The trans-activator gene of HTLV-III is essential for virus replication

The regulatory mechanisms of human immunodeficiency virus replication predict multiple expression rates

The qualitative nature of human immunodeficiency virus replication dynamics is examined by using the known action of regulatory proteins. The combination of activation of transcription by the Tat protein and the influence of the Rev protein on processing of RNA leads to a regulatory loop that can have multiple expression rates. In the lower state, the regulatory loop leads to low, or no, production of virus, which corresponds to its quiescent state. Conversely, when the regulatory loop is in the upper state, active production of virus is maintained over time. These features of the kinetic model have implications for both human immunodeficiency virus-related illness and therapeutics and predict testable experimental results in vitro.

Inhibition of antigen-induced lymphocyte proliferation by Tat protein from HIV-1

The purified human immunodeficiency virus type-l (HIV-l) Tat protein inhibited lymphocyte proliferation induced by tetanus toxoid or Candida antigens by 66 to 97% at nanomolar concentrations of Tat. In contrast, Tat did not cause a significant reduction of lymphocyte proliferation in response to mitogens such as phytohemagglutinin or pokeweed mitogen. Inhibition was blocked by oxidation of the cysteine-rich region of Tat or by incubation with an antibody to Tat before the assay. A synthetic Tat peptide (residues 1 to 58) also inhibited antigen-stimulated proliferation. Experiments with H9 and U937 cell lines showed that Tat can easily enter both lymphocytes and monocytes. The specific inhibition of antigen-induced lymphocyte proliferation by Tat mimics the effect seen with lymphocytes from HIV-infected individuals and suggests that Tat might directly contribute to the immunosuppression associated with HIV infection.

Regulation by HIV Rev depends upon recognition of splice sites

The ability of the Rev protein of HIV to regulate the cytoplasmic level of unspliced RNA from a beta-globin gene containing the Rev response element was dependent on the integrity of the 5' and 3' splice sites. A beta-globin pre-mRNA containing the Rev response element is not under regulation by Rev but is made Rev responsive by a mutation at either the 5' or 3' splice site. These mutant RNAs accumulated in the nucleus as unspliced precursors owing to recognition by splicing components. Only in the presence of Rev did these unspliced RNAs appear in the cytoplasm. Thus, regulation by Rev probably involves the dissociation of splicing components and pre-mRNA.

Functional analysis of the Tat trans activator of human immunodeficiency virus type 2

The trans-activator (Tat) proteins of the related but distinct type 1 and type 2 human immunodeficiency viruses (HIV-1 and HIV-2) display incomplete functional reciprocity. One possible explanation of this observation, suggested by computer analysis of potential RNA secondary structures within the viral trans-activation response (TAR) elements, is that HIV-2 Tat requires the presentation of two viral RNA stem-loop sequences for full activity whereas HIV-1 Tat is maximally active upon presentation of a single stem-loop structure. Here, we demonstrate that the HIV-2 long terminal repeat indeed contains two functionally independent TAR elements. However, the second (3') TAR element of HIV-2 is significantly less active than the 5' TAR element and is functionally masked in the context of an intact HIV-2 long terminal repeat. Evidence is presented suggesting that the activities of these two HIV-2 TAR elements reflect, at least in part, their relative distances from the site of transcription initiation. Although the HIV-2 TAR element proximal to the viral mRNA cap site appears to be sufficient for effective trans activation by HIV-2 Tat in vitro, this functional redundancy may nevertheless serve to enhance HIV-2 replication in infected cells in vivo.

Prospects for the control of AIDS patients by introducing defective-HIV harbouring leukocytes

Introduction of leukocytes harbouring an artificially constructed defective HIV provirus into AIDS patients may result in inducing superinfection resistance against HIV and interfering with HIV receptors or replication of HIV. All these may slow down progression of the disease.

trans activation of human immunodeficiency virus type 1 is sequence specific for both the single-stranded bulge and loop of the trans-acting-responsive hairpin: a quantitative analysis

We have used site-directed mutagenesis to delineate sequence specific domains within the human immunodeficiency virus type 1 (HIV-1) trans-acting-responsive (TAR) RNA element that are required for trans activation by the viral Tat protein. Our data in part corroborate a recent report [S. Feng and E. C. Holland, Nature (London) 334:165-167, 1988] that five nucleotides within the loop (+29 to +33) of the TAR hairpin are important for trans activation. We, however, found no absolute requirement for the CUGGG loop sequence. Mutants with substitutions within the loop retained between 9 and 50% activity compared with the wild type. A second sequence, important for trans activation, was found in the 3-base bulge loop (+22 to +24) of the TAR hairpin. Cross-trans-activation studies of mutant HIV-1 TAR elements with the HIV-2 Tat protein suggest that a similar recognition event(s) forms the basis for trans activation of HIV-1 and HIV-2.

Analysis of regulatory elements of the equine infectious anemia virus and caprine arthritis-encephalitis virus long terminal repeats

We analyzed the equine infectious anemia virus (EIAV) long terminal repeat (LTR) for sequences that influence its promoter activity and ability to be trans-activated by the EIAV tat gene product. A series of LTR deletion mutants and recombinants between LTR and simian virus 40 (SV40) regulatory sequences were used for these studies. We were able to identify the EIAV promoter region and showed that sequences within the U3 region significantly inhibited LTR-directed transcription. However, when placed in a heterologous context (SV40 promoter) these U3 sequences functioned as an enhancer. trans-activation of the EIAV LTR was found to depend upon sequences downstream of the transcription initiation site and also within U3. Deletion mutagenesis experiments showed that the major downstream element was present in a 46-nucleotide stretch (+4 to +50). An SV40 promoter construct containing these sequences could be trans-activated in cells expressing the EIAV tat gene product. For comparative purposes we also examined the LTR of another animal lentivirus, caprine arthritis-encephalitis virus (CAEV), for positive and negative transcriptional regulatory elements and demonstrated the presence of an enhancer within its U3 sequence. There is evidence that trans-activation of the CAEV LTR requires U3 sequences. When the EIAV U3 region was replaced by the CAEV U3 sequence, the promoter activity of the EIAV LTR was markedly elevated, but the responsiveness to the EIAV trans-activator could not be restored.

A region of basic amino-acid cluster in HIV-1 Tat protein is essential for trans-acting activity and nucleolar localization

The trans-acting factor of human immunodeficiency virus (HIV), Tat, has a basic amino-acid cluster that is highly conserved among different HIV isolates. We have examined the effects of mutations in the basic region of Tat on its trans-acting activity and cellular localization. Introduction of a stop codon immediately preceding the basic region abolished the activity, while the truncated mutant with the basic region retained some activity. The basic region of Tat was replaceable with that of Rev (another trans-acting factor of HIV) but not with that of adenovirus Ela nor cellular enzyme. The result of immunofluorescence analysis revealed a correlation between the nuclear, especially nucleolar, accumulation and the activities of mutant Tat proteins.

Functional comparison of the Rev trans-activators encoded by different primate immunodeficiency virus species

The known primate lentiviruses can be divided into two subgroups consisting of the human immunodeficiency virus type 1 (HIV-1) isolates and the related HIV type 2 (HIV-2) and simian immunodeficiency virus (SIV) isolates. HIV-1 has been shown to encode a post-transcriptional trans-activator of viral structural gene expression, termed Rev, that is essential for viral replication in culture. Here, we demonstrate that HIV-2 and SIVmac also encode functional Rev proteins. As in the case of HIV-1, these Rev trans-activators are shown to induce the cytoplasmic expression of the unspliced viral transcripts that encode the viral structural proteins. Unexpectedly, the Rev proteins of HIV-2 and SIVmac proved incapable of activating the cytoplasmic expression of unspliced HIV-1 transcripts, whereas HIV-1 Rev was fully functional in the HIV-2/SIV system. This nonreciprocal complementation may imply a direct role for Rev in mediating the recognition of its viral RNA target sequence.

Purification of biologically active human immunodeficiency virus rev protein from Escherichia coli

A genetic approach was used to facilitate purification of human immunodeficiency virus (HIV) rev protein. A recombinant protein containing a stretch of six histidine residues at the amino terminus was engineered and overexpressed in Escherichia coli. Purification of greater than 95% was achieved in a single step using an immobilized metal ion chromatography with a resin that has selectivity for proteins with neighboring histidine residues. We show that the modified protein is both properly modified and biologically active.

Multiple transcriptional regulatory domains in the human immunodeficiency virus type 1 long terminal repeat are involved in basal and E1A/E1B-induced promoter activity

The human immunodeficiency virus (HIV) type 1 long terminal repeat (LTR) is the site of activation of the HIV tat protein. However, additional transactivators, such as the adenovirus E1A and herpesvirus ICPO proteins, have also been shown to be capable of activating the HIV LTR. Analysis of adenovirus mutants indicated that complete transactivation of the HIV LTR was dependent on both the E1A and E1B proteins. To determine which regions of the HIV LTR were important for complete E1A/E1B activation, a variety of oligonucleotide-directed mutations in HIV transcriptional regulatory domains were assayed both in vivo and in vitro. S1 nuclease analysis of RNA prepared after transfection of these HIV constructs into HeLa cells infected with wild-type adenovirus indicated that the enhancer, SP1, TATA, and a portion of the transactivation-responsive element were each required for complete E1A/E1B-mediated activation of the HIV LTR. These same promoter elements were required for both basal and E1A/E1B-induced levels of transcription in in vitro transcription reactions performed with cellular extracts prepared from cells infected with dl434, an E1A/E1B deletion mutant, or wild-type adenovirus. No mutations were found that reduced only E1A/E1B-induced expression without proportionally reducing basal levels of transcription, suggesting that E1A/E1B-mediated induction of the HIV LTR requires multiple promoter elements which are also required for basal transcriptional levels. Unlike activation by the tat protein, there was not a rigid dependence on maintenance of the transactivation-responsive stem base pairing for E1A/E1B-mediated activation either in vivo or in vitro, indicating that activation occurs by a mechanism distinct from that of tat induction.

Human chromosome 12 is required for elevated HIV-1 expression in human-hamster hybrid cells

Host cell factors act together with regulatory genes of the human immunodeficiency virus (HIV) to control virus production. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV gene expression. DNA transfection experiments showed that 4 of 18 clones had high levels of HIV gene expression measured by both extracellular virus production and transactivation of the HIV long terminal repeat in the presence of the trans-activator (tat) gene. Karyotype analyses revealed a 94% concordance (17/18) between human chromosome 12 and HIV gene expression. Other chromosomes had an 11 to 72% concordance with virus production.

Tat trans-activates the human immunodeficiency virus through a nascent RNA target

Expression of the human immunodeficiency virus type 1 (HIV-1) genome is greatly dependent on the viral trans-activator protein Tat. Tat functions through the TAR element, which is represented in both viral DNA and RNA. At present, there is no definitive evidence that determines whether Tat acts through a DNA or RNA form of TAR. We have used an intramolecular mutagenesis approach to change selectively the RNA secondary structure of TAR without affecting its primary sequence. We show that a specific RNA secondary structure for TAR is needed for biological activity. Furthermore, transcripts that only transiently form a native TAR RNA hairpin, which is not maintained in the mature mRNA, are completely trans-activated by Tat, suggesting that TAR is recognized as a nascent RNA.

HIV-1 Gag mutants can dominantly interfere with the replication of the wild-type virus

The products of the human immunodeficiency virus (HIV) gag gene exist in a highly multimerized state in the mature virion. For that reason, they may represent a particularly suitable target for the generation of dominant negative mutants. A number of HIV site-directed Gag mutants did show interference with the production of infectious viral particles from cells in which they were cotransfected with a wild-type proviral DNA. Furthermore, cells constitutively expressing such HIV Gag mutants had an impaired ability to support HIV replication when infected with wild-type virus. The block was localized to the late stages of the virus life cycle. Such Gag variants could constitute prototypes for the development of anti-HIV intracellular immunization.

Activity of synthetic peptides from the Tat protein of human immunodeficiency virus type 1

To determine which of the 86 amino acids in the Tat protein of human immunodeficiency virus type 1 (HIV-1) are important for transactivation, peptides from Tat were synthesized and their activity was measured in cells containing a chloramphenicol acetyltransferase reporter gene under control of the HIV long terminal repeat promoter. Although the Tat sequence contains arginine- and cysteine-rich stretches that are difficult to synthesize, it was possible to prepare pure peptides in good yield by using fluoren-9-ylmethoxycarbonyl (Fmoc) chemistry. A peptide containing residues 1-58 had 5-10% the activity of full-length Tat. Deleting 4 amino acids from the N terminus of this peptide further reduced activity, while peptides with more extensive N-terminal deletions and peptides missing the basic region at the C terminus had no detectable activity. A peptide previously reported to transactivate, Tat-(37-62), was completely inactive in our assays. Inactive peptides were also tested as possible inhibitors of transactivation. Tat-(21-38), which contains the cysteine-rich region and can form heterodimers with intact Tat in vitro, showed inhibition at high peptide concentrations. However, this effect was not specific for Tat or for the HIV promoter, since the peptide also inhibited expression from the simian virus 40 early promoter.

Functional significance of phosphorylation to the human immunodeficiency virus Rev protein

The human immunodeficiency virus Rev protein is posttranslationally modified by a serine kinase activity present in the nucleus of the cell. Site-directed mutagenesis was used to identify the site of phosphorylation. Changing of serine residues 92 and 99 dramatically reduced Rev phosphorylation, suggesting that at least one, if not both, of these residues is the one recognized by the Rev-specific serine kinase. Similarly, a truncated Rev protein lacking the 25 carboxy-terminal amino acids was not phosphorylated. By using two independent assays, both the serine mutant proteins and the truncated form of Rev were found to be fully functional. Thus, phosphorylation and the 25 carboxy-terminal amino acids appear to be dispensable for protein function.

Comparative analysis of the HTLV-I Rex and HIV-1 Rev trans-regulatory proteins and their RNA response elements

The Rex proteins of types I and II human T-cell leukemia viruses (HTLV-I, HTLV-II) are required for expression of the viral structural gene products, gag and env and, thus, are essential for the replication of these pathogenic retroviruses. The action of Rex is sequence specific, requiring the presence of a cis-acting Rex response element located in the 3' long terminal repeat. This element corresponds to a predicted RNA secondary structure and functions in an orientation-dependent but position-independent manner. Rex acts through this response element to stimulate the nuclear export of the unspliced or singly spliced viral mRNA species encoding the virion structural proteins that are normally excluded from the cytoplasm. Although the Rex proteins of HTLV-I and HTLV-II can also function via the related Rev response element present in the env gene of the type I human immunodeficiency virus (HIV-1), the analogous HIV-1 Rev protein is unable to act on the HTLV-I Rex response element. This nonreciprocal pattern of genetic complementation by Rex and Rev suggests that these viral trans-regulators may interact directly with their RNA response elements.

A rapid, quantitative bioassay based on the human immunodeficiency virus trans-activator

We constructed a human immunodeficiency virus (HIV) trans-activator cDNA (tat) encoding the N-terminal 76 amino acids of the viral trans-activator followed by two additional amino acids (val and pro). This cDNA encoded a functional trans-activator (TAT) as shown by cotransfection into murine cells with a HIV promoter-chloramphenicol acetyltransferase DNA construct. The tat cDNA was cloned into an avian retroviral expression vector, a modified spleen necrosis virus (SNV), and high-titer infectious stocks of recombinant virus (SNV-tat) were recovered from dog cells. Hybridization analyses indicated that SNV-tat was stably propagated in these cells for months. We also prepared recombinant cells that stably carry reporter genes, either a human gene encoding a soluble CD4 receptor (sCD4) or the human preprorenin gene, under the transcriptional control of the HIV promoter. Medium obtained from these cell cultures after infection with control viruses or an SNV carrying an antisense tat contained only low background levels of sCD4 or prorenin (HRN) as determined by specific immunoassays (1-10 ng protein per 10(6) cells per ml medium). In contrast, cells infected with SNV carrying tat in the transcriptional sense orientation secreted 75 +/- 7 ng sCD4 and 73 +/- 4 ng HRN per 10(6) cells per ml medium. Moreover, these proteins were constitutively secreted at these levels during months of subculturing. The data indicate that sCD4 and HRN are secreted from these cells because of a TAT-mediated trans-activation of the HIV reporter gene DNA and/or RNA. This combination of recombinant cells, SNV-tat, and specific immunoassays provide a rapid, quantitative, and safe bioassay to seek inhibitors of TAT.

Mutational analysis of the human immunodeficiency virus vpr open reading frame

Mutations were introduced by recombinant DNA techniques into the vpr open reading frame of an infectious molecular clone of human immunodeficiency virus type 1. The effect of these changes on the replicative and cytopathologic properties of the virus recovered from transfected cells was studied in several human CD4+ lymphocyte cell lines. In all cases, mutant viruses were infectious and cytopathic. However, when a low-input dose was used, mutants grew significantly more slowly than the wild-type virus. The growth kinetics of vpr mutants were distinct from those of vif and vpu mutants.

Rapidly and slowly replicating human immunodeficiency virus type 1 isolates can be distinguished according to target-cell tropism in T-cell and monocyte cell lines

Human immunodeficiency virus type 1 (HIV-1) isolates from various patients were divided into two major groups, rapid/high and slow/low, according to their replication properties in vitro. Rapid/high isolates grow well in cell lines and induce the formation of syncytia in peripheral blood mononuclear cells. In contrast, slow/low isolates do not replicate in cell lines and rarely induce syncytia in peripheral blood mononuclear cells. To understand the differences in replicative capacity of these isolates, a panel of indicator cell lines was used. These cell lines were generated for sensitive detection of HIV-1 isolates and show characteristics of T-lymphoid or monocytoid cells. As a result of infection, chloramphenicol acetyltransferase expression is activated. Rapid/high viruses activate chloramphenicol acetyltransferase expression in T-cell and monocytoid indicator cell lines, whereas slow/low isolates activate chloramphenicol acetyltransferase expression only in monocytoid cell lines. The block in infection of T-lymphoid cells by the slow/low isolates appears to occur early in the infection cycle, prior to the production of the virally encoded tat protein. HIV-1 isolates can thus be distinguished according to target-cell tropism. Monocyte-derived cells seem to be a more general target for the various HIV-1 isolates.

Effects of long terminal repeat mutations on human immunodeficiency virus type 1 replication

The effects of deletions within three functional regions of the long terminal repeat of human immunodeficiency virus type 1 upon the ability of the long terminal repeat to direct production of the chloramphenicol acetyltransferase gene product and upon the ability of viruses that carry the mutations to replicate in human cell lines was investigated. The results show that the enhancer and TATAA sequences were required for efficient virus replication. Deletion of the negative regulatory element (NRE) yielded a virus that replicated more rapidly than did an otherwise isogeneic NRE-positive virus. The suppressive effect of the NRE did not depend upon the negative regulatory gene (nef), as both NRE-positive and NRE-negative viruses were defective for nef. We conclude that factors specified by the cell interact with the NRE sequences to retard human immunodeficiency virus type 1 replication.

The human immunodeficiency virus long terminal repeat is preferentially expressed in Langerhans cells in transgenic mice

Four lines of transgenic mice containing the HIV LTR linked to the bacterial gene encoding chloramphenicol acetyltransferase (CAT) were constructed. In each line, a characteristic tissue pattern of CAT expression was observed with detectable levels present in the eye, heart, spleen, thymus, and tail. Low levels of CAT were present in circulating lymphocytes, but CAT activity in these cells could be augmented following treatment with the mitogen phytohemagglutinin (PHA). Likewise, CAT expression was present at only low levels in circulating monocytes, but higher levels of CAT were observed in macrophages grown in the presence of various cytokines (CSF-1, GM-CSF, IL-1 alpha, IL-4, and IL-2). Furthermore, Langerhans cells recovered from skin showed higher levels of CAT activity than those observed in other cells of monocyte-macrophage lineage. These results indicate that LTR-CAT expression in cells of monocyte-macrophage lineage may increase in proportion to the degree of differentiation of these cells. These animals may be useful in the study of cell-specific determinants of LTR-directed gene activity and may serve to identify exogenous cofactors that promote the progression of HIV-related disease in vivo.

Functional characterization of a complex protein-DNA-binding domain located within the human immunodeficiency virus type 1 long terminal repeat leader region

Transcriptional trans activation of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) by the viral tat trans activator is mediated by an LTR-specific sequence located immediately 3' to the start of transcription initiation. We have used a range of molecular techniques to examine DNA-protein interactions that occur in the vicinity of this cis-acting sequence. Our results demonstrate the existence of a sequence-specific DNA-protein interaction involving the HIV-1 leader DNA and map this binding event to between -2 and +21 base pairs relative to the HIV-1 LTR transcription start site. Evidence suggesting that this interaction involves three distinct protein-DNA contact sites extending along one side of the DNA helix is presented. Mutation of these sites was found to ablate protein-DNA binding yet was observed to have no effect on either the basal or tat trans-activated level of HIV-1 LTR-specific gene expression. We therefore conclude that this DNA-protein interaction has a function distinct from the regulation of HIV-1 LTR-specific gene expression.

Rev-induced modulation of Nef protein underlies temporal regulation of human immunodeficiency virus replication

The replication of human immunodeficiency virus type 1 (HIV-1) requires the concerted action of two virus-encoded transactivator proteins, Tat and Rev, and is in turn moderated by the viral transcriptional repressor Nef. We show here that the phenotype of a Rev- HIV-1 provirus was nonreplicating and was distinguished by accumulation of Nef protein and reduced Tat function. Provirus defective in both the rev and nef genes (Rev-Nef-) was also nonreplicating but had normal Tat function. Trans-complementation of the Rev- mutant with Rev caused a decrease of both the steady-state level and the rate of synthesis of Nef. This was accompanied by enhanced synthesis of viral structural proteins. Rev induced even greater levels of virus production from the Rev-Nef- double mutant. In contrast, exogenous Rev did not augment virus production from wild-type provirus. Virus production from Rev- and Rev-Nef- mutants induced by Rev was repressed by exogenous Nef. The repression induced by Nef could not be reversed by exogenous Rev. The ability of Rev to modulate Nef expression solely from the provirus, and thereby relieve the Nef-mediated inhibition of transcription from the viral long terminal repeat, reveals a delicate balance of the functions of these two proteins that might underlie the switch between latency and reactivation.

Cytomegalovirus enhances lysis of HIV-infected T lymphoblasts

A T4+ lymphoblastoid cell line (CR-10) persistently infected with the human immunodeficiency virus (HIV) and designated CR-10/NIT was superinfected with cytomegalovirus (CMV) isolated from peripheral blood mononuclear cells of a patient with AIDS. A productive CMV cycle in the CR-10/NIT lymphoblasts was demonstrated by fluorescent antibody staining (IF) using a monoclonal antibody (MAb) to the 150-kDa major capsid protein, by infectivity assays and by electron microscopy (EM). Two-color IF analysis showed that a small percentage of the CR-10/NIT cells were producing both CMV and HIV at any one time. EM studies revealed that all doubly infected cells were lysed whereas most cells infected only with HIV appeared intact. Cell lysis appeared 24 hr after superinfection of the CR-10/NIT cells with CMV and progressed to complete destruction of the cell culture between days 9 and 10. Our results suggest that CMV may convert a mildly cytopathic HIV infection of T lymphoblasts into a highly lytic process.

Functional dissection of the HIV-1 Rev trans-activator--derivation of a trans-dominant repressor of Rev function

Human immunodeficiency virus type 1 (HIV-1) encodes a nuclear trans-activator, termed Rev, that is required for the expression of the viral structural proteins and, hence, for viral replication. The Rev protein acts posttranscriptionally to induce the sequence-specific nuclear export of unspliced HIV-1 mRNA species that are otherwise excluded from the cell cytoplasm. We have used site-directed mutagenesis to identify two distinct regions of the HIV-1 Rev protein that are required for in vivo biological activity. The larger and more N-terminal of these two regions includes, but extends beyond, an arginine-rich sequence element required for nuclear localization. Mutation of a second, more C-terminal Rev protein sequence element was found to yield defective Rev proteins that act as trans-dominant inhibitors of Rev function. These Rev mutants are shown to inhibit HIV-1 replication when expressed in transfected cells and may have potential application in the treatment of HIV-1 related disease.

Mutational analysis of HIV-1 Tat minimal domain peptides: identification of trans-dominant mutants that suppress HIV-LTR-driven gene expression

The HIV-1 Tat protein is a potent trans-activator essential for virus replication. We reported previously that HIV-1 Tat peptides containing residues 37-48 (mainly region II), a possible activating region, and residues 49-57 (region III), a nuclear targeting and putative nucleic acid binding region, possess minimal but distinct trans-activator activity. The presence of residues 58-72 (region IV) greatly enhances trans-activation. We postulate that Tat mutant peptides with an inactive region II and a functional region III can behave as dominant negative mutants. We synthesized minimal domain peptides containing single amino substitutions for amino acid residues within region II that are conserved among different HIV isolates. We identify four amino acid residues whose substitution within Tat minimal domain peptides leads to defects in transactivation. Some of these mutants are trans-dominant in several peptide backbones, since they strongly inhibit trans-activation by wild-type Tat protein added to cells or expressed from microinjected plasmid. Significantly, trans-activation of integrated HIV-LTRCAT is blocked by some trans-dominant mutant peptides. These results suggest an attractive approach for the development of an AIDS therapy.

Specific binding of a HeLa cell nuclear protein to RNA sequences in the human immunodeficiency virus transactivating region

The transactivator protein, tat, encoded by the human immunodeficiency virus is a key regulator of viral transcription. Activation by the tat protein requires sequences downstream of the transcription initiation site called the transactivating region (TAR). RNA derived from the TAR is capable of forming a stable stem-loop structure and the maintenance of both the stem structure and the loop sequences located between +19 and +44 is required for complete in vivo activation by tat. Gel retardation assays with RNA from both wild-type and mutant TAR constructs generated in vitro with SP6 polymerase indicated specific binding of HeLa nuclear proteins to the TAR. To characterize this RNA-protein interaction, a method of chemical "imprinting" has been developed using photoactivated uranyl acetate as the nucleolytic agent. This reagent nicks RNA under physiological conditions at all four nucleotides in a reaction that is independent of sequence and secondary structure. Specific interaction of cellular proteins with TAR RNA could be detected by enhanced cleavages or imprints surrounding the loop region. Mutations that either disrupted stem base-pairing or extensively changed the primary sequence resulted in alterations in the cleavage pattern of the TAR RNA. Structural features of the TAR RNA stem-loop essential for tat activation are also required for specific binding of the HeLa cell nuclear protein.

Stable indicator cell lines exhibiting HIV-1 tat function

Since HIV tat function is essential for the HIV infectious cycle, it represents an important possible target of therapeutic intervention for HIV infection. Stable human cell lines were derived that express high levels of beta-galactosidase under the combined control of the transacting HIV-1 tat gene product and the cis-acting HIV-1 LTR. The tat gene product induces LTR-linked gene expression approximately 1000-fold in this system. The high level of expression of beta-galactosidase under HIV tat and LTR control in stable cell lines allows rapid spectrophotometric quantitation of beta-galactosidase enzymatic activity from fewer than 5000 cells seeded in a microtiter plate well. Such cell lines provide a virus-free system for the high-capacity screening of compounds for the ability to interfere with HIV tat-mediated transactivation of gene expression.

Role of SP1-binding domains in in vivo transcriptional regulation of the human immunodeficiency virus type 1 long terminal repeat

Five regions of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) have been shown to be important in the transcriptional regulation of HIV in HeLa cells. These include the negative regulatory, enhancer, SP1, TATA, and TAR regions. Previous studies in which purified SP1 was used showed that the three SP1-binding sites in the HIV LTR were important in the in vitro transcription of this promoter. However, no studies to ascertain the role of each of these SP1-binding sites in basal and tat-induced transcriptional activation in vivo have been reported. To determine the role of SP1 sites in transcriptional regulation of the HIV LTR in vivo, these sites were subjected to oligonucleotide mutagenesis both individually and in groups. The constructs were tested by DNase I footprinting with both oligonucleotide affinity column-purified SP1 and partially purified HeLa extract and by chloramphenicol acetyltransferase assays in both the presence and absence of the tat gene. Mutagenesis of each SP1-binding site resulted in minimal changes in basal and tat-induced transcriptional activation. Mutations involving alterations of SP1 sites I and II, I and III, or II and III also resulted in minimal decreases in basal and tat-induced transcriptional activation. However, mutagenesis of all three SP1-binding sites resulted in a marked decrease in tat induction. The latter mutation also greatly decreased DNase I protection over the enhancer, TATA, and TAR regions when partially purified HeLa nuclear extract was used. Mutagenesis of the HIV LTR SP1 sites which converted them to consensus high-affinity SP1-binding sites with the sequence GGGGCGGGGC resulted in increased tat-induced gene expression compared with the wild-type HIV LTR template. These results suggest that SP1, through its interaction with other DNA-binding proteins, is critical for in vivo transcriptional regulation of HIV.

Human cell lines stably expressing HIV env and tat gene products

A DNA fragment containing the tat, rev and env genes of the human immunodeficiency virus type 1 was inserted into the retroviral vector pZIPneoAU3. The resulting plasmid penvAU3 was transfected into HeLa and psi CRIP cells. Resulting recombinant retroviruses were used to infect HeLa and Jurkat cells. Immunoprecipitation analysis of stable transformants showed the expression of HIV env glycoproteins gp160, gp120 and gp41. Transactivation assays with a plasmid containing the gene for chloramphenicol acetyltransferase linked to HIV promoter-enhancer sequences demonstrated the expression of functional tat. These cells constitute virus-free tools for functional and structural studies of native env and tat.

Chemical synthesis and expression of a gene encoding HIV-1 TAT protein

A gene encoding the human immunodeficiency virus-1 (HIV-1) TAT protein was chemically synthesized and expressed in HeLa cells and in a cell-free system. To facilitate both the assembly of the synthetic gene and further mutagenesis and gene fusion studies, several unique restriction endonuclease cleavage sites were included in the coding sequence without altering the encoded protein sequence. The synthetic TAT coding sequence was fused to a translation start signal and placed under SV40 early transcriptional control. Co-transfection of the TAT-encoding synthetic gene together with a reporter gene (chloramphenical acetyl transferase or beta-galactosidase) linked to an HIV LTR confirmed that the synthetic gene product exhibits similar activity to TAT expressed from HIV genomic DNA in the transactivation of the LTR. TAT mRNA prepared by cell-free transcription of the synthetic TAT coding sequence was also shown to produce functional TAT following microinjection into HeLa-derived cells containing an integrated reporter gene with the HIV LTR linked to beta-galactosidase.

Regulation of human immunodeficiency virus env expression by the rev gene product

A single simian virus 40 late replacement vector which expresses both the rev and envelope (env) genes of human immunodeficiency virus was used to examine the mechanism underlying the dependence of env gene expression on the rev protein. When rev was deleted from the vector, no envelope protein expression could be detected in transfected cells, and the levels of cytoplasmic env mRNA were dramatically reduced. In contrast to this, the levels of env RNA in total cellular RNA preparations were similar with or without rev coexpression, and analysis of nuclear RNA showed that the levels of nuclear env RNA were increased in the absence of rev. These results suggest that rev functions to regulate nuclear export of env mRNA. It was possible to restore env expression from the vector lacking rev by supplying rev in trans, provided that a cis-acting sequence was also present. This sequence was mapped to a 854-base-pair region within the env open reading frame, and it was shown that the sequence could be moved but that it worked only in its original orientation.

Formation of infectious hybrid virions with gibbon ape leukemia virus and human T-cell leukemia virus retroviral envelope glycoproteins and the gag and pol proteins of Moloney murine leukemia virus

The gibbon ape leukemia virus, SEATO strain, and human T-cell leukemia virus type I envelope glycoproteins can be functionally assembled with a Moloney murine leukemia virus core into infectious particles. The envelope-host cell receptor interaction is the major determinant of the host cell specificity for these hybrid virions.

HIV-1 core proteins expressed from recombinant vaccinia viruses

The sequences encoding the core proteins p55, p25, and p18 of the human immunodeficiency virus (HIV-1) have been inserted into the vaccinia virus genome. Infection of cultured cells with the live recombinant viruses led to the expression of proteins that were recognized by sera from HIV-seropositive individuals. Immunization of mice with the recombinant virus expressing the HIV p25 protein and the p55 precursor yielded high levels of antibodies directed against the corresponding HIV antigens. The data obtained are discussed in terms of the possible use of these live recombinant viruses in the development of a strategy toward an AIDS vaccine.

Structure, sequence, and position of the stem-loop in tar determine transcriptional elongation by tat through the HIV-1 long terminal repeat

The human immunodeficiency virus (HIV-1)-encoded trans-activator (tat) increases HIV gene expression and replication. Previously, we demonstrated that tat facilitates elongation of transcription through the HIV-1 long terminal repeat (LTR) and that short transcripts corresponding to prematurely terminated RNA are released and accumulate in the absence of tat. Here, using a transient expression assay, we tested clustered and compensatory mutations, as well as 3' deletions, in the trans-acting responsive region (tar) and observed that the primary sequence in the loop and secondary structure in the stem of the stem-loop in tar are required for trans-activation by tat. Insertions in the 5' region of tar revealed that tar must be near the site of HIV-1 initiation of transcription for trans-activation by tat. Deletions (3') and an insertion in tar demonstrated that an intact stem-loop is required for the recovery of prematurely terminated transcripts. Short and full-length transcripts were observed also with HIV type 2 (HIV-2) in the absence and presence of tat, respectively. We conclude that an intact stem-loop in tar is essential for trans-activation by tat and that initiation of transcription by HIV-1 promoter factors and elongation of transcription by tat are coupled.

Mutational analysis of the conserved basic domain of human immunodeficiency virus tat protein

The tat trans-activators encoded by the known strains of primate immunodeficiency virus share a conserved, highly basic protein domain. Mutagenesis of this sequence in the tat gene of human immunodeficiency virus type 1 is shown here to reduce, but not eliminate, the trans-activation of human immunodeficiency virus type 1-specific gene expression. The degree of inhibition is shown to vary in a dose-dependent manner and is most marked at low levels of tat expression. Multiple mutations of the basic domain of tat were found to impair both the in vivo stability and the nuclear localization of the tat protein. It is proposed that this protein domain serves to efficiently target the tat gene product to its appropriate site or substrate within the nucleus of expressing cells.

rev protein of human immunodeficiency virus type 1 affects the stability and transport of the viral mRNA

rev (trs/art) is an essential human immunodeficiency virus type 1 (HIV-1) regulatory protein. rev increases the levels of the gag- and env-producing mRNAs via a cis-acting element in the env region of HIV-1, named rev-responsive element. Our results show that rev increases the stability of the unspliced viral mRNA, while it does not affect the stability of the multiply spliced viral mRNAs that do not contain the rev-responsive element. The study of mutated proviral constructs producing mRNA that cannot be spliced revealed that the effect of rev on stability is independent of splicing. Our experiments also indicate that rev promotes the transport of the viral mRNA containing the rev-responsive element from the nucleus to the cytoplasm. The proposed functions of rev are consistent with its nuclear localization as shown by immunofluorescence. The selective effects of rev on the levels of the viral mRNA suggest a model for feedback regulation by rev leading to a steady state of viral expression.

Hepatitis B virus X gene can transactivate heterologous viral sequences

The smallest open reading frame of hepatitis B virus (HBV) has been designated the X gene and its biological function during HBV infection and replication is not known. Experiments described here demonstrate that expression of the HBV X gene in HepG2 cells containing a plasmid with the chloramphenicol acetyltransferase (CAT) gene under control of the human immunodeficiency virus (HIV-1) long terminal repeat (LTR) sequence leads to a marked increase in CAT gene transcription as well as expression of the gene product (CAT). The HIV-1 tatIII gene and the HBV X gene together increased HIV-1 LTR-regulated CAT expression above that observed with either gene alone, suggesting a synergistic effect of the X gene and tat. HBV X gene also stimulated expression of the CAT gene under control of the simian virus 40 enhancer and early promoter but not the visna virus LTR or the human T-cell lymphotropic virus type I (HTLV-I) LTR, indicating that the HBV X gene can transactivate some but not other heterologous viral sequences. Transactivation of the HIV-1 LTR by the HBV X gene varied in different cell lines, suggesting that it may be mediated by a cellular factor(s).

Human immunodeficiency virus type 1 negative factor is a transcriptional silencer

The negative factor (nef) of human immunodeficiency virus (HIV) type 1 acts to down-regulate virus replication. To decipher the step in the virus life cycle affected by nef, functional proviral clones with (pHIV F-) or without (pHIV F+) a deletion mutation in the nef gene were constructed. In CD4+ cells, 30- to 50-fold more virus was produced over the course of 18-20 days with cultures infected with F- compared to F+ virus. In CD4- cell lines, 2- to 10-fold greater virus production was found from cultures transfected with pHIV F- than those transfected with pHIV F+. The negative regulatory effects of nef on pHIV F- could be supplied in trans with a plasmid expressing only the nef gene product. Virus produced by COS-1 cells transfected with pHIV F- or pHIV F+ showed similar binding, uptake, uncoating, and reverse transcription. Analysis of HIV-1 RNA and structural protein levels and rates of viral RNA synthesis in CD4- cells also showed 2- to 10-fold higher levels in cells transfected with pHIV F- compared to pHIV F+. The activity of a HIV-1-chloramphenicol acetyltransferase (CAT) plasmid was also suppressed by nef, whereas other CAT plasmids were unaffected. These findings demonstrate that nef acts as a specific silencer of HIV-1 transcription. This activity may be critical for maintenance of HIV-1 latency in vivo.

Bioassay for trans-activation using purified human immunodeficiency virus tat-encoded protein: trans-activation requires mRNA synthesis

Expression of the human immunodeficiency virus tat-encoded protein (Tat) is required for virus replication. A genetic approach was used to facilitate the purification of biologically active Tat. A recombinant Tat protein containing a stretch of six histidine residues and a protease cleavage site was engineered and purified to greater than 95% homogeneity in a single step by immobilized metal-ion chromatography with a special affinity resin that has selectivity for proteins with neighboring histidine residues. A modified scrape loading method for introduction of protein into cell monolayers was used to demonstrate that the purified Tat retained biological activity. Tat function was completely blocked in the presence of transcription inhibitors, which demonstrates the requirement of ongoing mRNA synthesis for trans-activation. These studies indicate that the mechanism of trans-activation is unlikely to involve a direct action of Tat on mRNA stability, transport, or translation and provides the basis for a rapid assay that can be used to identify inhibitors of trans-activation. The methods described herein should be useful for the functional analysis of other proteins that do not confer activity through a receptor-mediated pathway.

Productive dual infection of human CD4+ T lymphocytes by HIV-1 and HHV-6

Although infection by HIV-1 has been implicated as the primary cause of AIDS and related disorders, cofactorial mechanisms may be involved in the pathogenesis of the disease. For example, several viruses commonly detected in AIDS patients and capable of transactivating the long terminal repeat of HIV-1, such as herpesviruses, papovaviruses, adenoviruses and HTLV-I have been suggested as potential cofactors. Another candidate is human herpesvirus-6 (HHV-6, originally designated human B-lymphotropic virus), which has not only been identified in most patients with AIDS by virus isolation, DNA amplification techniques and serological analysis, but is also predominantly tropic and cytopathic in vitro for CD4+ T lymphocytes. Here we demonstrate that HHV-6 and HIV-1 can productively co-infect individual human CD4+ T lymphocytes, resulting in accelerated HIV-1 expression and cellular death. We also present evidence that HHV-6 transactivates the HIV-1 long terminal repeat (LTR). These observations indicate that HHV-6 might contribute directly or indirectly to the depletion of CD4+ T cells in AIDS.

Targets for antiviral therapy of human immunodeficiency virus infection

The development of potent anti-retroviral drugs is central to the control of human immunodeficiency virus (HIV) infection and the prevention of disease. Despite the benefit (albeit limited) shown by the early trials of zidovudine in patients with acquired immune deficiency syndrome (AIDS), there is general agreement that the best prospects for therapeutic intervention lie in the use of agents early in the infectious process. There is a definite possibility that this can be achieved if compounds acting specifically against virus encoded events can be found or developed. Although relatively simple in its structure, HIV is highly sophisticated in its mode of replication. The unique nature of the replication cycle of the retroviridae and the specific controlling mechanisms operative in HIV offer a number of possible targets for chemotherapeutic agents. The details of the structure and replication cycle of HIV will be briefly reviewed with comments on the possible virus specific and non-specific sites for potential antiviral drug development. The first specific target to be recognised was the unique, virus-associated enzyme, the reverse transcriptase (RNA directed DNA polymerase). Several inhibitors of reverse transcriptase were identified during the 1970s (e.g. suramin, HPA23, phosphonoformate). These have been found, in early trials, to be either insufficiently potent or too toxic to consider for development as anti-retroviral drugs. Indeed, knowledge of the pathogenesis of HIV infection led to the realisation that any putative drug would need to satisfy several important criteria; namely potency, low toxicity, easy administration, penetration of the blood-brain barrier and hopefully, low production costs.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional characterization of human immunodeficiency virus tat protein

Site-directed mutagenesis was used to identify functional domains present within the human immunodeficiency virus (HIV) tat protein. Transient cotransfection experiments showed that derivatives of tat protein with amino acid substitutions either at the amino-terminal end or at cysteine residue 22, 37, 27, or 25 were no longer able to transactivate HIV long terminal repeat-directed gene expression. Incubation of Tat expressed in Escherichia coli with zinc demonstrated that both authentic Tat and cysteine mutation derivatives could form metal-protein complexes. The tat proteins that contained alterations within the cluster of positively charged amino acid residues retained their ability to transactivate gene expression, albeit at markedly reduced levels. Indirect immunofluorescence showed that the authentic tat protein and the amino-terminal and cysteine substitution mutants all localized in the nucleus, with accumulation being most evident in the nucleolus. In contrast, nuclear accumulation was greatly reduced with the basic-substitution mutations. Consistent with this result, a fusion protein that contained amino acids GRKKR, derived from the basic region, fused to the amino-terminal end of beta-galactosidase also accumulated within the nucleus. These results demonstrate that the 14-kilodalton tat protein contains at least three distinct functional domains affecting localization and transactivation.

Characterization of a cDNA clone encoding the visna virus transactivating protein

The unique pathogenesis of lentiviral infections in humans and ruminant animals may be explained, in part, by the complex mechanisms regulating transcription and translation of their viral genes. This report demonstrates that a visna virus-encoded protein transactivates viral gene expression. A 1.4-kilobase cDNA clone encodes two distinct proteins with apparent molecular masses of 21.5 and 10 kDa. We demonstrate that the 10-kDa species is the visna virus transactivating (Tat) protein; the other species may be analogous to the rev (formerly art or trs) gene product of human immunodeficiency virus.