Characterization of the DNA-binding properties of herpes simplex virus regulatory protein ICP4

The herpes viral transcription factor ICP4 forms a novel DNA recognition complex

The transcription factor ICP4 from herpes simplex virus has a central role in regulating the gene expression cascade which controls viral infection. Here we present the crystal structure of the functionally essential ICP4 DNA binding domain in complex with a segment from its own promoter, revealing a novel homo-dimeric fold. We also studied the complex in solution by small angle X-Ray scattering, nuclear magnetic resonance and surface-plasmon resonance which indicated that, in addition to the globular domain, a flanking intrinsically disordered region also recognizes DNA. Together the data provides a rationale for the bi-partite nature of the ICP4 DNA recognition consensus sequence as the globular and disordered regions bind synergistically to adjacent DNA motifs. Therefore in common with its eukaryotic host, the viral transcription factor ICP4 utilizes disordered regions to enhance the affinity and tune the specificity of DNA interactions in tandem with a globular domain.

ICP4-induced miR-101 attenuates HSV-1 replication

Hepes simplex Virus type 1 (HSV-1) is an enveloped DNA virus that can cause lytic and latent infection. miRNAs post-transcriptionally regulate gene expression, and our previous work has indicated that HSV-1 infection induces miR-101 expression in HeLa cells. The present study demonstrates that HSV-1-induced miR-101 is mainly derived from its precursor hsa-mir-101-2, and the HSV-1 immediate early gene ICP4 (infected-cell polypeptide 4) directly binds to the hsa-mir-101-2 promoter to activate its expression. RNA-binding protein G-rich sequence factor 1 (GRSF1) was identified as a new target of miR-101; GRSF1 binds to HSV-1 p40 mRNA and enhances its expression, facilitating viral proliferation. Together, ICP4 induces miR-101 expression, which downregulates GRSF1 expression and attenuates the replication of HSV-1. This allows host cells to maintain a permissive environment for viral replication by preventing lytic cell death. These findings indicate that HSV-1 early gene expression modulates host miRNAs to regulate molecular defense mechanisms. This study provides novel insight into host-virus interactions in HSV-1 infection and may contribute to the development of antiviral therapeutics.

High-mobility group protein A1 binds herpes simplex virus gene regulatory sequences and affects their expression

The high-mobility group protein A1 (HMGA1), which regulates mammalian gene expression by altering chromatin architecture, was found to bind at multiple sites within the promoter regions of all of the herpes simplex virus type 1 (HSV-1) immediate early genes, as well as a representative early (tk) gene and one late (gC) gene, both in vitro and in vivo. Infected cell polypeptide (ICP) 4, the major HSV-1 regulatory protein, binds these promoters both in vitro and in vivo, and HMGA1 enhances its in vitro binding. In transient expression experiments, HMGA1 modified the effects of both ICP4 and ICP0, another virus transactivator, on virus gene expression in a promoter-specific manner, but it had no effect on the transactivation of immediate-early promoters by VP16. These data indicate that host-cell architectural chromatin proteins could influence the interactions of host-cell and viral transcription factors with the virus DNA regulatory elements and affect HSV-1 gene expression.

Binding of ICP4, TATA-binding protein, and RNA polymerase II to herpes simplex virus type 1 immediate-early, early, and late promoters in virus-infected cells

The binding of herpes simplex virus type 1 ICP4, TATA-binding protein (TBP), and RNA polymerase II (polII) to the promoter regions of representative immediate-early (IE) (ICP0), early (E) (thymidine kinase [tk]), and late (L) (glycoprotein C [gC]) genes on the viral genome was examined as a function of time postinfection, viral DNA replication, cis-acting sites for TFIID in the tk and gC promoters, and genetic background of ICP4. The binding of TBP and polII to the IE ICP0 promoter was independent of the presence of ICP4, whereas the binding of TBP and polII to the tk and gC promoters occurred only when ICP4 also bound to the promoters, suggesting that the presence of ICP4 at the promoters of E and L genes in virus-infected cells is crucial for the formation of transcription complexes on these promoters. When the TATA box of the tk promoter or the initiator element (INR) of the gC promoter was mutated, a reduction in the amount of TBP and polII binding was observed. However, a reduction in the amount of ICP4 binding to the promoters was also observed, suggesting that the binding of TBP-containing complexes and ICP4 is cooperative. The binding of ICP4, TBP, and polII was also observed on the gC promoter at early times postinfection or when DNA synthesis was inhibited, suggesting that transcription complexes may be formed early on L promoters and that additional events or proteins are required for expression. The ability to form these early complexes on the gC promoter required the DNA-binding domain but in addition required the carboxyl-terminal 524 amino acids of ICP4, which is missing the virus n208. This region was not required to form TBP- and polII-containing complexes on the tk promoter. n208 activates E but not L genes during viral infection. These data suggest that a region of ICP4 may differentiate between forming TBP- and polII-containing complexes on E and L promoters.

Mitotic transcription repression in vivo in the absence of nucleosomal chromatin condensation

All nuclear RNA synthesis is repressed during the mitotic phase of the cell cycle. In addition, RNA polymerase II (RNAP II), nascent RNA and many transcription factors disengage from DNA during mitosis. It has been proposed that mitotic transcription repression and disengagement of factors are due to either mitotic chromatin condensation or biochemical modifications to the transcription machinery. In this study, we investigate the requirement for chromatin condensation in establishing mitotic transcription repression and factor loss, by analyzing transcription and RNAP II localization in mitotic cells infected with herpes simplex virus type 1. We find that virus-infected cells enter mitosis and that mitotic viral DNA is maintained in a nucleosome-free and noncondensed state. Our data show that RNAP II transcription is repressed on cellular genes that are condensed into mitotic chromosomes and on viral genes that remain nucleosome free and noncondensed. Although RNAP II may interact indirectly with viral DNA during mitosis, it remains transcriptionally unengaged. This study demonstrates that mitotic repression of transcription and loss of transcription factors from mitotic DNA can occur independently of nucleosomal chromatin condensation.

Herpes simplex virus immediate-early proteins ICP0 and ICP4 activate the endogenous human alpha-globin gene in nonerythroid cells

Globin genes are normally expressed only in erythroid cell lineages. However, we found that the endogenous alpha-globin gene is activated following infection of human fibroblasts and HeLa cells with herpes simplex virus (HSV), leading to accumulation of correctly initiated transcripts driven by the alpha-globin promoter. The alpha1- and alpha2-globin genes were both induced, but expression of beta- or zeta-globin genes could not be detected. Experiments using HSV mutants showed that null mutations in the genes encoding the viral immediate-early proteins ICP4 and ICP22 reduced induction approximately 10-fold, while loss of ICP0 function had a smaller inhibitory effect. Transient transfection experiments showed that ICP0 and ICP4 are each sufficient to trigger detectable expression of the endogenous gene, while ICP22 had no detectable effect in this assay. ICP4 also strongly enhanced expression of transfected copies of the alpha2-globin gene. In contrast, the adenovirus E1a protein did not activate the endogenous gene and inhibited expression of the plasmid-borne alpha2-globin gene. Previous studies have led to the hypothesis that chromosomal alpha-globin genes are subject to chromatin-dependent repression mechanism that prevents expression in nonerythroid cells. Our data suggest that HSV ICP0 and ICP4 either break or bypass this cellular gene silencing mechanism.

Analysis of phosphorylation sites of herpes simplex virus type 1 ICP4

The herpes simplex virus ICP4 protein is required for induction of early and late viral gene transcription as well as for repression of expression of its own gene and several other viral genes. Several electrophoretic forms of ICP4 have been observed, and phosphorylation is thought to contribute to this heterogeneity and possibly to the multiple functions of ICP4. To define the complexity of the site(s) of phosphorylation of ICP4 and to initiate mapping of this site(s), we have performed two-dimensional phosphopeptide mapping of wild-type and mutant forms of ICP4 labeled in infected cells or in vitro. Wild-type ICP4 labeled in infected cells shows a complex pattern of phosphopeptides, and smaller mutant forms of ICP4 show progressively fewer phosphopeptides, arguing that multiple sites on ICP4 are phosphorylated. The serine-rich region of ICP4, residues 175 to 198, was shown to be a site for phosphorylation. Furthermore, the serine-rich region itself or the phosphorylation of this region increases phosphorylation of all phosphopeptides. A mutant ICP4 molecule lacking the serine-rich region showed low levels of phosphorylation by protein kinase A or protein kinase C in vitro. These results suggest that there may be a sequential phosphorylation of ICP4, with phosphorylation of the serine-rich region stimulating phosphorylation of the rest of the molecule. In addition, purified ICP4 showed an associated kinase activity or an autophosphorylation activity with properties different from those of protein kinase A or protein kinase C.

Role of protein kinase A and the serine-rich region of herpes simplex virus type 1 ICP4 in viral replication

Efficient expression of herpes simplex virus genes requires the synthesis of functional ICP4, a nuclear phosphoprotein that contains a prominent serine-rich region between amino acids 142 and 210. Residues in this region not only are potential sites for phosphorylation but also are involved in the functions of ICP4. By comparing the growth of a virus in which this region is deleted (d8-10) with wild-type virus (KOS) in PC12 cells or PC12 cells that are deficient in cyclic AMP-dependent protein kinase (PKA), two observations were made: (i) the growth of wild-type virus was impaired by 1 to 2 orders of magnitude in the PKA-deficient cells, indicating the involvement of PKA in the growth cycle of herpes simplex virus type 1, and (ii) while the growth of d8-10 was impaired by almost 2 orders of magnitude in wild-type cells, it was not further impaired (as was that of wild-type virus) in PKA-deficient cells, implicating the region deleted in d8-10 as a possible target for cellular PKA. In trigeminal'ganglia of mice, the d8-10 mutant virus grew poorly; however, it established latency in nearly 90% of ganglia tested. Studies of the phosphorylation of wild-type and d8-10 ICP4 proteins revealed that the serine-rich region is a major determinant for phosphorylation of ICP4 in vivo and that the phosphorylation state could change as a function of the PKA activity. Consistent with this observation, the serine-rich region of ICP4 was shown to be a target for PKA in vitro. While intact ICP4 was readily phosphorylated by ICP4 in vitro, the d8-10 mutant ICP4 was not. Moreover, a synthethic peptide representing a sequence in the serine tract that is predicted to be a substrate for PKA was phosphorylated by PKA in vitro, having a Km within the physiological range. These data suggest that PKA plays a role in viral growth through phosphorylation of one or more sites on the ICP4 molecule.

Polyamines alter sequence-specific DNA-protein interactions

The polyamines are abundant biogenic cations implicated in many biological processes. Despite a plethora of evidence on polyamine-induced DNA conformational changes, no thorough study of their effects on the activities of sequence-specific DNA binding proteins has been performed. We describe the in vitro effects of polyamines on the activities of purified, representative DNA-binding proteins, and on complex protein mixtures. Polyamines at physiological concentrations enhance the binding of several proteins to DNA (e.g. USF, TFE3, Ig/EBP, NF-IL6, YY1 and ICP-4, a herpes simplex virus gene regulator), but inhibit others (e.g. Oct-1). The degree of enhancement correlates with cationic charge; divalent putrescine is ineffective whereas tetravalent spermine is more potent than trivalent spermidine. Polyamine effects on USF and ICP-4 result from increased rate of complex formation rather than a decreased rate of dissociation. DNAse I footprint analysis indicated that polyamines do not alter DNA-protein contacts. Polyamines also facilitate formation of complexes involving binding of more than one protein on a DNA fragment.

Nucleotide sequence of infectious laryngotracheitis virus (gallid herpesvirus 1) ICP4 gene

The infectious laryngotracheitis virus (ILTV) gene encoding a homologue to the ICP4 protein of herpes simplex virus (HSV) has been mapped to the inverted repeat region. The complete nucleotide sequence of ILTV ICP4 has been determined. The ILTV ORF encoding ICP4 is 4386 nucleotides long, calculated from the first of four ATG codons, and has an overall G+C content of 59%. The ILTV ICP4 contains two domains of high homology which have been reported in other studies to be conserved in the ICP4 homologues of alphaherpesviruses, and to be functionally important. Several regulatory features were identified including a serine-rich domain in region one. A more extensive serine-rich domain was located in region five which is also found in varicella-zoster virus (VZV) and bovine herpesvirus 1. A 5.4 kb immediate early transcript was identified in infected primary kidney cells.

Effect of the transcription start region of the herpes simplex virus type 1 latency-associated transcript promoter on expression of productively infected neurons in vivo

It has been previously reported that the latency-associated transcript (LAT) promoter contains a DNA sequence at the LAT transcription start site which resembles the ICP4 consensus DNA binding site and that this site allows ICP4-mediated downregulation of the LAT promoter in transient assays (A. H. Batchelor and P. O'Hare, J. Virol. 64:3269-3279, 1990). We have confirmed these data by showing that an ICP4-expressing plasmid will downregulate lacZ expression from a plasmid containing the LAT promoter and transcription start site (pJA1) and does not downregulate lacZ expression from a plasmid in which the start site has been mutagenized (pWAG15). To determine the role of the LAT transcription start site in regulating LAT promoter activity in the context of the virus, two recombinant viruses, KOS-1 and KOS-15, were studied. KOS-1 contains an 863-bp portion of the LAT promoter, including the LAT cap site, fused to the lacZ gene and inserted into the gC locus (T.P. Margolis, F. Sedarati, A.T. Dobson, L.T. Feldman, and J.G. Stevens, Virology 189:150-160, 1992). The second virus (KOS-15) was constructed in identical fashion, using plasmid pWAG-15, which is not downregulated by ICP4. Vero cells productively infected with KOS-15 produce 10-fold more beta-galactosidase than do those infected with KOS-1. In murine dorsal root ganglia acutely infected with KOS-1, only 1.2% of dorsal root ganglion neurons that expressed viral antigen also expressed beta-galactosidase. In contrast, in KOS-15-infected mice, beta-galactosidase was detected in 18% of viral antigen-positive neurons. Similar findings were observed in trigeminal ganglia acutely infected with KOS-1 and KOS-15. Thus, the region encompassing the LAT transcription start site appears to play an important role in repression of the LAT promoter activity not only in vitro but also in acutely infected neurons in vivo. These results suggest that during productive infection with HSV-1, LAT expression is tightly regulated.

Localization of a 34-amino-acid segment implicated in dimerization of the herpes simplex virus type 1 ICP4 polypeptide by a dimerization trap

The herpes simplex virus type 1 immediate-early protein ICP4 plays an essential role in the regulation of the expression of all viral genes. It is the major trans activator of early and late genes and also has a negative regulatory effect on immediate-early gene transcription. ICP4 is a sequence-specific DNA-binding protein and has always been purified in a dimeric form. The part of the protein that consists of the entire highly conserved region 2 and of the distal portion of region 1 retains the ability to specifically associate with DNA and to form homodimers in solution. In an attempt to map the dimerization domain of ICP4, we used a dimerization trap assay, in which we screened deletion fragments of this 217-amino-acid stretch for sequences that could confer dimerization properties on a heterologous cellular transcription factor (LFB1), which binds to its cognate DNA sequence only as a dimer. The analysis of these chimeric proteins expressed in vitro ultimately identified a stretch of 34 amino acids (343 to 376) that could still confer DNA-binding activity on the LFB1 reporter protein and thus apparently contained the ICP4 dimerization motif. Consistent with this result, a truncated ICP4 protein containing amino acids 343 to 490, in spite of the complete loss of DNA-binding activity, appeared to retain the capacity to form a heterodimer with a longer ICP4 peptide after coexpression in an in vitro translation system.

The DNA binding domains of the varicella-zoster virus gene 62 and herpes simplex virus type 1 ICP4 transactivator proteins heterodimerize and bind to DNA

The product of varicella-zoster virus gene 62 (VZV 140k) is the functional counterpart of the major transcriptional regulatory protein of herpes simplex virus type 1 (HSV-1), ICP4. We have found that the purified bacterially expressed DNA binding domain of VZV 140k (residues 417-647) is a stable dimer in solution. As demonstrated by the appearance of a novel protein--DNA complex of intermediate mobility in gel retardation assays, following in vitro co-translation of a pair of differently sized VZV 140k DNA binding domain peptides, the 140k DNA binding domain peptide binds to DNA as a dimer. In addition, the DNA binding domain peptide of HSV-1 ICP4 readily heterodimerizes with the VZV 140k peptide on co-translation, indicating that HSV-1 ICP4 and VZV 140k possess very similar dimerization interfaces. It appears that only one fully wild type subunit of the dimer is sufficient to mediate sequence specific DNA recognition in certain circumstances. Co-immunoprecipitation analysis of mutant DNA binding domain peptides, co-translated with an epitope-tagged ICP4 DNA binding domain, shows that the sequence requirements for dimerization are lower than those necessary for DNA binding.

Mutation of a single lysine residue severely impairs the DNA recognition and regulatory functions of the VZV gene 62 transactivator protein

The product of varicella-zoster virus gene 62 (VZV 140k) is a potent transactivator protein. We have identified a region within the DNA binding domain of VZV 140k that shows a striking similarity to the DNA recognition helix of the homeodomain, with an especially highly conserved quartet of residues, WLQN. The 140k protein has functional counterparts within the other alphaherpesviruses, which include the major transcriptional regulatory protein of HSV-1, (ICP4), and the WLQN region is highly conserved among the members of this family of viral transactivators. Substitution of VZV 140k residue lysine 548, just adjacent to the WLQN region, drastically reduces the DNA binding activity of the 140k DNA binding domain and the intact 140k mutant protein fails to activate gene expression. Substitutions of two other VZV 140k residues in this conserved WLQN region result in alterations to the DNA binding interaction and reduced transactivation activities. All three mutations act at the level of DNA recognition, as they have no apparent effect on the dimerization state, solubility or efficiency of expression of the mutant peptides.

Amino acid substitution mutations in the herpes simplex virus ICP27 protein define an essential gene regulation function

ICP27 is an essential herpes simplex virus type 1 (HSV-1) alpha protein that is required for the transition from the beta to the gamma phase of infection. To identify functional regions of ICP27, we constructed 16 plasmids that contain nucleotide substitution mutations in the ICP27 gene. The mutations created XhoI restriction sites, altered one or two codons, and were spaced at semiregular intervals throughout the coding region. Three mutations completely inactivated an essential function of ICP27, as demonstrated by the inability of the transfected plasmids to complement the growth of an HSV-1 ICP27 deletion mutant. These mutations, M11, M15, and M16, mapped in the carboxyl-terminal one-third of ICP27 at residues 340 and 341, 465 and 466, and 488, respectively. In cotransfection assays, all three defective-plasmid mutants retained the transrepression function of ICP27 but were defective at transactivation. To define the lytic functions that are mediated by the transactivation activity of ICP27, we engineered HSV-1 recombinants containing the M11, M15, or M16 mutation. All three viral mutants failed to grow in Vero cells and possessed similar phenotypes. The viral mutants replicated their DNA similarly to the wild-type virus but showed several defects in viral gene expression. These were a failure to down-regulate alpha and beta genes at late times after infection and an inability to induce certain gamma-2 genes. Our results demonstrate that the transactivation function of ICP27 (as it is defined in cotransfection assays) mediates an essential gene regulation function during the HSV-1 infection. This activity is not required for ICP27-dependent enhancement of viral DNA replication. Our work supports and extends previous studies which suggest that ICP27 carries out two distinct regulatory activities during the HSV-1 infection.

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.

Direct interaction of the human cytomegalovirus IE86 protein with the cis repression signal does not preclude TBP from binding to the TATA box

The human cytomegalovirus major immediate-early gene encodes several protein isoforms which autoregulate the major immediate-early promoter (MIEP). One of these isoforms, the IE86 protein, represses the MIEP through a DNA sequence located between the TATA box and the transcription initiation site, designated the cis repression signal (crs). Through mutational analysis, amino acid domains within IE86 responsible for binding the crs element were located at the C terminus. Mutation of the putative zinc finger domain, which precluded IE86 from binding DNA, converted the protein from a repressor of MIEP transcription into an activator. DNase I protection analysis demonstrated that the IE86 footprint overlapped the sequence protected by the TATA-binding protein (TBP). Investigation of whether IE86 was able to displace TBP from DNA revealed that both proteins could bind DNA simultaneously. However, higher concentrations of IE86 were required to obtain protection of the crs element in the presence of prebound TBP. Similarly, higher concentrations of TBP were required to obtain protection in the presence of prebound IE86. These observations indicate that steric hinderance impairs but does not prevent both proteins from binding DNA synchronously.

The acidic amino-terminal region of herpes simplex virus type 1 alpha protein ICP27 is required for an essential lytic function

The herpes simplex virus type 1 (HSV-1) alpha protein ICP27 regulates the transition between the delayed-early and late phases of the viral infection. Previous genetic analyses have suggested that the important functional domains of ICP27 map to its carboxyl-terminal half. One striking feature of the primary sequence of ICP27, however, is an extremely acidic region near its amino terminus. To determine whether this region is required for ICP27 function, we deleted the sequences in the ICP27 gene which encode it (codons 12 through 63). In transient expression assays, the deletion mutant was unable to efficiently repress the expression of a cotransfected reporter gene or to efficiently complement the growth of d27-1, an HSV-1 ICP27 null mutant. These results suggested that the acidic region of ICP27 is involved in a regulatory function required for lytic growth. To test this possibility further, we introduced the mutant allele into the HSV-1 genome by marker transfer. Two independently derived isolates of the mutant virus, designated d1-2a and d1-2b, were recovered and analyzed. Both isolates were defective for growth in Vero cells, exhibiting a 100-fold reduction in virus yield compared with the wild-type infection. Vero cells infected with the d1-2 isolates showed a three- to eightfold reduction in viral DNA replication, a moderate reduction in the expression of viral gamma genes, and a delay in the repression of beta genes. The phenotype of the d1-2 isolates differs substantially from the phenotypes of previously isolated ICP27 mutants, which show much more severe defects in viral gene expression. Our results demonstrate that the amino-terminal half of ICP27 participates in its regulatory activities in both infected and transfected cells.

Repression of the herpes simplex virus 1 alpha 4 gene by its gene product occurs within the context of the viral genome and is associated with all three identified cognate sites

The infected cell protein 4 (ICP-4), the major regulatory protein encoded by the a4 gene of the herpes simplex virus 1, binds two sites (alpha 4-1 proximal, alpha 4-1 distal) at the 5'-untranscribed domain and at the transcription initiation site (alpha 4-2) of the alpha 4 gene. Chimeric genes consisting of the 5'-untranscribed and transcribed noncoding domains of the alpha 4 gene fused to the coding sequences of the thymidine kinase gene were mutagenized to abolish binding of ICP-4 by substitution of bases, including the guanines whose methylation interferes with binding of the protein, and recombined into the viral genome. The cytoplasmic RNAs extracted from infected cells treated with cycloheximide, from untreated infected cells maintained for 4 or 8 hr, and from cells infected first with a virus deleted in the alpha 22 gene and 3 hr later with the test viruses were tested in RNase protection assay for amounts of the chimeric gene RNA relative to amounts of alpha 22 gene RNA. We report the following: (i) Mutation of the alpha 4-2 binding site resulted in a 5-to 6-fold higher accumulation of chimeric gene RNA at 4 hr and as much as 15-fold higher accumulation by 8 hr after infection. (ii) Mutations of alpha 4-1 sites by themselves had no effect on RNA accumulation. However, mutagenesis of all three sites significantly increased mRNA amounts above the levels seen in cells infected with alpha 4-2 site mutants. (iii) The mutations have no effect on accumulation of alpha 4 mRNA in the absence of ICP-4 synthesis and, therefore, the mutations had no effect on RNA stability or transcription rate. (iv) Accumulation of alpha 4 mRNA relative to that of alpha 22 mRNA is highest in the presence of cycloheximide and decreases with time after infection. We conclude that ICP-4 autoregulates the transcription of its own gene in infected cells and that binding of ICP-4 to three sites in its promoter is additive in its effects on this process.

Differences in the poly(ADP-ribosyl)ation patterns of ICP4, the herpes simplex virus major regulatory protein, in infected cells and in isolated nuclei

Infected-cell protein 4 (ICP4), the major regulatory protein in herpes simplex viruses 1 and 2, was previously reported to accept 32P from [32P]NAD in isolated nuclei. This modification was attributed to poly(ADP-ribosyl)ation (C. M. Preston and E. L. Notarianni, Virology 131:492-501, 1983). We determined that an antibody specific for poly(ADP-ribose) reacts with ICP4 extracted from infected cells, electrophoretically separated in denaturing gels, and electrically transferred to nitrocellulose. Our results indicate that all forms of ICP4 observed in one-dimensional gel electrophoresis are poly(ADP-ribosyl)ated. Poly(ADP-ribose) on ICP4 extracted from infected cells was resistant to cleavage by purified poly(ADP-ribose) glycohydrolase unless ICP4 was in a denatured state. Poly(ADP-ribose) added to ICP4 in isolated nuclei was sensitive to this enzyme. This result indicates that the two processes are distinct and may involve different sites on the ICP4 molecule.

The herpes simplex virus type 1 immediate-early polypeptide ICP4 is required for expression of globin genes located in the viral genome

We infected Vero cells with ICP4-deficient herpes simplex virus recombinants bearing the rabbit beta-globin and human alpha 2-globin genes under the control of their own promoters and found that globin gene expression occurred only when ICP4 was provided in trans. These results demonstrate that ICP4 is required for the activity of globin promoters located in the viral genome and support the hypothesis that these cellular promoters are functionally equivalent to HSV early regulatory regions.

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 ICP4 binding sites in the herpes simplex virus type 1 glycoprotein D (gD) promoter are not essential for efficient gD transcription during virus infection

Activation of the early and late genes of herpes simplex virus type 1 during infection in tissue culture requires functional immediate-early regulatory protein ICP4. ICP4 is a specific DNA-binding protein which recognizes a variety of DNA sequences, many of which contain the consensus ATCGTC. In general, mutations which impair the ability of ICP4 to bind to DNA also eliminate its ability to activate viral early and late promoters both in transfection assays and in the infected cell. However, the role of ICP4 binding sites in the viral genome is unclear; many early and late promoters do not contain consensus binding sites in their vicinity. The glycoprotein D (gD) gene contains two well-characterized ICP4 binding sites upstream of its promoter and a third downstream of the transcription start site. Multimerization of one of these sites has been shown to increase the response of the gD promoter to ICP4 in transfection assays, while their removal reduces stimulation of the gD promoter by ICP4 in vitro. To assess the role of these binding sites during virus infection, we have constructed a recombinant viral genome which has mutations affecting all three. Comparison of the amounts of gD RNA synthesized by the recombinant and wild-type viruses indicated that the mutations had little or no effect on the activity of the gD promoter. Therefore, either the sites have no essential role in gD promoter regulation in the presence of all of the herpes simplex virus type 1 IE polypeptides during a normal infection or they can be functionally substituted by other ICP4 binding sites elsewhere in the genome.

Purification of the DNA binding domain of herpes simplex virus type 1 immediate-early protein Vmw175 as a homodimer and extensive mutagenesis of its DNA recognition site

The herpes simplex virus type 1 (HSV-1) Immediate-Early (IE) polypeptide Vmw175 is essential for the activation of transcription from viral early and late promoters. Vmw175 also reduces the activity of its own (IE-3) promoter in transfection assays. Both transactivation and repression mediated by Vmw175 require the integrity of a conserved domain of the polypeptide which has been shown to bind to specific DNA sequences. We have investigated the DNA sequence requirements for Vmw175 binding using a randomly mutated target. The results indicate that the binding site covers a region of 13 nucleotides divided into proximal and distal parts which are consistent with the consensus ATCGTNNNNNYSG. We have also expressed several different constructs encompassing the DNA binding domain of Vmw175 in bacteria, and obtained preparations of greater than 90% purity. The DNA binding domain is a dimer in solution, and binds DNA with a specificity similar to that of the intact protein, although the smallest DNA binding competent protein has a slightly reduced specificity.

Transactivation by herpes simplex virus proteins ICP4 and ICP0 in vaccinia virus infected cells

Vaccinia virus recombinants containing the sequences from herpes simplex virus type 1 (HSV-1) encoding the immediate early (IE)(alpha) proteins ICP4 and ICP0, under the control of a mutated vaccinia virus 11K late promoter, were constructed. A cDNA copy of the gene encoding ICPO and an ICP4-encoding genomic segment were each inserted into the vaccinia virus genome at the thymidine kinase (TK) locus by homologous recombination. Steady-state analyses revealed that RNAs homologous to the IE-0 and IE-4 sequences accumulated in cells infected by recombinants with the kinetics of a typical vaccinia late mRNA. Western blot analyses demonstrated that the expression level of both ICPO and ICP4, produced by the recombinant viruses, was comparable to that in HSV-1-infected cells at late times postinfection. Both proteins synthesized in cells infected by the recombinants were located in the nucleus as revealed by immunofluorescence. Although in vitro studies reveal that extracts from vaccinia-virus-infected cells lose the ability to transcribe genes that contain RNA polymerase II promoters (Puckett and Moss (1983), Cell 35, 441-448) both ICPO and ICP4 expressed by the recombinant viruses can transactivate plasmids containing a reporter gene driven by the promoters for the HSV-1 TK and glycoprotein C genes. Nuclear extracts prepared from cells infected with the vaccinia virus vector expressing ICP4 exhibited sequence-specific DNA-binding activity.

ICP4, the major regulatory protein of herpes simplex virus, shares features common to GTP-binding proteins and is adenylated and guanylated

Infected cell protein 4 (ICP4), the product of the alpha 4 gene, regulates herpes simplex virus 1 and herpes simplex virus 2 gene expression at the transcriptional level both positively and negatively. Previous studies have shown that ICP4 is extensively modified posttranslationally. We report that ICP4 was labeled in isolated nuclei of infected cells by [alpha-32P]GTP or [alpha-32P]ATP. The labeling of ICP4 by [alpha-32P]GTP or [alpha-32P]ATP required excess GTP, ATP, GDP, and ADP and occurred also in the presence of excess GTP(gamma)S. While GDP and ADP activated the labeling process, only GTP and ATP labeled ICP4. Accumulation of labeled ICP4 was favored at temperatures from 15 to 27 degrees C and in the presence of okadaic acid. The conditions for labeling ICP4 with [alpha-32P]GTP or [alpha-32P]ATP and the stability of the labeled protein were different from those of ICP4 labeled with [gamma-32P]ATP. Labeling studies with tritiated ATP and GTP showed that ICP4 is nucleotidylated, and chemical degradation of ICP4 labeled with [alpha-32P]GTP yielded ribose-5-phosphate. Pulse-chase experiments indicated that the adenylation and guanylation are independent processes. These results, and the observation that ICP4 contains four regions which possess consensus GTP-binding elements, suggest that ICP4 may belong to a class of GTP-binding proteins which function in transcriptional transactivation.

A predictive model for DNA recognition by the herpes simplex virus protein ICP4

The herpes simplex virus (HSV) type 1 immediate early protein ICP4 is an essential regulatory enzyme that binds DNA directly in order to stimulate or repress gene expression. The degree of transaction is related to the locations and affinities of the ICP4 binding sites. A number of binding sites have been identified; some sites showed obvious homology to one another, and these were called consensus ICP4 binding sites. Other binding sites did not appear to be related, and these were termed non-consensus sites. We hypothesized, however, that a single model could describe all ICP4 binding sites, given the appropriate characterizations of sites. We performed statistical analyses on a set of ICP4 binding sites and found that the bases important for defining binding were located within a 13 base region. Missing contact analyses on several high-affinity binding sites revealed the same 13 base region as important for critical protein-DNA contacts. From these data we derived the consensus sequence RTCGTCNNYNYSG, where R is purine, Y is pyrimidine, S is C or G, and N is any base. In addition, we found that a better profile for ICP4 binding sites involves use of a matrix of base proportions from the binding site data; sites are analyzed by calculating the Matrix Mean score. We show that this Matrix Mean model could accurately predict the locations of novel ICP4 binding sites. Finally, we analyzed the entire HSV-1 genome for potential ICP4 binding sites and speculate about what these results suggest for the role of ICP4 in viral gene regulation.

Association of a major transcriptional regulatory protein, ICP4, of herpes simplex virus type 1 with the plasma membrane of virus-infected cells

A major transcriptional regulatory protein, ICP4, of herpes simplex virus type 1 (HSV-1) is localized primarily within the nucleus soon after its synthesis. Recent studies have shown that approximately 100 to 200 molecules of ICP4 are located in the tegument region of purified virions (F. Yao and R. J. Courtney, J. Virol. 63:3338-3344, 1989). As an extension to these studies, we present data suggesting that ICP4 may also associate with the plasma membrane of HSV-1-infected cells. The experimental approaches used included the isolation and purification of plasma membranes from HSV-1-infected cells, the isolation of purified vesicular stomatitis virus containing ICP4, and immunofluorescence of HSV-1-infected cells following selective permeabilization with detergent. The results from the above studies support the suggestion that detectable amounts of ICP4 are associated with the inner surface of the plasma membrane of HSV-1-infected cells.

The conserved DNA-binding domains encoded by the herpes simplex virus type 1 ICP4, pseudorabies virus IE180, and varicella-zoster virus ORF62 genes recognize similar sites in the corresponding promoters

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), pseudorabies virus (PRV), varicella-zoster virus (VZV), and equine herpesvirus 1 (EHV-1) are all classified as Alphaherpesvirinae. Each of these five viruses encodes an essential immediate-early (IE) regulatory protein referred to as HSV-1 ICP4, HSV-2 ICP4, PRV IE180, VZV ORF62 protein, and EHV-1 IE1, respectively. These five proteins share extensive homology with each other in domains referred to as regions 2 and 4. The HSV-1 ICP4 region 2 domain contains residues that are required for the DNA-binding capability of ICP4. In this report, we describe the expression of region 2 domains from the ICP4, IE180, and ORF62 genes as fusion proteins in Escherichia coli. DNA-binding assays revealed that each of these region 2 fusion proteins binds to a sequence that overlaps the transcription start site in the promoter for the gene encoding the corresponding protein. Each of the sites with high affinity for one or more of these fusion proteins contains the sequence 5'-ATCGT-3'. This sequence spans the mRNA cap site in the HSV-2 ICP4 gene promoter and is immediately upstream from the transcription start site in the EHV-1 IE1 gene. These results suggest that formation of a specific complex between an IE protein and its own gene promoter may be a common mechanism used by Alphaherpesvirinae to autoregulate transcription of an essential IE gene.

Nucleotides within both proximal and distal parts of the consensus sequence are important for specific DNA recognition by the herpes simplex virus regulatory protein ICP4

The herpes simplex virus type 1 regulatory protein ICP4 is a sequence specific DNA binding protein which associates with a number of different sites, some of which include the consensus ATCGTCnnnnYCGRC. In order to investigate the involvement in DNA binding of conserved bases within the consensus, we have synthesised a family of mutant oligonucleotides and tested their ability to form a complex with ICP4. We have also compared the binding specificities of bacterially expressed fragments of ICP4 which include the DNA binding domain. Mutation of most (but not all) bases in the proximal part of the consensus greatly reduced binding by ICP4, as did a mutation affecting the distal part. Most (but not all) G residues identified in methylation interference assays were required for efficient binding. While a bacterially expressed ICP4 peptide encompassing amino acid residues 252-523 bound to DNA with a specificity similar to that of the whole protein, a shorter protein (residues 275-523) had a slightly relaxed DNA binding specificity.

The Vmw175 binding site in the IE-1 promoter has no apparent role in the expression of Vmw110 during herpes simplex virus type 1 infection

The immediate-early (IE) genes of herpes simplex virus type 1 (HSV-1) are the first to be expressed during infection in tissue culture. Since they are transcribed at abnormally high levels in the absence of IE protein synthesis they appear to be subject to repression during normal infection. One of the major HSV-1 regulatory proteins, Vmw175 (the product of IE gene 3), is required for normal IE gene regulation since mutations which inactivate it lead to abnormally high levels of IE gene expression. The mechanism of repression of the IE-3 promoter requires both the ability of Vmw175 to bind to DNA and the presence of a Vmw175 recognition DNA binding sequence at the cap site of the IE-3 promoter. A similar Vmw175 DNA binding sequence has been defined within the IE-1 promoter. This paper describes the construction of a variant of HSV-1 with a mutation within the IE-1 Vmw175 DNA binding site. Although the mutation destroyed the ability of Vmw175 to bind to the site, and greatly reduced the ability of Vmw175 to repress the IE-1 promoter in transfection assays, the mutation had no effect on the levels of Vmw110 expression during normal HSV-1 infection.

A second-site revertant of a defective herpes simplex virus ICP4 protein with restored regulatory activities and impaired DNA-binding properties

A mutant of herpes simplex virus type 1, vi12, encodes a DNA-binding- and transactivation-deficient ICP4 polypeptide. Because of the mutation, the vi12 virus does not grow on Vero cells but must be propagated on cells that express complementing levels of wild-type ICP4 (E5 cells). A pseudorevertant of vi12, designated pri12, was isolated on the basis of the restored ability to replicate on Vero cells. In addition to the original i12 insertion mutation at amino acid 320, the ICP4 molecule expressed from pri12 possesses an alanine to valine substitution at amino acid 342 within the ICP4 gene. The infectivity of pri12 on Vero cells as measured by burst size is elevated by 5 orders of magnitude relative to that observed for vi12, reflecting the restored ability of the mutant ICP4 molecule possessing the alanine to valine substitution to activate transcription and thus support viral replication. Despite the restored regulatory activities of the pri12 ICP4 molecule, the ability of the pseudorevertant ICP4 molecule to form a high-affinity, specific interaction with the consensus binding site was still impaired relative to that of wild-type ICP4. This observation suggests that the in vitro-measured DNA-binding properties of ICP4 may not reflect the functional interactions occurring in vivo that mediate transcriptional activation.

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.

Activities of heterodimers composed of DNA-binding- and transactivation-deficient subunits of the herpes simplex virus regulatory protein ICP4

Two mutant strains (vi12 and vi13) of herpes simplex virus that contain insertion mutations in the sequences that encode the DNA-binding domain of viral regulatory protein ICP4 were generated. Both mutations disrupted specific DNA binding and resulted in transcriptionally inactive molecules; however, the ability of the mutant proteins to form dimers was retained. The mutant proteins formed heterodimers with an ICP4 deletion mutant (X25) that is nonfunctional but retains the ability to bind to consensus sites. Significantly elevated levels of early (E or beta) and "leaky late" (beta gamma or gamma 1) gene expression were observed upon coexpression of the insertion mutant and X25 ICP4 polypeptides. While the heterodimers composed of the vi13 and X25 peptides possessed DNA-binding activity, those composed of vi12 and X25 did not, indicating that DNA binding by the heterodimers may not be required for restored activity. Despite significant levels of early gene expression and viral DNA synthesis in vi12-infected X25 cells, true late (gamma 2) mRNA was not synthesized. This indicates that the structural requirements for ICP4 induction of different classes of viral genes may be different.

The major transcriptional regulatory protein of herpes simplex virus type 1 includes a protease resistant DNA binding domain

Herpes simplex virus type 1 expresses five immediate-early (IE) polypeptides. In the absence of functional Vmw175 (the product of IE gene 3) activation of transcription of later classes of viral genes and repression of IE gene expression does not occur. The recognition of specific DNA sequences by Vmw175 requires, as determined by sensitivity to mutation, a part of the protein highly conserved in the corresponding proteins of related herpes viruses. However, mutations in other parts of the protein can also disrupt specific DNA binding. This paper shows that the DNA binding domain of Vmw175 can be liberated as a functional unit by digestion with proteinase K. Analysis of mutant Vmw175 proteins showed that the proteinase K resistant domain has an amino terminus between amino acid residues 229 and 292, while its carboxy terminus is between residues 495 and 518. Mutations outside this region which affect DNA binding by the intact protein do not eliminate binding of the proteinase K resistant domain. This implies that direct DNA binding by Vmw175 involves a linear subsection of the polypeptide, and that mutations in other parts of the polypeptide which affect DNA binding of the whole protein do so by indirect means.

Genetic evidence for two distinct transactivation functions of the herpes simplex virus alpha protein ICP27

Infected-cell protein 27 (ICP27) is a herpes simplex virus type 1 alpha, or immediate-early, protein involved in the regulation of viral gene expression. To better understand the function(s) of ICP27 in infected cells, we have isolated and characterized viral recombinants containing defined alterations in the ICP27 gene. The mutant virus d27-1 contains a 1.6-kilobase deletion which removes the ICP27 gene promoter and most of the coding sequences, while n59R, n263R, n406R, and n504R are mutants containing nonsense mutations which encode ICP27 molecules truncated at their carboxyl termini. All five mutants were defective for lytic replication in Vero cells. Analysis of the mutant phenotypes suggests that ICP27 has the following regulatory effects during the viral infection: (i) stimulation of expression of gamma-1 genes, (ii) induction of expression of gamma-2 genes, (iii) down regulation of expression of alpha and beta genes late in infection, and (iv) stimulation of viral DNA replication. Cells infected with the mutant n504R expressed wild-type levels of gamma-1 proteins but appeared to be unable to efficiently express gamma-2 mRNAs or proteins. This result suggests that ICP27 mediates two distinct transactivation functions, one which stimulates gamma-1 gene expression and a second one required for gamma-2 gene induction. Analysis of the mutant n406R suggested that a truncated ICP27 polypeptide can interfere with the expression of many viral beta genes. Our results demonstrate that ICP27 has a variety of positive and negative effects on the expression of viral genes during infection.

Codons 262 to 490 from the herpes simplex virus ICP4 gene are sufficient to encode a sequence-specific DNA binding protein

The HSV-1 immediate early (IE) protein ICP4 (alpha 4, IE175, Vmw175) is an oligomeric molecule which activates transcription of viral early genes, represses transcription of viral IE genes, and binds to specific sequences in certain viral promoters. The extent to which these functions are interrelated has not been fully established. We have expressed truncated portions of the ICP4 gene in E. coli as trpE fusion proteins. DNA-binding studies with these hybrid proteins revealed that ICP4 residues 262 to 490 are sufficient for sequence-specific DNA-binding. DNA-binding was not detected with polypeptides extending from residue 262 to 464 or from residue 306 to 490. Multiple bands of protein-DNA complexes observed in gel mobility shift assays indicate that residues 262 to 490 may also contribute to the oligomerization of ICP4.

Mapping of two varicella-zoster virus-encoded genes that activate the expression of viral early and late genes

A transient assay system was used to identify varicella-zoster virus (VZV)-encoded genes whose products are able to activate the expression of an early gene promoter, the thymidine kinase (tk) promoter, and a late gene promoter, and the glycoprotein I (gpI) promoter. Vero cells were cotransfected with individual cloned DNA fragments spanning the entire VZV genome and with the recombinant construct p1tkCAT which contained the chloramphenicol acetyl transferase (CAT) gene under the control of putative regulatory sequences. Five- to 20-fold increases in the expression p1tkCAT was observed in cotransfections with plasmids containing VZV open reading frame (ORF)4 (map location 0.02-0.03) or ORF62 (0.82-0.86). Expression of p68CAT (contains -682 to +222 bp relative to the AUG of gpI) was also enhanced by the products of ORF4 and ORF62. Synergy between ORF4 and ORF62 products was observed in the activation of p68CAT, resulting in a 22-fold increase in CAT activity. RNA analysis indicated that activation of these promoters was at the transcriptional level. A VZV-encoded "repressor" sequence, containing ORF60 and ORF61, was also identified which repressed expression of p1tkCAT and modulated its activation by ORF4 and ORF62.

Binding of the herpes simplex virus major regulatory protein to viral DNA

Infected-cell protein 4 (ICP4), the major regulatory protein specified by herpes simplex virus 1 in infected cells, binds to homologs of the sequence ATCGTCnnnnYCGRC (A sites, where n is any nucleotide, Y is a pyrimidine, and R is a purine) and to unrelated sequences for which no consensus sequence has been derived (B sites). We have examined the binding of ICP4 to each of two A and two B binding sites by using Fab fragments of a monoclonal antibody that is reactive with an epitope located at the N terminus of ICP4 and that decreases the mobility of ICP4-DNA complexes in non-denaturing gels. The results indicate that each type of site binds two monomers of ICP4. Methylation-interference studies on the type B sites mapped the guanines whose methylation interfered with the binding of ICP4. The methylation-interference pattern obtained with one of the B sites was similar to that obtained on an A site but differed from that of the other B site. The ability of ICP4 to bind to DNA fragments containing the binding site appears to be dependent on length and on the proximity of the binding site to the fragment end. Short DNA fragments did not form stable complexes with ICP4 even though they contained all of the purines whose methylation interfered with the binding of the regulatory protein.

Separation of primary structural components conferring autoregulation, transactivation, and DNA-binding properties to the herpes simplex virus transcriptional regulatory protein ICP4

A truncated ICP4 peptide which contains the amino-terminal 774 amino acids of the 1,298-amino-acid polypeptide is proficient for DNA binding, autoregulation, and transactivation of some viral genes (N. A. DeLuca and P. A. Schaffer, J. Virol. 62:732-743, 1988) and hence exhibits many of the properties characteristic of intact ICP4. To define the primary sequence important for the activities inherent in the amino-terminal half of the ICP4 molecule, insertional and deletion mutagenesis of the sequences encoding these residues were conducted. The DNA-binding activity of the molecule as assayed by the association with a consensus binding site was sensitive to insertional mutagenesis in two closely linked regions of the molecule. One region between amino acids 445 and 487 is critical for DNA binding and may contain a helix-turn-helix motif. The second region between amino acids 263 and 338 reduces the binding activity to a consensus binding site. When analyzed in the viral background, the DNA-binding activity of a peptide containing an insertion at amino acid 338 to a consensus binding site was reduced while the association with an alternative sequence was eliminated, suggesting a possible mechanism by which ICP4 may recognize a broader range of sequence elements. Mutations which eliminated DNA binding also eliminated or reduced both transactivation and autoregulation, supporting the requirement for DNA binding for these activities. Peptides that retained the deduced DNA-binding domain but lacked amino acids 143 through 210 retained the ability to associate with the consensus site and autoregulatory activity but were deficient for transactivation, demonstrating that the structural requirements for transactivation are greater than those required for autoregulation.

DNA binding and gene regulation by the herpes simplex virus type 1 protein ICP4 and involvement of the TATA element

We report the results of fine mapping the sequences responsible for negative regulation of immediate-early (IE) gene 3 by its own gene product, ICP4. Affinity-purified ICP4 binds the transcriptional start site of IE gene 3 and protein-protein interactions induce a secondary mobility shift that footprints exactly as the primary complex. Since these DNA-protein complexes contain ICP4, it is likely that the two differ only in stoichiometry of protein. Additional data show that the DNA-binding domain recognized by ICP4 can be embedded as a cassette in foreign DNA and that native ICP4 will recognize and bind the resulting DNA. In two different immediate-early promoters, the ICP4 binding site can be located either 3' or 5' of the TATA box; however, the ICP4 site is rotationally displaced from the transcription factor IID (TFIID) site by a roughly one-half helical turn, suggesting that ICP4 and TFIID are on the opposite helical face when bound at their respective sites. In the IE1 and IE3 promoters, binding of ICP4 causes an alteration in the helical geometry of the minor groove of the TATA region as visualized by copper footprinting. In contrast, TATA hypersensitivity was not detected in the glycoprotein D promoter (an early gene promoter containing the ICP4 site separated from TATA by eight helical turns) or in an artificial IE3 promoter construct in which the TATA-A4 separation was increased from 2.5 to roughly 5 helical turns. Such stereospecific and distance-dependent conformational alterations in the TATA box under the influence of ICP4 binding may be important in the repression of immediate-early genes.

ICP4-binding sites in the promoter and coding regions of the herpes simplex virus gD gene contribute to activation of in vitro transcription by ICP4

The herpes simplex virus immediate-early gene product ICP4 activates the transcription of viral early and late genes. We characterized the DNA sequence elements of the early glycoprotein D (gD) gene that play a role in the response to ICP4 in vitro. Using gel mobility shift assays and DNase I footprinting, we identified three ICP4-binding sites, two 5' to the mRNA start site and a third within the coding region. Site II, which gave a footprint between nucleotides -75 and -111 relative to the RNA start site, was previously identified by Faber and Wilcox and contained the reported consensus ICP4-binding site. Site III, which was located between nucleotides +122 and +163, was very similar to the site II sequence, including a core consensus binding sequence, TCGTC. The site I sequence (nucleotides -308 to -282), however, did not share significant homology with either site II or site III. In vitro transcription experiments from mutant constructs of the gD promoter indicated that all three ICP4-binding sites contribute to the stimulation of transcription by ICP4. DNase I footprinting of the gD promoter with uninfected nuclear extracts of HeLa cells showed protection of two very G-rich sequences between nucleotides -33 and -75. We propose that optimal transcription of the gD gene depends on the interaction of ICP4 with multiple binding sites across the gene and cellular factors that recognize specific sequence elements in the promoter.

The role of viral and cellular nuclear proteins in herpes simplex virus replication

Following infection of cells by herpes simplex virus, the cell nucleus is subverted for transcription and replication of the viral genome and assembly of progeny nucleocapsids. The transition from host to viral transcription involves viral proteins that influence the ability of the cellular RNA polymerase II to transcribe a series of viral genes. The regulation of RNA polymerase II activity by viral gene products seems to occur by several different mechanisms: (1) viral proteins complex with cellular proteins and alter their transcription-promoting activity (e.g., alpha TIF), (2) viral proteins bind to specific DNA sequences and alter transcription (e.g., ICP4), and (3) viral proteins affect the posttranslational modification of viral or cellular transcriptional regulatory proteins (e.g., possibly ICP27). Thus, HSV may utilize several different approaches to influence the ability of host-cell RNA polymerase II to transcribe viral genes. Although it is known that viral transcription uses the host-cell polymerase II, it is not known whether viral infection causes a change in the structural elements of the nucleus that promote transcription. In contrast, HSV encodes a new DNA polymerase and accessory proteins that complex with and reorganize cellular proteins to form new structures where viral DNA replication takes place. HSV may encode a large number of DNA replication proteins, including a new polymerase, because it replicates in resting cells where these cellular gene products would never be expressed. However, it imitates the host cell in that it localizes viral DNA replication proteins to discrete compartments of the nucleus where viral DNA synthesis takes place. Furthermore, there is evidence that at least one specific viral gene protein can play a role in organizing the assembly of the DNA replication structures. Further work in this system may determine whether assembly of these structures is essential for efficient viral DNA replication and if so, why assembly of these structures is necessary. Thus, the study of the localization and assembly of HSV DNA replication proteins provides a system to examine the mechanisms involved in morphogenesis of the cell nucleus. Therefore, several critical principles are apparent from these discussions of the metabolism of HSV transcription and DNA replication. First, there are many ways in which the activity of RNA polymerase II can be regulated, and HSV proteins exploit several of these in controlling the transcription of a single DNA molecule. Second, the interplay of these multiple regulatory pathways is likely to control the progress of the lytic cycle and may play a role in determining the lytic versus latent infection decision.(ABSTRACT TRUNCATED AT 400 WORDS)