strands of the viral DNA. The UL37 gene encodes a 1,123-amino-acid protein of unknown function, while the 465-amino-acid UL38 protein is involved in capsid assembly. Previous work from our laboratory indicated that the transcripts encoding these proteins are expressed with significantly different kinetics in productive infection. In the present communication we confirm the kinetic classes and precisely map the cap sites of the UL37 and UL38 mRNAs. A bifunctional reporter gene vector was used to demonstrate that divergent promoters control the expression of these reporter genes in trans-activation assays. The UL38 promoter is functionally separable from that controlling UL37 in a recombinant virus. We used deletion analysis to demonstrate that as few as 29 bases 5' of the mRNA cap site are adequate for full activity of the UL38 promoter in trans-activation assays. Finally, we analyzed the protein-binding properties of the UL38 promoter; several sites that form complexes containing ICP4, with clear homology to those identified in the HSV-1 gamma 42 promoter, are present. Thus, in general, the properties of this promoter are quite similar to those of other gamma promoters.