Co-ordinate regulation of herpes simplex virus gene expression is mediated by the functional interaction of two immediate early gene products

The herpes simplex virus immediate early protein ICP27 encodes a potential metal binding domain and binds zinc in vitro

The herpes simplex virus type 1 (HSV-1) immediate-early regulatory proteins ICP27 and ICP0 each encode putative zinc-finger metal-binding domains. We utilized the technique of metal chelate affinity chromatography to demonstrate that ICP27 and ICP0 were able to bind to zinc in vitro. This property was further exploited to purify ICP27 from extracts of HSV-1-infected cells. The purification procedure also revealed that ICP27 possessed single-stranded DNA-binding activity. Analysis of ICP27 truncated peptides produced by in vitro translation verified that the zinc-binding region of ICP27 resides in the carboxy terminal 105 amino acids spanning the putative metal binding motif. However, a specific configuration of cysteine and histidine residues in this region was not required for binding to occur as demonstrated by the ability of a frame-shift mutation to bind with an efficiency similar to wild type. The mutated peptide retained four histidine and cysteine residues but in a configuration different from the consensus proposed for zinc-finger motifs. Therefore, while the region spanning the metal binding domain of ICP27 is essential for both the activator and repressor functions, and ICP27 binds zinc in vitro, it is not clear whether zinc binding in vivo is necessary for function.

A herpesvirus regulatory protein appears to act post-transcriptionally by affecting mRNA processing

Post-transcriptional control mechanisms play an important role in regulating gene expression for a number of viruses, especially in the regulation of late gene products. In this study we have investigated the mode of action of ICP27, an immediate-early regulatory protein of herpes simplex virus 1 (HSV-1) required for late gene expression. Transfection experiments have demonstrated that ICP27 can activate or repress expression depending on the target gene. Here, we show that the regulatory activity of ICP27 is independent of the target gene promoter sequences but, instead, depends on the presence of different mRNA processing signals. The activation function correlated with different polyadenylation sites, whereas the repressor function correlated with the presence of introns either 5' or 3' to the target gene-coding sequences. Poly(A)+ RNA levels were increased by ICP27 in transfections with a target gene having only an AATAAA recognition signal but no G/U box within the usual distance. In contrast, in the presence of ICP27, spliced target mRNAs were decreased 5- to 10-fold in transfections with target genes containing a 5' or 3' intron. These results suggest that this essential HSV-1 regulatory protein acts post-transcriptionally to affect mRNA processing and point to possible interactions between splicing and polyadenylation factors.

Association of ICP0 but not ICP27 with purified virions of herpes simplex virus type 1

Recent studies have shown that ICP4, one of the major immediate-early proteins of herpes simplex virus type 1 is present within the tegument region of the virion (F. Yao and R. J. Courtney, J. Virol. 63:3338-3344, 1989). With monoclonal antibodies to two additional immediate-early proteins, ICP0 and ICP27, and Western blot (immunoblot) analysis, ICP0, but not ICP27, was also found to be associated with purified virus particles. In an effort to localize the ICP0 within the virion, purified virions were treated with trypsin in the presence and absence of detergent. The data suggest that ICP0 is located within the tegument region of the virion and is not localized in the envelope or within the nucleocapsid. The number of molecules of ICP0 per virion was estimated to be approximately 150.

Herpes simplex virus type 1 ICP0 regulates expression of immediate-early, early, and late genes in productively infected cells

The herpes simplex virus type 1 protein, ICP0, can activate expression of all kinetic classes of viral promoters in transient expression assays. To examine the role of ICP0 in the regulation of viral gene expression during productive infection, we characterized the wild-type virus, an ICP0 null mutant (7134), and several ICP0 nonsense mutant viruses with regard to virus replication and protein synthesis in Vero cells. Relative to wild-type virus, 7134 was severely deficient in viral growth and protein synthesis at low multiplicities of infection but exhibited a nearly wild-type phenotype at high multiplicities. The phenotypes of the ICP0 nonsense mutants were intermediate between those of the wild-type virus and 7134 in that the more ICP0-coding sequence expressed by a given nonsense mutant, the more wild type-like was its phenotype. The location of the ICP0 domain responsible for transactivation during productive infection was confirmed to be within the N-terminal portion of the protein, as previously shown in transient expression assays. Immunoprecipitation and immunofluorescence tests were used to detect low-level expression of selected immediate-early (IE), early (E), and late (L) proteins by mutant and wild-type viruses following low-multiplicity infection. The 7134 deletion mutant and several nonsense mutants expressed markedly reduced levels of E and L proteins but wild-type levels of the IE protein, ICP4. Because the latency-associated transcripts (LATs) are specified by the strand opposite that which encodes ICP0, the ICP0 deletion and nonsense mutants are by definition ICP0-LAT double mutants. The ability of a LAT- ICP0+ mutant to replicate as efficiently as wild-type virus at low multiplicities and the ability of ICP0-expressing 0-28 cells to complement the defects of the mutants in E and L protein synthesis indicates that the phenotypes of the mutants are caused by mutations in ICP0 and not the LATs. Thus, we conclude that ICP0 up-regulates E and L but not necessarily IE gene expression during productive infection. The activation of IE gene expression by ICP0 during productive infection is likely overshadowed by the activity of the virion-associated protein, VP16. This hypothesis was tested by transfection of Vero cells with infectious mutant and wild-type viral DNAs. In such tests, no VP16 is present at early times posttransfection. Significantly fewer cells transfected with infectious 7134 DNA expressed ICP4 than cells transfected with KOS DNA. This reduction was fully reversed by cotransfection with an ICP0-expressing plasmid.(ABSTRACT TRUNCATED AT 400 WORDS)

Herpes simplex viruses with mutations in the gene encoding ICP0 are defective in gene expression

Herpes simplex virus type 1 (HSV-1) mutants with codon insertions and deletions in IE-0, the gene encoding ICP0, were constructed. The HSV-1 deletion mutant dl1403 (N. D. Stow and E. C. Stow, J. Gen. Virol. 67:2571-2585, 1986) and an IE-0:lacZ transplacement vector isolated in this study were used to facilitate the construction of mutant viruses. Mutant viruses, all of which produced stable ICP0, were examined for their ability to plaque and grow on both Vero and HeLa cells because previous results showed that HSV-1 immediate-early (IE) gene promoters and their products are differentially expressed in these cells (J. Chen, X. Zhu, and S. Silverstein, Virology 180:207-220, 1991; I. H. Gelman and S. Silverstein, J. Virol. 61:2286-2296, 1987). Viruses with IE-0 genes that only poorly activated reporter genes in transient expression assays plaqued less efficiently on Vero cells and consistently accumulated decreased levels of late proteins. These mutants were also examined in single-step growth curve experiments and for the dependence of virus yield on multiplicity of infection (MOI). At low MOIs, their yields were less in Vero cells than in HeLa cells; by contrast, at high MOIs, there was no apparent difference in yield in either cell type, although each virus produced considerably fewer progeny than wild-type virus. Analysis of steady-state levels of RNA from genes representing each of the three major kinetic classes demonstrated that lower levels of RNAs accumulate in these mutants. We conclude from these studies that while ICP0 is not essential for virus growth in tissue culture, defects in this gene result in impairment of virus replication and delay the expression of early and late gene transcripts.

Differential dependence of herpes simplex virus immediate-early gene expression on de novo-infected cell protein synthesis

The time course of accumulation of herpes simplex virus immediate-early (IE) mRNA and the requirement for infected cell protein synthesis for mRNA transcription and accumulation were compared. Measurements of transcription in nuclear run-on assays, accumulation of cytoplasmic mRNA by Northern (RNA) blot hybridization, and rates of infected cell protein synthesis by pulse-labeling did not indicate differences among the five IE gene, consistent with previous studies. However, as a result of varying the amount of de novo protein synthesis after infection, at least three patterns of maximal expression of the IE genes were revealed. Addition of the protein synthesis inhibitor anisomycin to cells coincident with infection resulted in maximal rates of transcription and accumulation of functional ICP0 mRNA, while 0.5 h of infected cell protein synthesis prior to addition of the drug was required for maximal expression of ICP22/47 and ICP27 mRNAs. Maximal expression of ICP4 mRNA occurred only when 1 h of de novo protein synthesis occurred prior to the addition of the drug. These results are discussed in the context of alternative mechanisms for regulating IE gene expression.

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.

A cellular function can enhance gene expression and plating efficiency of a mutant defective in the gene for ICP0, a transactivating protein of herpes simplex virus type 1

ICP0 transactivates herpes simplex virus type 1 genes of all classes as well as a number of heterologous viral and cellular genes, yet it is not essential for virus replication in vitro or in vivo. Stocks of ICP0 deletion mutants, however, exhibit significantly lower plating efficiencies on standard 24-h-old Vero cell monolayers than do stocks of wild-type virus. In an attempt to determine whether the growth status of cells in the monolayer affects the ability of ICP0 mutants to initiate plaque formation, the plating efficiencies and abilities of an ICP0 null mutant (7134) and of wild-type virus (KOS) to express selected viral proteins were determined on Vero cell monolayers whose growth had been arrested either by contact inhibition-trypsinization or by isoleucine deprivation and had then been released from growth arrest. The proportion of cells cycling synchronously after release from growth arrest was assessed by flow cytometry. The results of these studies indicate that the plating efficiency of 7134 was greatest on Vero cell monolayers 8 h after release from growth arrest induced by either treatment. Monolayers of both types released from growth arrest at other times supported 7134 plaque formation less efficiently. In contrast, the plating efficiency of KOS was nearly equal on monolayers at all times after release from growth arrest. Notably, both KOS and 7134 were equally efficient in entering cells and inducing expression of the immediate-early protein ICP4 in either 8- or 24-h monolayers. Relative to KOS, however, 7134 was significantly impaired in the expression of selected early and late genes in cells at 24 h postrelease. When the plating efficiencies of 7134 and KOS were examined in 0-28 cells (Vero cells that are stably transformed with the ICP0 gene) whose growth had been arrested and then released, no differences in the plating efficiencies of the two viruses as a function of growth status were noted. These findings suggest that a cellular function expressed maximally in cells 8 h after release from growth arrest can substitute operationally for ICP0 to enhance plaque formation and viral gene expression by 7134. They further suggest that one role of ICP0 in viral infection is to facilitate virus replication in cells that do not express this function.

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.

Isolation and characterization of a functional cDNA encoding ICP0 from herpes simplex virus type 1

The IE-0 gene of herpes simplex virus type 1 (HSV-1) contains two introns and encodes ICP0, a powerful transcriptional activator. We have isolated a cDNA clone that encodes ICP0 from a lambda gt10 cDNA library constructed from RNAs made from HSV-1-infected HeLa cells. DNA sequence analysis of this clone confirmed the predicted intron/exon boundaries (L. J. Perry, F. J. Rixon, R. D. Everett, M. C. Frame, and D. J. McGeoch, J. Gen. Virol. 67:2365-2380, 1986). Following transfection, a plasmid containing the cDNA copy of IE-0 directed the synthesis of ICP0, which was appropriately compartmentalized and distributed in the nucleus, as revealed by immunofluorescence. A transient expression assay was used to demonstrate that this cDNA copy retained the ability to transactivate the HSV-1 promoters for the IE-0 gene (an immediate-early gene), the thymidine kinase gene (an early gene), and the glycoprotein C gene (a late gene). The product of this cDNA clone cooperated with ICP4 to activate expression from the thymidine kinase gene promoter in a synergistic manner. The availability of a functional cDNA copy encoding ICP0 provides the opportunity to express this protein in vector systems that do not recognize eucaryotic donor and acceptor splicing signals to overexpress ICP0.

Mutational analysis of the sequence encoding ICP0 from herpes simplex virus type 1

In-frame codon insertion and deletion mutants were constructed in a plasmid containing the sequence that encodes ICP0, a transcriptional activator of herpes simplex virus type 1 (HSV-1). The effect of these mutations was analyzed in a transient expression assay using the promoters for, the IE-0 gene (an immediate early (alpha) gene), the thymidine kinase gene (an early (beta) gene), and the glycoprotein C gene (a late (gamma) gene) fused to reporter cassettes that encoded either beta-galactosidase or chloramphenicol acetyl transferase. Assays were performed in the presence or absence of a plasmid encoding ICP4, the major regulatory protein of HSV-1. Our results demonstrate that ICP0-mediated transactivation varied depending on the position of the insertion in the gene. One region of this protein was consistently shown to be required for full activation of each promoter examined either in the presence or in the absence of ICP4. This region overlaps with a cysteine-rich region and coincides with a transactivator domain identified in another extensive mutational analysis of this sequence. Analysis of the deletion mutants generated in this study demonstrated that the carboxy-terminal regions were required for activation in certain circumstances and that this varied depending on the promoter being assayed and the cell type in which the analysis was performed.

Reactivation of latent herpes simplex virus by adenovirus recombinants encoding mutant IE-0 gene products

We have previously shown that adenovirus recombinants expressing functional ICP0 reactivate latent herpes simplex virus type 2 (HSV-2) in an in vitro latency system. This study demonstrated that ICP0, independent of other HSV gene products, is sufficient to reactivate latent HSV-2 in this in vitro system. To assess the effects of defined mutations in the sequence encoding ICP0 (IE-0) on reactivation, seven in-frame insertion and three in-frame deletion mutants were moved into an adenovirus expression vector. Each recombinant directed the synthesis of stable ICP0 of the correct size. The transactivation activity of the mutated sequences in these recombinants was similar to that when they were tested in plasmids. When these recombinants were examined for their ability to reactivate in the in vitro latency system, mutants with dramatic defects in transactivation (Ad-0/125, Ad-0/89, Ad-0/2/7, and Ad-0/88/93) were unable to reactivate latent HSV-2 independent of the multiplicity of infection. An exception to this correlation was the finding that Ad-0/89, which transactivated poorly, was able to reactivate latent virus after prolonged incubation whereas other transactivation-deficient mutants could not. Moreover, the presence of ICP4 did not compensate for the inability of any of the recombinants tested to reactivate HSV-2. These results show that (i) the transactivation domains of ICP0 are also used in reactivation, (ii) the presence of another essential HSV regulatory protein ICP4 does not alter the pattern of reactivation by ICP0, and (iii) mutations in some regions of IE-0 previously shown to affect viral growth and plaque formation did not alter its ability to reactivate in this in vitro system.

Functional substitution of the basic domain of the HIV-1 trans-activator, Tat, with the basic domain of the functionally heterologous Rev

The tat gene of HIV is a strong activator of the viral LTR. The Tat protein contains a highly basic domain that is important for its transport to the nuclear/nucleolar locations. The Tat basic domain when fused to Escherichia coli beta-galactosidase directed the chimeric protein to the nucleus and nucleolus. Tat mutants lacking the entire basic domain were severely defective in trans-activation. Substitution of the basic domain of Tat with that of the functionally unrelated HIV-1 Rev protein targeted the chimeric protein to the nucleolus and restored the function of Tat. In contrast, substitution with the nuclear targeting signal (NLS) of SV40 T antigen targeted the chimeric protein to the nucleus and accumulation in the nucleolar region was excluded. The Tat-NLS chimeric protein did not restore the trans-activation function of Tat efficiently. These results indicate that the arginine-rich basic domain of the trans-activator, Tat, and post-transcriptional trans-regulator, Rev, are functionally similar with regard to trans-activation of HIV-1 LTR.

Guinea pig cytomegalovirus immediate-early transcription

Guinea pig cytomegalovirus (GPCMV) immediate-early (IE) gene expression was analyzed. GPCMV IE RNA was defined as RNA obtained from GPCMV-infected guinea pig cells treated with cycloheximide for 1 h before infection and for 4 h postinfection. Mapping studies showed that GPCMV IE genes are located at several distinct sites on the GPCMV genome. A total of 17 GPCMV IE transcripts were identified, and 9 IE transcripts coded for by three specific regions of the genome (regions I, II, and III) were characterized in detail. A series of recombinant DNA clones were generated to identify the nine IE transcripts. Three of the IE transcripts from region I and three from region III were transcribed in the same direction from overlapping sequences. The 2.0-kilobase (kb) transcript encoded by the EcoRI E DNA fragment (region II) was the most abundant IE GPCMV transcript. The cloned GPCMV DNA subfragment that was used to identify the region II EcoRI E 2.0-kb transcript did not hybridize to GPCMV early or late RNA, indicating that this transcript is expressed only under IE conditions. Expression of RNAs from the IE genes was also measured during a natural GPCMV infection in the absence of cycloheximide. During the natural infection, the transcripts previously identified under IE cycloheximide block conditions were expressed, and the region II EcoRI E 2.0-kb transcript was the most abundant transcript at 1 h postinfection. In addition, a rise and fall in RNA levels was observed during the natural infection, demonstrating the transient nature of expression of these transcripts. We conclude that GPCMV IE gene expression is complex, involving a reasonably large number of genes, and demonstrates some similarities with IE transcription by other CMVs.

Variable requirements for herpes simplex virus immediate-early proteins in the expression of the adenovirus E2 gene

Regulation of the adenovirus E2 gene by trans-activating proteins encoded by herpes simplex virus was investigated. Coinfection of Vero cells was performed with Ad5 dl312 (an E1 deletion mutant) and either wildtype HSV, mutant virus encoding a temperature-sensitive ICP4 protein (tsK), or mutants carrying deletions in the ICP4 (d120) or ICP0 (dl x 3.1) gene. As detected by the presence of E2 mRNA, or the product of the E2 gene, 72-kDa DNA binding protein (DBP), functional ICP4 was sufficient for expression of the E2 gene. Regulation of E2 gene expression was at the level of transcription activation as judged by nuclear run-on assay. In contrast to results when Vero cells were coinfected, expression of 72-kDa DBP in CN3 cells, carrying an integrated copy of the E2 gene, required expression of both HSV immediate-early proteins. These results suggest that the DNA-protein organization of the target gene sequence may play a significant role in the ability of viral regulatory proteins to activate expression of heterologous as well as homologous genes.

The herpes simplex virus 1 gene for ICP34.5, which maps in inverted repeats, is conserved in several limited-passage isolates but not in strain 17syn+

In a previous study, it was reported that herpes simplex virus 1 (HSV-1) strain F contains a transcribed open reading frame situated in the inverted repeats of the L component between the terminal a sequence and the open reading frame that encodes the alpha 0 gene (J. Chou and B. Roizman, J. Virol. 57: 629-637, 1986). By means of an antibody to repeats of the trimer Ala-Thr-Pro predicted to be specified by the open reading frame, it was shown that the open reading frame specifies a protein (M. Ackermann, J. Chou, M. Sarmiento, R. A. Lerner, and B. Roizman, J. Virol. 58: 843-850, 1986). This open reading frame is absent from the reported sequence of HSV-1(17)syn+ (D. J. McGeoch, M. A. Dalrymple, A. J. Davison, A. Dolan, M. C. Frame, D. McNab, L. J. Perry, J. E. Scott, and P. Taylor, J. Gen. Virol. 69: 1531-1574, 1988; L. J. Perry and D. J. McGeoch, J. Gen. Virol. 69: 2831-2846, 1988). To define the extent of variability in this open reading frame, we compared the sequences of the ICP34.5-encoding open reading frames of the genomes of three strains characterized by limited passage in cell culture with that of the HSV-1(17)syn+ strain. Furthermore, to establish unambiguously that the antibody to the Ala-Thr-Pro repeats reacts with the product of this open reading frame, we inserted a short sequence that encodes a known epitope in frame at the 5' terminus of the coding domain. Our results indicate that with minor variations, the open reading frame is conserved in the three HSV-1 genomes analyzed but not in HSV-1(17)syn+. Thus, two strains contain an inserted amino acid and one strain, isolated from a case of human encephalitis, lacks a seven-amino-acid sequence. The recombinant virus carrying the foreign epitope expressed a slightly slower-migrating protein which reacted with both the rabbit polyclonal antibody to the Ala-Thr-Pro trimer repeats and the monoclonal antibody to the inserted epitope. The implications of the results are discussed.

Trans-activation of viral tk promoters by proteins encoded by varicella zoster virus open reading frames 61 and 62

Plasmids containing the varicella zoster virus (VZV) open reading frames (ORFs) 61 and 62 were used in a transient co-transfection assay to test for trans-activation of the VZV and herpes simplex virus type 1 (HSV-1) thymidine kinase (tk) promoters. The trans-activating potential of the polypeptides encoded by these VZV ORFs, designated p51 and p140, was compared to that of their HSV-1 homologs ICP0 and ICP4, respectively. VZV p51 was functionally inactive in this system while p140 appeared to be a much stronger transcriptional activator than ICP4. Co-transfection of plasmids encoding VZV p140 and HSV-1 ICP0 resulted in a synergistic activation of the reporter gene as has been shown for the combination of ICP4 and ICP0.

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.

Differential regulation by varicella-zoster virus (VZV) and herpes simplex virus type-1 trans-activating genes

Transient expression assays were performed in Vero cells in order to compare varicella-zoster virus (VZV)-encoded trans-activating proteins [defined by the products of open reading frames (ORF) 4 and 62] with herpes simplex virus type-1 (HSV-1) trans-activating proteins, ICP4 and ICP0, with respect to activation of gene expression. We demonstrate that the product of VZV ORF4 and ORF62 (which are the HSV-1 analogs of ICP27 and ICP4, respectively) stimulate a variety of viral and cellular gene promoters, including the HSV-1 thymidine kinase (tk) promoter. On the other hand, expression of a recombinant vector containing the VZV tk promoter could not be stimulated, by HSV-1 infection or by the HSV-1 ICP4 or ICP0 proteins expressed during cotransfection experiments. These data suggest different mechanisms of activation of the VZV and the HSV-1 tk gene promoters by "trans-activating" factors.

The regions important for the activator and repressor functions of herpes simplex virus type 1 alpha protein ICP27 map to the C-terminal half of the molecule

The herpes simplex virus type 1 (HSV-1) alpha or immediate-early proteins ICP4 (IE175), ICP0 (IE110), and ICP27 (IE63) are trans-acting proteins which affect HSV-1 gene expression. We previously showed that ICP27 in combination with ICP4 and ICP0 could act as a repressor or an activator in transfection assays, depending on the target gene (R. E. Sekulovich, K. Leary, and R. M. Sandri-Goldin, J. Virol. 62:4510-4522, 1988). To investigate the regions of the ICP27 protein which specify these functions, we constructed a series of in-frame insertion and deletion mutants in the ICP27 gene. These mutants were analyzed in transient expression assays for the ability to repress or to activate two different target genes. The target plasmids used consisted of the promoter regions from the HSV-1 beta or early gene which encodes thymidine kinase and from the beta-gamma or leaky late gene. VP5, which encodes the major capsid protein, each fused to the chloramphenicol acetyltransferase gene. Our previous studies showed that induction of pTK-CAT expression by ICP4 and ICP0 was repressed by ICP27, whereas the stimulation of pVP5-CAT expression seen with ICP4 and ICP0 was significantly increased when ICP27 was also added. In this study, a series of transfection assays was performed with each of the ICP27 mutant plasmids in combination with plasmids containing the ICP4 and ICP0 genes with each target. The results of these experiments showed that mutants containing insertions or deletions in the region from amino acids 262 to 406 in the carboxy-terminal half of the protein were unable to stimulate expression of pVP5-CAT but were able to repress induction of pTK-CAT activity by ICP4 and ICP0. Mutants in the carboxy-terminal 78 amino acids lost both activities; that is, these mutants did not show repression of pTK-CAT activity or stimulation of pVP5-CAT activity, whereas mutants in the hydrophilic amino-terminal half of ICP27 were able to perform both functions. These results show that the carboxy-terminal half of ICP27 is important for the activation and repression functions. Furthermore, the carboxy-terminal 62 amino acids are required for the repressor activity, because mutants with this region intact were able to repress. Analysis of the DNA sequence showed that there are a number of cysteine and histidine residues encoded by this region which have some similarity to zinc finger metal-binding regions found in other eucaryotic regulatory proteins. These results suggest that the structural integrity of this region is important for the function of ICP27.

Herpes simplex virus type 1 ICP0 plays a critical role in the de novo synthesis of infectious virus following transfection of viral DNA

As a first step in identifying the functions and intramolecular functional domains of herpes simplex virus type 1 infected cell protein 0 (ICP0) in productive infection and latency, a series of mutant plasmids specifying varying amounts of the ICP0 primary amino acid sequence were constructed. In transient expression assays with mutant and wild-type plasmids, the N-terminal half of the ICP0 molecule was found to be sufficient to transactivate a variety of viral promoters. Although promoters representing the immediate-early, early, and late kinetic classes were transactivated by wild-type ICP0, individual promoters responded to mutant forms of ICP0 in a manner consistent with the possibility that ICP0 transactivates different promoters by different mechanisms. Unlike infection with virus particles, which contain the 65-kilodalton transcriptional transactiovator, the initiation of viral replication after transfection of cells with purified viral DNA requires de novo protein synthesis. In order to assess the role of ICP0 in the de novo synthesis of infectious virus, Vero cells were transfected with purified DNA of wild-type virus or an ICP0 null mutant and the production of infectious virus was monitored. In cells transfected with mutant DNA, virus production was delayed by 2 days and the level of virus was reduced by several orders of magnitude relative to Vero cells transfected with wild-type viral DNA, suggesting an important role for ICP0 in the de novo synthesis of infectious particles. In cotransfection experiments with infectious DNA of the ICP0 null mutant and a plasmid specifying wild-type ICP0 titers of infectious virus were significantly enhanced relative to transfection with mutant DNA alone, confirming the role of ICP0 in de novo synthesis. These findings are consistent with the proposed role of ICP0 in reactivation of herpes simplex virus from latency (D. A. Leib, D. M. Coen, C. L. Bogard, K. A. Hicks, D. R. Yager, D. M. Knipe, K. L. Tyler, and P. A. Schaffer, J. Virol. 63:759-768, 1989), a process also thought to require de novo protein synthesis. The complementing activities of ICP0 mutant plasmids for ICP0 null mutant DNA in cotransfection assays correlated well with their transactivating activities for viral promoters in transient assays, indicating that the transactivating function of ICP0 is a critical factor in the de novo synthesis of infectious particles.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A 165 kd protein of the herpes simplex virion shares a common epitope with the regulatory protein, ICP4

We have investigated the possibility that immediate-early (IE) protein ICP4 could be a part of herpes simplex virus type 1 (HSV-1) virion particle. Immunodetection with a monoclonal antibody against ICP4 reveals that a component of the virion, migrating at 165 kd, shares a common epitope with this immediate-early protein. Immunolocalization studies on purified virions indicate that the antigen can be detected only in virions without membranes, and is located outside the capsid, most probably in the tegument. Ultrastructural localizations on HSV-1 infected BHK cells extracted with a nonionic detergent confirm that the protein immunoreacting with anti-ICP4 is present in virions.

Identification of immediate-early-type cis-response elements in the promoter for the ribonucleotide reductase large subunit from herpes simplex virus type 2

Regulation of the expression of the herpes simplex virus (HSV) type 2 large subunit of ribonucleotide reductase (ICP10) gene was studied directly by immunofluorescence or by chloramphenicol acetyltransferase analysis with hybrid ICP10 promoter constructions. In Vero cells, cotransfection with DNA encoding HSV IE110 or Vmw65 proteins or HCMV IE2 enhanced expression at least 10-fold. In contrast, expression was minimally enhanced by DNA encoding IE175 at low doses and slightly reduced at high doses. IE110-mediated trans-activation was minimal in primary astrocytes and cells from line 293. However, Vmw65 enhanced expression 20-fold in all cell types. cis-Response elements in the ICP10 promoter include a TAATGARAT-like element and other sequences associated with regulation of IE gene expression and potential SP-1, consensus AP-1, and octamer transcription factor 1 binding elements. Factors that bind to the ICP10 promoter were identified in mock and HSV-infected cell extracts. DNA-protein complex formation, presumably involving Vmw65, was demonstrated by gel retardation analysis with mixtures of uninfected cell nuclear extracts and virion lysates. The octamer transcription factor 1 motif (ATGCAAAT) was necessary for optimal Vmw65 binding to the ICP10 promoter as evidenced by competition experiments with oligonucleotides overlapping the consensus IE110 promoter virion response element. The data suggest that ICP10 can be regulated as an immediate-early gene.

DNA binding by the herpes simplex virus type 1 ICP4 protein is necessary for efficient down regulation of the ICP0 promoter

The herpes simplex virus type 1 ICP4 and ICP0 polypeptides are immediate-early proteins that positively and negatively regulate expression of other viral genes in trans. ICP4 has recently been shown to bind DNA bearing the consensus sequence 5'-ATCGTCNNNN(T/C)CG(A/G)C-3', present upstream of a number of viral genes. To test the hypothesis that this DNA-binding activity is involved in ICP4-mediated gene regulation, site-specific mutagenesis was employed to mutate the version of this sequence in the promoter of the ICP0 gene. The mutation eliminated detectable binding of ICP4 to the promoter as measured in vitro by a gel electrophoresis band shift assay. The ability of the mutated ICP0 promoter to direct synthesis of a reporter gene was also investigated in a transient transfection assay. Whereas ICP4 was found to transactivate the wild-type ICP0 promoter two- to threefold, the mutated promoter was transactivated seven- to ninefold. In assays containing the ICP0 transactivator gene, ICP4 down regulated the wild-type promoter far more efficiently than the mutated promoter. Finally, both the wild-type and mutated ICP0 promoters exhibited a similar response to ICP4 in transfections that included a vector expressing the viral transactivator protein VP16. These experiments suggest that the sequence-specific DNA-binding activity of ICP4 is an essential element of its role as a negative regulator of gene expression.

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.

Stimulation of estrogen receptor mRNA levels in MCF-7 cells by herpes simplex virus infection

Infection of estrogen-responsive cells (MCF-7) with herpes simplex virus type 1 or 2 stimulates expression of the estrogen receptor message. Experiments on infection with the mutant virus, tsK, together with transfection studies implicate the virion protein, Vmw65, in the response. Cellular protein synthesis is essential for estrogen receptor mRNA expression.

The herpes simplex virus type 1 alpha protein ICP27 can act as a trans-repressor or a trans-activator in combination with ICP4 and ICP0

The herpes simplex virus type 1 (HSV-1) alpha proteins ICP4, ICP0, and ICP27 are trans-acting proteins which affect HSV-1 gene expression. To investigate potential interactions between these alpha products and to determine the specificity of action of the alpha proteins in combination with each other compared with their activities individually, we performed a series of transient-expression assays. In these assays we used plasmids containing the alpha genes encoding ICP4, ICP0, and ICP27 either singly or in combination as effectors and HSV-1 genes of different kinetic classes and heterologous genes as targets. The HSV-1 targets consisted of promoter-regulatory domains from alpha (ICP0 and ICP27), beta (thymidine kinase and alkaline exonuclease), beta-gamma (glycoprotein D, glycoprotein B, and VP5), and gamma (glycoprotein C) genes, each fused to the chloramphenicol acetyltransferase (CAT) gene. The heterologous target genes consisted of the simian virus 40 early promoter with enhancer and the Rous sarcoma virus long terminal repeat promoter and enhancer each fused to the CAT gene. Target promoter activity was measured by the assay of CAT activity in extracts of transfected cells and by Northern (RNA) blot hybridization of CAT mRNA. The results of these experiments showed that ICP4 activated only HSV-1 target genes, whereas ICP0 activated all of the targets and ICP27 had little effect on any of the targets. ICP4 and ICP0 had a synergistic effect when inducing HSV-1 targets, but they did not have this effect on the heterologous targets pSV2-CAT or pRSV-CAT. In fact, lower levels of CAT activity and CAT mRNA were found in the presence of both effectors than with ICP0 alone. Most interestingly, although the effector plasmid containing the ICP27 gene had little effect on its own, two different and marked effects depending on the target were observed when ICP27 was combined with ICP4 or ICP0 or both. A trans-repression of the induction seen with ICP4 and ICP0 was found when ICP27 was present in the transfections with pSV2-CAT, pRSV-CAT, pICP0-CAT, pICP27-CAT, pTK-CAT, pgD-CAT, pgB-CAT, and pgC-CAT. This resulted in CAT activity levels which were similar to or lower than the basal level of expression of the target genes in the absence of effector plasmids. This trans-repression occurred over a wide range of concentrations of input ICP27 plasmid. In contrast to this repressive effect of ICP27, a trans-activation was seen when ICP4, ICP0, and ICP27 plasmids were combined in transfections with pAE-CAT and pVP5-CAT as targets. This trans-activation also occurred over a 10-fold range of input ICP27 plasmid. These results suggest that ICP27 can facilitate both down

Role for DNA-protein interaction in activation of the herpes simplex virus glycoprotein D gene

On the basis of experiments with mutant virus and transfection with isolated genes, the herpes simplex virus immediate-early gene product ICP4 is known to positively regulate the transcription of viral early and late genes and negatively regulate expression from its own promoter. Binding of ICP4 to DNA sequences in several viral genes has been reported, yet the significance of ICP4-DNA interaction in transcriptional activation remains unclear. We have studied this problem by using the early glycoprotein D (gD) gene, which possesses a binding site at approximately -100 relative to the RNA initiation site. We linked this promoter and various mutant constructs to the chloramphenicol acetyltransferase gene in order to measure promoter activity in transient transfections both in the presence and in the absence of an ICP4-encoding plasmid. The natural promoter was activated 3.3-fold, and a deletion construct lacking the binding site was activated minimally (1.7-fold). Constructs containing multiple tandem repeats of the binding site (three or five inserts) demonstrated higher expression in the presence of ICP4 than did the natural promoter while retaining low levels of expression when unstimulated. Gel mobility shift assays and DNase I footprinting analyses indicated that ICP4 associated with multiple binding sites. In vitro transcription from a gD promoter construct containing multiple binding sites showed increased RNA synthesis in the presence of partially purified ICP4. These data provide the first direct evidence that binding of ICP4 to a specific DNA sequence in the gD gene contributes to activation of transcription.

Adenovirus vector expressing functional herpes simplex virus ICP0

ICP0, one of the five immediate-early (IE) gene products of herpes simplex virus (HSV), is a potent activator of transcription. To assess the biological activities of ICP0 and to explore its mechanisms of action, two helper-independent recombinant adenoviruses were constructed. In each recombinant, the E1 region was substituted with the ICP0-encoding genomic segment under the control of either the adenovirus major late promoter (MLP-0) or the HSV IE-0 promoter (0PRO-0). Infection of HeLa cells or 293 cells (a human embryo kidney cell line expressing adenovirus 5 E1a and -b functions) with the MLP-0 recombinant results in the synthesis of more IE-0 mRNA and ICP0 protein than did infection with the 0PRO-0 recombinant. Although 293 cells infected with MLP-0 accumulate 5- to 10-fold more IE-0 mRNA late in the infection than cells infected with HSV, the level of the protein product, ICP0, increased only slightly. In 293 cells, both recombinants could replicate, albeit at a slower rate and with lower final yields than wild-type adenovirus. Neither virus replicates its DNA in HeLa cells, and thus ICP0 cannot substitute for adenovirus E1a; however, the level of ICP0 that accumulates in MLP-0-infected HeLa cells was comparable to that of HSV-infected HeLa cells. In a functional test, we demonstrated that the adeno-ICP0 recombinant viruses can transactivate a transfected TK-CAT cassette, indicating that the ICP0 is biologically active.

Direct correlation between a negative autoregulatory response element at the cap site of the herpes simplex virus type 1 IE175 (alpha 4) promoter and a specific binding site for the IE175 (ICP4) protein

In transient-expression assays, the IE175 (alpha 4) promoter region of herpes simple virus is down-regulated after cotransfection with DNA encoding its own protein product (IE175 or ICP4). The inhibition by IE175 proved to be highly specific for its own promoter region and did not act on either the herpes simplex virus type 1 IE110 (alpha 0) or human cytomegalovirus major immediate-early promoters. Furthermore, the inhibition was still exhibited by IE175 effector plasmids driven by strong heterologous promoters and therefore must be a direct autoregulatory response that cannot be explained by promoter competition effects. In gel mobility retardation assays with infected-cell nuclear extracts, a prominent and specific DNA-protein complex was formed with DNA fragments containing sequences from -108 to +30 in the IE175 promoter region. This activity was not present in mock-infected samples. Even stronger binding occurred with a fragment containing sequences from -128 to +120 in the IE110 promoter, but this second locus was not associated with any detectable response phenotype in cotransfection assays. Supershift experiments with an anti-IE175 monoclonal antibody confirmed the presence of the IE175 protein in both DNA-protein complexes. In the IE175 promoter, specific binding correlated closely with the presence of an intact autoregulatory signal near the cap site as judged by the loss of both activities in a 3'-deleted promoter fragment lacking sequences from -7 to +30. Insertion of a cloned 30-mer synthetic oligonucleotide sequence from positions -8 to +18 in IE175 restored both IE175 binding activity and the down-regulation phenotype. Direct shift-up assays with a similar 30-base-pair (bp) oligonucleotide containing 21 bp from positions -75 to -55 of IE110 (which encompasses a consensus ATCGTC motif) also produced a specific DNA-protein complex containing the IE175 protein. This ATCGTC motif proved to be a necessary component of both the IE110 and IE175 binding sites, but was insufficient on its own for complex formation. Finally, deletion of 2 bp from positions -3 and -4 within the ATCGTC sequence in the IE175 cap site region abolished both binding activity and the IE175-dependent autoregulation phenotype.

Promoter sequence and cell type can dramatically affect the efficiency of transcriptional activation induced by herpes simplex virus type 1 and its immediate-early gene products Vmw175 and Vmw110

The activation of transcription of the early and late classes of viral genes during infection by herpes simplex virus type 1 (HSV-1) requires the prior expression of immediate-early (IE) gene products. The IE gene products can also activate certain cellular and heterologous viral promoters. This paper presents a thorough analysis of transactivation of the HSV-1 glycoprotein gD and simian virus 40 early promoters, and two other promoters that are hybrids of both, under a variety of experimental conditions. Two methods of transactivation (superinfection with virus and co-transfection with isolated IE genes) have been used with all four target promoters in a variety of cell types. The conclusions are: (1) promoter sequence affects the efficiency of promoter activation by infectious HSV-1 virus, but this activation is not restricted to HSV promoters; (2) cell type affects the efficiency of promoter activation by HSV-1, and this can lead to a failure to activate a promoter in one cell type but not in others in which activation is generally more efficient; (3) a promoter can be activated to different extents in co-transfection experiments using plasmids carrying isolated IE genes that express Vmw110 or Vmw175 or when both are used together; (4) the pattern of activation of a promoter by the IE gene products in cotransfection experiments varies in different cell types; (5) changes in promoter sequence can alter the pattern of activation by the different IE polypeptides, and this pattern can again differ in different cell types; (6) other apparently minor experimental variables, as might exist between the standard methods used in different laboratories, can also affect the patterns of activation observed. The results are discussed in terms of the mechanism of action of the HSV-1 IE gene products and the limitations of the co-transfection assay.

trans-activation and autoregulation of gene expression by the immediate-early region 2 gene products of human cytomegalovirus

The major immediate-early (IE) gene region mapping at coordinates 0.71 to 0.74 in the genome of human cytomegalovirus (HCMV) gives rise to a series of overlapping spliced IE mRNAs that are all under the transcriptional control of the complex IE68 promoter-enhancer region. We show here that one of the phosphorylated nuclear proteins encoded by this region behaves as a powerful but nonspecific trans-activator of gene expression. In transient chloramphenicol acetyltransferase (CAT) assay experiments with Vero cells all relatively weak heterologous target promoters tested, including those of herpes simplex virus IE175 and delayed-early genes, adenovirus E3, the enhancerless simian virus 40 early gene, and the human beta interferon gene, were stimulated between 30- and 800-fold by cotransfection with the HindIII C fragment of HCMV (Towne) DNA. In contrast, expression of the homologous HCMV IE68-CAT gene but not SV2-CAT was specifically repressed. Inactivation mapping studies of the effector DNA, together with dose-response comparisons with subclones from the region, revealed that an intact 7.1-kilobase sequence encompassing both the IE1 and IE2 coding regions (exons 1 to 5) in the major IE transcription complex was required for both the nonspecific trans-activation and autoregulatory responses. The IE1 coding region alone (exons 1 to 4) was inactive, but both functions were restored by insertion of the IE2 coding region (exon 5) in the correct orientation downstream from the IE1 coding region. Internal deletions or inserted terminator codons in IE1 (exon 4) still gave efficient trans-activation and autoregulation, whereas the insertion of terminator codons in IE2 (exon 5) abolished both activities. Finally, IE2 (exon 5) sequences only (under the direct transcriptional control of the strong simian CMV IE94 promoter) were still able to specifically down regulate IE68-CAT expression but failed to exhibit trans-activation properties. Therefore, the IE2 gene product(s) of HCMV appear likely to be key control proteins involved in gene regulation during HCMV infection.

Association of herpes simplex virus regulatory protein ICP4 with sequences spanning the ICP4 gene transcription initiation site

The HSV gene encoding ICP4 is negatively regulated and the HSV gene encoding thymidine kinase is positively regulated by ICP4 in vivo. We report that ICP4 is a component of a stable complex that contains protein and a sequence of approximately 28 nucleotides that span the ICP4 gene transcription initiation site. The association of ICP4 with DNA sequences between positions -103 and +32 relative to the ICP4 mRNA start site was demonstrated by DNA binding immunoassays. DNase footprinting revealed that nucleotides between positions -8 and +20 are protected by ICP4. In contrast, binding of ICP4 to sequences flanking the mRNA start site in the thymidine kinase gene was not observed. Models for ICP4-mediated positive or negative regulation of HSV gene transcription are discussed.

Dissection of immediate-early gene promoters from herpes simplex virus: sequences that respond to the virus transcriptional activators

The immediate-early promoters of herpes simplex virus give rise to the first series of transcripts after infection. These promoters are composed of compound sequence elements that govern basal level and regulated transcription. The response of three core (truncated) promoters from the herpes simplex virus type 1 IE-4, IE-0, and IE-27 genes to a battery of virus-encoded trans-acting proteins was examined in a short-term transient expression assay system. The results of this study reveal (i) a role for a sequence, 5'---GGGGG---3', flanked by 3 to 5 base pairs of symmetry (the G box), which is present in the upstream region of all immediate-early gene promoters, (ii) a requirement for the consensus sequence protected by ICP4 for autoregulation by this immediate-early gene product, and (iii) an alternative, sequence-independent mechanism for the augmentation of alpha gene expression by the virion-associated transcriptional activator Vmw65, now designated as TIF.

Trans-activation of human immunodeficiency virus gene expression is mediated by nuclear events

Human immunodeficiency virus encodes a gene product termed tat that is able to activate viral gene expression when present in trans. The mechanism of action of the tat gene product appears to be bimodal, resulting in both an increase in the steady-state level of viral mRNA and the enhanced translation of that RNA. In this report we have examined the mechanism by which tat elevates viral mRNA levels. Data are presented demonstrating that tat acts by increasing the rate of viral transcription, rather than by modulating the stability of viral mRNA. Indirect immunofluorescence was used to show that tat is predominantly localized in the nucleus of expressing cells, a location consistent with a role in the regulation of viral transcription. These results suggest that tat could play a role in human immunodeficiency virus replication essentially similar to that proposed for the trans-acting nuclear gene products described for several other virus species.

Herpes simplex virus immediate-early promoters are responsive to virus and cell trans-acting factors

The promoters for each of the immediate-early genes from herpes simplex virus type 1 were cloned and fused to a chloramphenicol acetyltransferase cassette. These chimeric genes were used as targets in a transient expression assay to determine how the immediate-early gene products ICP4 and ICP0 and the virion-associated stimulatory protein Vmw65 affected their expression in HeLa and Vero cells. The basal level of expression from these cassettes differed significantly depending on the extent of 5'-flanking sequence and the cell line that served as host. The promoters from IE-4 and IE-0 behaved in a qualitatively similar fashion independent of the host cell. However, the promoter for ICP27 had a unique response pattern: in Vero cells it acted as an alpha gene promoter, whereas in HeLa cells its response was more like that of a beta gene promoter. The promoter sequences for ICP22 and ICP47 behaved as the IE-4 and IE-0 promoters did in HeLa cells, but their response to the effector molecules in Vero cells was unlike that of other alpha gene promoters we have studied. Evidence is also presented for a role for ICP27 in autoregulation.

Activities of herpes simplex virus type 1 (HSV-1) ICP4 genes specifying nonsense peptides

Synthetic oligonucleotide linkers containing translational termination codons in all possible reading frames were inserted at various positions in the cloned gene encoding the herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein, ICP4. It was determined that the amino-terminal 60 percent of the ICP4 gene was sufficient for trans-induction of a thymidine kinase promoter-CAT chimera (pTKCAT) and negative regulation of an ICP4 promoter-CAT chimera (pIE3CAT); however, it was relatively inefficient in complementing an ICP4 deletion mutant. The amino-terminal ninety amino acids do not appear to be required for infectivity as reflected by the replication competence of a mutant virus containing a linker insertion at amino acid 12. The size of the ICP4 molecule expressed from the mutant virus was consistent with translational restart at the next methionine codon corresponding to amino acid 90 of the deduced ICP4 amino acid sequence.

RNA from an immediate early region of the type 1 herpes simplex virus genome is present in the trigeminal ganglia of latently infected mice

Transcription of the type 1 herpes simplex virus (HSV-1) genome in trigeminal ganglia of latently infected mice was studied using in situ hybridization. Probes representative of each temporal gene class were used to determine the regions of the genome that encode the transcripts present in latently infected cells. Probes encoding HSV-1 sequences of the five immediate early genes and representative early (thymidine kinase), early-late (major capsid protein), and late (glycoprotein C) genes were used in these experiments. Of the probes tested, only those encoding the immediate early gene product infected-cell polypeptide (ICP) 0 hybridized to RNA in latently infected tissues. Probes containing the other immediate early genes (ICP4, ICP22, ICP27, and ICP47) and the representative early, early-late, and late genes did not hybridize. Two probes covering approximately equal to 30% of the HSV-1 genome and encoding over 20 early and late transcripts also did not hybridize to RNA in latently infected tissues. These results, with probes spanning greater than 60% of the HSV-1 genome, suggest that transcription of the HSV-1 genome is restricted to one region in latently infected mouse trigeminal ganglia.