Does the human immunodeficiency virus Tat trans-activator contain a discrete activation domain?

Insights into the mechanisms underlying the inactivation of HIV-1 proviruses by CRISPR/Cas

DNA editing using CRISPR/Cas has emerged as a potential treatment for diseases caused by pathogenic human DNA viruses. One potential target is HIV-1, which replicates via a chromosomally integrated DNA provirus. While CRISPR/Cas can protect T cells from de novo HIV-1 infection, HIV-1 frequently becomes resistant due to mutations in the chosen single guide RNA (sgRNA) target site. To address this problem, we asked whether an sgRNA targeted to a conserved, functionally critical HIV-1 sequence might prevent the selection of escape mutants. We report that two sgRNAs specific for the HIV-1 transactivation response (TAR) element produce opposite results: the TAR2 sgRNA rapidly selects for mutants that retain TAR function, but are no longer inhibited by Cas9, while the TAR1 sgRNA fails to select any replication competent TAR mutants, most probably because it is targeted to a region of TAR that is disrupted by even minor mutations.

Systematic Analysis of the Functions of Lysine Acetylation in the Regulation of Tat Activity

The Tat protein of HIV-1 has several well-known properties, such as nucleocytoplasmic trafficking, transactivation of transcription, interaction with tubulin, regulation of mitotic progression, and induction of apoptosis. Previous studies have identified a couple of lysine residues in Tat that are essential for its functions. In order to analyze the functions of all the lysine residues in Tat, we mutated them individually to alanine, glutamine, and arginine. Through systematic analysis of the lysine mutants, we discovered several previously unidentified characteristics of Tat. We found that lysine acetylation could modulate the subcellular localization of Tat, in addition to the regulation of its transactivation activity. Our data also revealed that lysine mutations had distinct effects on microtubule assembly and Tat binding to bromodomain proteins. By correlation analysis, we further found that the effects of Tat on apoptosis and mitotic progression were not entirely attributed to its effect on microtubule assembly. Our findings suggest that Tat may regulate diverse cellular activities through binding to different proteins and that the acetylation of distinct lysine residues in Tat may modulate its interaction with various partners.

Analysis of the interaction of primate retroviruses with the human RNA interference machinery

The question of whether retroviruses, including human immunodeficiency virus type 1 (HIV-1), interact with the cellular RNA interference machinery has been controversial. Here, we present data showing that neither HIV-1 nor human T-cell leukemia virus type 1 (HTLV-1) expresses significant levels of either small interfering RNAs or microRNAs in persistently infected T cells. We also demonstrate that the retroviral nuclear transcription factors HIV-1 Tat and HTLV-1 Tax, as well as the Tas transactivator encoded by primate foamy virus, fail to inhibit RNA interference in human cells. Moreover, the stable expression of physiological levels of HIV-1 Tat did not globally inhibit microRNA production or expression in infected human cells. Together, these data argue that HIV-1 and HTLV-1 neither induce the production of viral small interfering RNAs or microRNAs nor repress the cellular RNA interference machinery in infected cells.

Inhibition of human immunodeficiency virus type 1 replication in primary macrophages by using Tat- or CCR5-specific small interfering RNAs expressed from a lentivirus vector

Although several groups have demonstrated that RNA interference, induced by transfection of small interfering RNA (siRNA) duplexes, can protect cells against a viral challenge in culture, this protection is transient. Here, we describe lentivirus expression vectors that can stably express siRNAs at levels sufficient to block virus replication. We have used these vectors to stably express siRNAs specific for the essential human immunodeficiency virus type 1 (HIV-1) Tat transcription factor or specific for a cellular coreceptor, CCR5, that is required for infection by the majority of primary HIV-1 isolates. These lentivirus vectors are shown to protect cells, including primary macrophages, against HIV-1 infection in culture by inducing selective degradation of their target mRNA species. These data suggest that it should be possible to block the expression of specific viral or cellular genes in vivo by using viral vectors to stably express the appropriate siRNAs.

HIV-1 protein Tat reduces the glutamate-induced intracellular Ca2+ increase in cultured cortical astrocytes

The trans-activator protein Tat of the human immunodeficiency virus type 1 (HIV-1) is regarded as an injurious molecule in the pathogenesis of HIV-1 associated encephalopathy (HIVE). We investigated the effects of Tat on neuroligand-induced intracellular Ca2+ increase in cultured astroglial cells. Rat cortical astrocytes, human glioblastoma cells and glial restricted precursor cells, from a human embryonic teratocarcinoma cell line, were incubated with recombinant Tat (100 ng/mL for 60 min) which induced a significant reduction of glutamate or ATP-induced intracellular Ca2+ increase ("glutamate response", "ATP response"). The reduction of the glutamate response was also observed following cell incubation with cell extracts of HeLa-T4+ cells transiently transfected with an expression plasmid coding for Tat. However, inactivation of the transcriptional trans-activity of Tat, by using a mutant form of Tat, as well as inhibition of de novo protein synthesis by cycloheximide abolished the effect on the glutamate response. This suggests that Tat acts upon induction of a so far unknown cellular gene whose gene product causes the reduction of glutamate responses. As the effect of Tat resembles the effect of TNFalpha on glutamate responses [Köller et al. (2001) Brain Res., 893, 237-243] which is locally released within the brains of HIVE patients, we also tested for synergistic effects of Tat and TNFalpha on the glutamate response. Low concentrations of Tat in combination with subthreshold concentrations of TNFalpha also elicited a marked reduction of astroglial glutamate responses. Our data suggest that Tat and TNFalpha, both by itself and synergistically, induce astroglial dysfunction.

HIV Tat protein requirements for transactivation and repression of transcription are separable

The HIV Tat protein, primarily characterized as a transcriptional activator of the viral long terminal repeat (LTR), is also a potent repressor of major histocompatibility complex (MHC) class I transcription. In the present study, we demonstrate that these two functional activities are distinct and mediated by discrete, but overlapping, structural domains of Tat. Tat repressor activity depends on C-terminal sequences, whereas transactivation depends on N-terminal sequences; both functions require core sequences. The repressor activity requires a domain encompassing the region encoded by the second exon of the Tat gene, beginning at amino acid 73, with a C-terminal limit between amino acids 80 and 83. Tat repressor function also depends on the presence of a lysine at position 41, located within the core of the protein. Tat repressor activity is independent of two N-terminal domains essential for transactivation: the acidic segment and the cysteine-rich region. Conversely, Tat transactivation is independent of the second exon-encoded region of Tat. As further support for this novel model of separable Tat functions, we show that in murine fibroblasts, Tat represses class I promoter activity, but does not transactivate the HIV LTR. We propose that distinct structural domains mediate the two functionally distinct activities associated with the Tat protein.

The human immunodeficiency virus type 1 Tat protein potentiates zidovudine-induced cellular toxicity in transgenic mice

3'-Azido-2',3'-dideoxythymidine (AZT, zidovudine) is the principal antiretroviral agent in the treatment of AIDS. Although beneficial, AZT remains restricted for human usage because of its severe toxic effects. We examined the AZT sensitivity in transgenic mice expressing HIV-1 one-exon-encoded 72 amino acid Tat (Tat72) and full-length 86 amino acid Tat (Tat86) proteins. Administration of AZT (1 mg/ml) in drinking water for 1 week resulted in a three- to fourfold decrease in hematopoietic progenitors from bone marrow in Tat mice compared to AZT-treated nontransgenic controls as determined by erythroid and granulocyte/macrophage colony-forming unit assays. In liver and thymus, two of the tissues examined, AZT treatment of Tat mice resulted in as much as 80-90% suppression of Mn-superoxide dismutase (Mn-SOD) activity. Other parameters associated with loss of Mn-SOD such as increase in carbonyl proteins and decrease of sulfhydryl content were also significantly enhanced by AZT in Tat mice. Our in vivo study suggests that AZT therapy is associated with oxidative damage affecting cellular functions in several tissues and that Tat is one of the contributory factors in AZT-induced toxicities. The findings of AZT-induced oxidative damage may help to improve the therapeutic index of AZT and other related drugs in AIDS patients.

Inhibition of human immunodeficiency virus type 1 and type 2 Tat function by transdominant Tat protein localized to both the nucleus and cytoplasm

We introduced various mutations into the activation and RNA binding domains of human immunodeficiency virus type 1 (HIV-1) Tat in order to develop a novel and potent transdominant Tat protein and to characterize its mechanism of action. The different mutant Tat proteins were characterized for their abilities to activate the HIV LTR and inhibit the function of wild-type Tat in trans. A Tat protein containing a deletion of the basic domain (Tat(delta)49-57) localized exclusively to the cytoplasm of transfected human cells was nonfunctional and inhibited both HIV-1 and HIV-2 Tat function in a transdominant manner. Tat proteins containing mutations in the cysteine-rich and core domains were nonfunctional but failed to inhibit Tat function in trans. When Tat nuclear or nucleolar localization signals were fused to the carboxy terminus of Tat(delta)49-57, the chimeric proteins localized to the nucleus or nucleolus, respectively, and remained capable of acting in a transdominant manner. Introduction of secondary mutations in the cysteine-rich and core domains of the various transdominant Tat proteins completely eliminated their abilities to act in a transdominant fashion. Our data best support a mechanism in which these transdominant Tat proteins squelch a cellular factor or factors that interact with the Tat activation domain and are required for Tat to function.

Tat-SF1: cofactor for stimulation of transcriptional elongation by HIV-1 Tat

Tat may stimulate transcriptional elongation by recruitment of a complex containing Tat-SF1 and a kinase to the human immunodeficiency virus-type 1 (HIV-1) promoter through a Tat-TAR interaction. A complementary DNA for the cellular activity, Tat-SF1, has been isolated. This factor is required for Tat trans-activation and is a substrate of an associated cellular kinase. Cotransfection with the complementary DNA for Tat-SF1 specifically modulates Tat activation. Tat-SF1 contains two RNA recognition motifs and a highly acidic carboxyl-terminal half. It is distantly related to EWS and FUS/TLS, members of a family of putative transcription factors with RNA recognition motifs that are associated with sarcomas.

Wild-type and transactivation-defective mutants of human immunodeficiency virus type 1 Tat protein bind human TATA-binding protein in vitro

SUMMARY

Tat regulates human immunodeficiency virus type 1 (HIV-1) gene expression by increasing both the rate of transcription initiation and the efficiency of transcription elongation. The ability of Tat to facilitate HIV-1 transcription preinitiation complex formation suggests that components of the basal transcriptional machinery may be targeted by Tat. Previous studies have demonstrated that Tat interacts directly with the human TATA-binding protein (TBP) and specific TBP-associated factors (TAFS) that comprise the TFIID complex. Here, in vitro glutathione S-transferase protein binding assays containing fully functional or transactivation-defective mutant Tat proteins have been used to investigate the functional significance of the direct interaction between Tat and TBP relative to Tat transactivation. Results demonstrate that full-length Tat, as well as the activation domain of Tat alone, binds human TBP in vitro. Site-directed mutations within the activation domain of Tat (C22G and P18IS) that abrogate transactivation by Tat in vivo fail to inhibit Tat-TBP binding. Full-length Tat, the activation domain of Tat alone, and a transactivation-defective mutant of Tat that lacks N-terminal amino acid residues 2-36 bind with equal efficiencies to TBP provided that the H1 alpha helical domain that maps to amino acids 167-220 within the highly conserved carboxyl terminus of TBP is maintained. These data indicate that an activity mapped within the activation domain of Tat, which is distinct from Tat-TBP binding. is required for transactivation by Tat.

Aqueous solution structure of a hybrid lentiviral Tat peptide and a model of its interaction with HIV-1 TAR RNA

Human immunodeficiency virus, type 1, (HIV-1) encodes a transactivating regulatory protein, called Tat, which is required for efficient transcription of the viral genome. Tat acts by binding to a specific RNA stem-loop element, called TAR, on nascent viral transcripts. The specificity of binding is principally determined by residues in a short, highly basic domain of Tat. The structure in aqueous solution of a biologically active peptide, comprised of the ten-amino acid HIV-1 Tat basic domain linked to a 15-amino acid segment of the core regulatory domain of another lentiviral Tat, i.e., that from equine infectious anemia virus (EIAV), has been determined. The restraint data set includes interproton distance bounds determined from two-dimensional nuclear Overhauser effect (2D NOE) spectra via a complete relaxation matrix analysis. Thirty structures consistent with the experimental data were generated via the distance geometry program DIANA. Subsequent restrained molecular mechanics calculations were used to define the conformational space subtended by the peptide. A large fraction of the 25-mer peptide assumes a structure in aqueous solution with the lysine- and arginine-rich HIV-1 basic domain being separated from the basic domain by a turn and characterized by a nascent helix as well. The Tat peptide/TAR complex could be modeled with the basic alpha-helix lying in the major groove of TAR such that important interactions of a putative specificity-endowing arginine are maintained and very slight widening of the major groove is entailed.

Sp1 transcription factor is required for in vitro basal and Tat-activated transcription from the human immunodeficiency virus type 1 long terminal repeat

Sp1-DNA binding sites have been reported to be essential for basal and Tat-activated transcription of the human immunodeficiency virus type 1 long terminal repeat (LTR). The role of the Sp1 transcription factor itself in regulation of the retroviral LTR, however, has not been clearly defined. It is now known, for instance, that the Sp1-DNA binding sites function also as thyroid hormone receptor response elements (V. Desay-Yajnik and H. H. Samuels, Mol. Cell. Biol. 13:5057-5069, 1993). In this report, we present data that demonstrate a strict requirement for Sp1 for both basal transcription and Tat-mediated trans activation of the human immunodeficiency virus type 1 LTR in vitro.

Transcriptional trans activation by human immunodeficiency virus type 1 Tat requires specific coactivators that are not basal factors

Expression of human immunodeficiency virus type 1 (HIV-1) genes is regulated by the trans activator Tat. Tat exerts its effects by increasing the rate of transcription, but the mechanism by which it does so is still unknown. To study the cellular factors required for Tat trans activation, we have expressed functional Gst-Tat fusion protein and used it to construct affinity columns. Our findings are as follows. (i) A Gst-Tat affinity matrix depleted HeLa nuclear extracts of a factor(s) required for Tat function. A Tat mutant bearing the missense mutation lysine to alanine at position 41 was incapable of this depletion. (ii) Tat trans activation was recovered by addition of unfractionated nuclear extract, the 0.5 M KCl elution fraction from the Tat affinity column, or sedimentation gradient fractions of HeLa extracts. The activity from the gradients sedimented with an apparent molecular mass of 200 kDa. (iii) Tat trans activation could not be recovered by use of recombinant human TATA-binding protein or partially purified TFIID. (iv) trans activation by Tat was blocked by heating of the nuclear extract under conditions in which basal transcription was not decreased. Our data demonstrate for the first time the existence of unique Tat coactivators distinct from factors required for general basal transcription.

Molecular cloning and characterization of a cellular protein that interacts with the human immunodeficiency virus type 1 Tat transactivator and encodes a strong transcriptional activation domain

The mechanism by which human immunodeficiency virus type 1 Tat transactivates the long terminal repeat promoter is not understood. It is generally believed that Tat has one or more transcription factors as its cellular target. One might expect a cellular target for Tat to possess several properties, including (i) the ability to bind to the Tat activation region, (ii) the possession of a transcriptional activation domain, and (iii) the ability to contact the cellular transcription machinery. Here we describe the cloning, expression, and characterization of a human protein, termed TAP (Tat-associated protein), which possesses some of these properties. TAP is highly conserved in eukaryotes and is expressed in a variety of human tissues. The major intracellular species of TAP is a highly acidic 209-amino-acid protein that likely is formed by removal of a highly basic 70-amino-acid N-terminal segment from a primary translation product. By deletion analysis, we have identified a TAP C-terminal region rich in acidic amino acids and leucine residues which acts as a strong transcriptional activator when bound through GAL4 sites upstream of the core long terminal repeat promoter, as well as flanking sequences that mask the activation function. Amino acid substitution of two leucine residues within the core activation region results in loss of the TAP activation function. Two lines of evidence suggest that Tat interacts with TAP in vivo. First, promoter-bound Tat can recruit a TAP/VP16 fusion protein to the promoter. Second, transiently expressed Tat is found associated with endogenous TAP, as demonstrated by coimmuno-precipitation analysis. As shown in an accompanying report, the TAP activation region binds the Tat core activation region and general transcription factor TFIIB (L. Yu, P.M. Loewenstein, Z. Zhang, and M. Green, J. Virol. 69:3017-3023, 1995). These combined results suggest the hypothesis that TAP may function as a coactivator that bridges Tat to the general transcription machinery of the cell via TFIIB.

A Simple and Rapid TAR-Dependent in vitro Transcription Assay Using T Cell Nuclear Extracts and Synthetic tat1-86 Protein

A 'G-less' cassette was cloned downstream of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) to facilitate rapid identification of authentic LTR-directed transcription in T cell nuclear extracts. Despite a high constitutive level of transcription, addition of chemically synthesized full-length HIV-1(bru) tat (amino acids 1-86) stimulated transcription 3-fold but only if the template included the TAR region of the LTR. Suppression of basal transcription in T cell nuclear extracts by sodium citrate increased the relative level of tat stimulation, but neither potassium chloride nor histone H1 had an effect. A mutant synthetic tat polypeptide, lacking residues 22-32 in the cysteine-rich domain, was inactive in uptake assays and failed to stimulate transcription. Short basic peptides that competed full-length tat from complexes with TAR RNA also inhibited tat stimulation of transcription, whereas short basic peptide unable to bind TAR or compete tat from complexes were also unable to inhibit tat stimulation of transcription. These data confirm that active HIV-1 tat must first interact with TAR RNA via basic amino acid residues in order to stimulate transcription of downstream sequences. Copyright 1995 S. Karger AG, Basel

Exon2 of HIV-2 Tat contributes to transactivation of the HIV-2 LTR by increasing binding affinity to HIV-2 TAR RNA

Human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) express related Tat proteins that are encoded in two exons. Tat proteins bind directly to the TAR RNA element contained in the 5' ends of viral transcripts and thereby stimulate transcription through an as yet unidentified mechanism. We have investigated the functional significance of exon2 of the HIV-2 Tat protein by examining properties of proteins consisting of exon1 alone or exon1 + 2. In transactivation assays in vivo, exon2 modestly increased HIV-2 Tat stimulation of transcription from the HIV-2 long terminal repeat (LTR) but had no effect on transcription from the HIV-1 LTR. In HeLa cells, exon2 increased transactivation of the HIV-2 LTR by approximately three-fold, while in COS and Jurkat cells this value was less than two-fold. In binding assays in vitro, exon2 increased the binding affinity of the HIV-2 Tat protein to HIV-2 TAR RNA. Results with GAL4 fusion proteins and a synthetic promoter containing GAL4 DNA binding sites indicated that exon2 does not contribute to the HIV-2 Tat activation domain. These observations suggest that exon2 of HIV-2 Tat contributes to transactivation of the HIV-2 LTR by increasing the binding affinity to HIV-2 TAR RNA.

The activation region of the Tat protein of human immunodeficiency virus type-1 functions in yeast

The N-terminal 48 amino acids of the Tat protein of human immunodeficiency virus type (HIV)-1 constitute its activation region. This region can autonomously activate transcription when targeted to the HIV-1 long terminal repeat or certain heterologous promoters either through DNA binding sites located upstream of the transcription initiation site or via downstream RNA binding sites in mammalian cells. To determine whether the Tat activation region can function in yeast, we have assayed the effect of a chimeric gene (GAL-Tat48) expressing the DNA binding domain of the yeast transcription factor Gal4 (residues 1-147) and the activation region of Tat on GAL1 promoter-directed expression of the lacZ reporter gene in Saccharomyces cerevisiae. Our results indicate that the Gal-Tat48 fusion protein can induce significant activation of the GAL1 promoter. Analysis of a number of Tat mutants located within the activation region indicate that the amino acid residues of Tat essential for trans-activation in mammalian cells are also required for transactivation in yeast. Our results suggest that Tat-mediated transcriptional activation may involve a mechanism conserved among yeast and mammalian cells.

Direct interaction of human TFIID with the HIV-1 transactivator tat

The tat gene of the human immunodeficiency virus (HIV) plays a central role in the activation and life cycle of HIV. The tat protein (Tat) specifically transactivates HIV transcription in vivo and in vitro, exerting its effects at the level of transcriptional initiation and elongation. Here we report that Tat binds directly to the basal transcription factor TFIID. The transcriptional activity of HeLa extracts was depleted after chromatography on a Tat affinity column, which specifically retained the polymerase II-specific factor TFIID. Direct interaction of Tat with holo-TFIID, composed of TATA-binding protein (TBP) and associated factors (TAFs), was observed. Tat binds, through amino acids 36-50, directly to the TBP subunit of TFIID. Our results suggest that Tat may transduce upstream or downstream regulatory signals by direct interaction with the basal transcription factor TFIID.

Human immunodeficiency virus type 2 (HIV-2) trans-activator (Tat): functional domains and the search for trans-dominant negative mutants

Human immunodeficiency virus type 2 (HIV-2) trans-activator (Tat) is an important trans-regulator of viral gene expression. It differs from the related HIV-1 Tat in certain aspects of its structure and function. HIV-2 Tat is composed of 130 amino acids versus 86 amino acids for HIV-1 Tat. Apart from certain conserved regions, there is little homology between the two Tats. They also differ in their ability to trans-activate HIV-2 and HIV-1 long terminal repeat (LTR)-directed gene expression. As an aid to understanding its mechanism of action, the functional domains important for HIV-2 Tat trans-activation of HIV-2 and HIV-1 LTR-directed gene expression were investigated. Like HIV-1 Tat, HIV-2 Tat contains conserved cysteine- and arginine-rich domains important for its function. However, HIV-2 Tat differs from HIV-1 Tat in that about 20% of the HIV-2 Tat at the amino terminus was not essential for its trans-activation function while HIV-1 Tat amino terminus is reportedly a part of its activation domain. Similarly, about 30% of the protein at the carboxy terminus of HIV-2 Tat was not essential. A domain critical for HIV-2 Tat-mediated trans-activation was located just upstream of the cysteine-rich domain. This segment is predicted to adopt an alpha-helical conformation and also contains acidic amino acid residues; thus, it may resemble amphipathic helix-type activation domains found in some transcriptional factors. A region with predicted hydrophobic alpha-helical character located between the cysteine- and arginine-rich domains was also important for HIV-2 Tat function. HIV-2 Tat mutants that were analogs of HIV-1 Tat trans-dominant negative mutants did not display such a phenotype.

Genetic evidence that the Tat proteins of human immunodeficiency virus types 1 and 2 can multimerize in the eukaryotic cell nucleus

The formation of dimers or higher-order multimers is critical to the biological activity of many eukaryotic regulatory proteins. However, biochemical analyses of the multimerization capacity of the Tat trans activator of human immunodeficiency virus types 1 (HIV-1) and 2 (HIV-2) have yielded contradictory results. We used the two-hybrid genetic assay for protein-protein interactions in the eukaryote Saccharomyces cerevisiae (S. Fields and O.-K. Song, Nature [London] 340:245-246, 1989) to examine the multimerization of Tat in vivo. Both HIV-1 and HIV-2 Tat are shown to form specific homo- but not heteromultimers in the yeast cell nucleus. Mutational analysis indicates a critical role for the essential core motif of Tat in mediating this interaction but demonstrates that efficient Tat multimerization does not require an intact cysteine motif. These data raise the possibility that the multimerization of Tat may be important for Tat function in higher eukaryotes.

Genetic analysis of the cofactor requirement for human immunodeficiency virus type 1 Tat function

The Tat protein of human immunodeficiency virus type 1 is a potent transcriptional trans activator of the viral long terminal repeat promoter element. Tat function requires the direct interaction of Tat with a cis-acting viral RNA target sequence termed the trans-activation response (TAR) element and has also been proposed to require at least one cellular cofactor. We have used a genetic approach to attempt to experimentally define the role of the cellular cofactor in Tat function and TAR binding. Our data suggest that neither Tat nor the cellular cofactor binds to TAR alone in vivo and indicate, instead, that the interaction of Tat with its cellular cofactor is a prerequisite for TAR binding. The known species tropism of lentivirus Tat proteins appears to arise from the fact that not only Tat but also the cellular cofactor can markedly influence the RNA sequence specificity of the resultant protein complex. These data also suggest that the Tat cofactor is likely a cellular transcription factor that has been highly conserved during vertebrate evolution. We hypothesize that the primary function of Tat is to redirect this cellular factor to a novel viral RNA target site and to thereby induce activation of viral gene expression.

Comparison of trans-dominant inhibitory mutant human immunodeficiency virus type 1 genes expressed by retroviral vectors in human T lymphocytes

trans-Dominant inhibitory mutant versions of the human immunodeficiency virus type 1 (HIV-1) regulatory genes tat and rev have previously been described. We have constructed a series of retroviral vectors to transduce these genes and compare their inhibitory activities. The inhibitory activities were measured with transient transfection assays by using a reporter which expresses an HIV-1 gag-Escherichia coli lacZ fusion protein with strict dependence on coexpression of both tat and rev. Additionally, the vectors were packaged as amphotropic virions and used to stably transduce human CEM T lymphocytes. The transduced CEM cells were challenged with HIV-1, and the effects of the mutant HIV-1 genes were determined by measuring the levels of HIV-1 p24gag produced. A tat gene substituted at amino acid 41 (tatk41a) retained partial trans-activating activity and lacked inhibitory activity. A tat gene with a premature stop codon at amino acid 54 (tat54ter) showed moderate trans-dominant inhibition of the reporter plasmid but failed to significantly inhibit HIV-1 replication. The M10 rev mutant, with a 2-amino-acid substitution, showed strong trans-dominant inhibitory activity both in the reporter plasmid and in the HIV-1 infection assay. The greatest inhibition of HIV-1 growth was seen when M10 was expressed under the transcriptional control of a human cytomegalovirus promoter; slightly less inhibition was achieved when expression of M10 was controlled by the Moloney murine leukemia virus long terminal repeat, and minimal inhibition was seen when the HIV-1 long terminal repeat controlled the M10 gene. These results demonstrate the potential utility of retroviral vectors expressing trans-dominant inhibitory mutant HIV-1 genes for gene therapy approaches to AIDS.

The VP16 transcription activation domain is functional when targeted to a promoter-proximal RNA sequence

Among eukaryotic transcription trans-activators, the human immunodeficiency virus type 1 (HIV-1) Tat protein is exceptional in that its target site TAR is an RNA rather than a DNA sequence. Here, we confirm that fusion of Tat to the RNA-binding domain of the HIV-1 Rev protein permits the efficient activation of an HIV-1 long terminal repeat (LTR) promoter in which critical TAR sequences have been replaced by RNA sequences derived from the HIV-1 Rev response element (RRE). An RRE target sequence as small as 13 nucleotides is shown to form an effective in vivo target for Rev binding. More important, a fusion protein consisting of Rev attached to the VP16 transcription activation domain was also observed to efficiently activate the HIV-1 LTR from this nascent RNA target. These data demonstrate that trans-activation of transcription by acidic activation domains does not require a stable interaction with the promoter DNA and suggest that VP16, like Tat, can act on steps subsequent to the formation of the HIV-1 LTR preinitiation complex. The finding that the activation domains of VP16 and Tat are functionally interchangeable raises the possibility that these apparently disparate viral trans-activators may nevertheless act via similar mechanisms.

Random mutagenesis of the human immunodeficiency virus type-1 trans-activator of transcription (HIV-1 Tat)

A new method is described for the direct construction of randomly mutagenized genes by applying the polymerase chain reaction (PCR) to an oligonucleotide synthesized using doped nucleotide reservoirs. We have demonstrated the utility of this method by generating a library of mutant HIV-1 tat genes. Several arbitrarily selected, inactive tat clones were sequenced to evaluate the extent of the mutagenesis. Moreover, fourteen recombinants encoding varying levels of transcriptional trans-activator activity were isolated by transient transfection of sub-library pools into a HeLa cell line bearing an HIV-LTR-chloramphenicol acetyltransferase (CAT) reporter gene. Sequence data revealed a spectrum of alterations including nucleotide substitutions, insertions, and deletions, suggesting that mutations arose from both the doped DNA synthesis and the subsequent PCR 'rescue' of full-length product. Sequence comparison between inactive and active Tat clones revealed a selection pressure against amino-acid substitutions within the N-terminal domains of Tat, indicating the importance of this region to trans-activation competence. In addition, single and double missense mutations within the basic-rich, TAR RNA-binding domain were seen to be tolerated within active Tat clones.

A comparison of regulatory features in primate lentiviruses

Historically, research into the regulation of gene expression in primate lentiviruses has focused on human immunodeficiency virus type 1 (HIV-1), the primary cause of acquired immunodeficiency syndrome (AIDS) in humans. The increasing emergence of HIV-2 as a human pathogen, and the importance of the various simian immunodeficiency viruses (SIV) as models for the treatment and prevention of HIV-1-induced disease, suggest that an understanding of gene regulation in these related viruses will become increasingly important. Here, the present state of knowledge in this latter field is reviewed. In general, while the data support the hypothesis that viral gene expression is regulated by very similar mechanisms in all primate lentiviruses, it also is clear that differences in detail do exist. These differences may influence the pathogenic potential of the different strains of primate lentiviruses and must be considered in evaluating SIV as an appropriate in vivo model for HIV-1.

Transactivation of heterologous promoters by HIV-1 tat

To determine whether HIV-1 tat can transactivate a heterologous promoter lacking HIV sequences other than the TAR element, TAR was placed downstream of the chicken beta-actin promoter. Tat increased expression directed by the actin-TAR promoter to a degree equal to tat induction of the HIV-1 LTR. Optimal transactivation was observed when TAR was positioned downstream of the actin promoter such that the expected cap site of transcripts from this promoter would be the same as in transcripts directed by the HIV-1 LTR. Tat was able to transactivate, though to a lesser extent, a promoter consisting solely of a TATA element fused to TAR. Thus, tat induction does not require HIV-1 LTR promoter sequences other than TAR. Tat, when fused to the DNA binding domain of BPV-1 E2, was able to transactivate a truncated SV40 promoter containing upstream E2 binding sites, indicating that tat may be capable of transactivation when directed by a DNA binding protein to an upstream site in a heterologous promoter lacking all HIV sequences. Substitution of Ala for Lys at position 41 of tat in the tat-E2 fusion, a mutation which dramatically decreases tat transactivation of the HIV-1 LTR, eliminated this transactivation.

Sp1-dependent activation of a synthetic promoter by human immunodeficiency virus type 1 Tat protein

The Tat protein coded by human immunodeficiency virus (HIV) is a strong activator of viral gene expression from the long terminal repeat (LTR). It appears that Tat-mediated trans-activation of the HIV LTR is predominantly a transcriptional event. It has been reported that Tat acts at the level of both transcriptional initiation and elongation through interaction with a nascent RNA target sequence termed TAR (for trans-activation response element). However, the precise mechanism(s) by which Tat mediates TAR-dependent transcriptional activity is not known. To determine whether Tat functions similarly to other eukaryotic transcriptional activators through any of the conventional promoter elements, we tested Tat activity on synthetic promoters containing consensus sequences required for binding transcription factor Sp1 and a TATA box. Here, we report that a chimeric Tat protein targeted to the promoter region by the DNA-binding domain of yeast transcription factor GAL4 activates the synthetic promoter. Because this trans-activation depends on Sp1-binding sites, Tat can apparently mediate transcriptional activation through its interaction with Sp1. Mutational analysis of the gal4-tat chimeric gene reveals that the N-terminal 48-amino acid region of Tat constitutes the activation region for Sp1-dependent trans-activation. This region of Tat exhibits substantially more activity than the N-terminal 58 amino acids of Tat, which includes the arginine-rich basic region. Effects of specific mutations in the 48-amino acid Tat region of GAL4-Tat on trans-activation of the synthetic promoter mimic the effects of these specific mutations on Tat-mediated trans-activation of the HIV-1 LTR, suggesting that trans-activation of both the synthetic promoter and the intact LTR occurs by a common mechanism.