Human chromosome 12 encodes a species-specific factor which increases human immunodeficiency virus type 1 tat-mediated trans activation in rodent cells

Genetic mapping in human and mouse of the locus encoding TRBP, a protein that binds the TAR region of the human immunodeficiency virus (HIV-1)

Productive infection with HIV-1, the virus responsible for AIDS, requires the involvement of host cell factors for completion of the replicative cycle, but the identification of these factors and elucidation of their specific functions has been difficult. A human cDNA, TRBP, was recently cloned and characterized as a positive regulator of gene expression that binds to the TAR region of the HIV-1 genome. Here we demonstrate that this factor is encoded by a gene, TARBP2, that maps to human chromosome 12 and mouse chromosome 15, and we also identify and map one human pseudogene (TARBP2P) and two mouse TRBP-related sequences (Tarbp2-rs1, Tarbp2-rs2). The map location of the expressed gene identifies it as a candidate for the previously identified factor encoded on human chromosome 12 that has been shown to be important for expression of HIV-1 genes. Western blotting indicates that despite high sequence conservation in human and mouse, the TARBP2 protein differs in apparent size in primate and rodent cells.

Transcriptional activation in vitro by the human immunodeficiency virus type 1 Tat protein: evidence for specific interaction with a coactivator(s)

The Tat protein encoded by human immunodeficiency virus type 1 is a strong transcriptional activator of gene expression from the viral long terminal repeat and is essential for virus replication. We have investigated the molecular mechanism of Tat trans-activation by using a cell-free transcription system. We find that the trans-activation domain of Tat, amino acid residues 1-48 [Tat-(1-48)], can inhibit specifically--i.e., "squelch," transcriptional activation by full-length Tat [Tat-(1-86)]. Squelching depends upon the functional integrity of the Tat trans-activation domain because the mutant [Ala41]Tat-(1-48), which is defective in Tat trans-activation in vivo and in vitro, does not squelch in vitro Tat trans-activation. Inhibition is selective because Tat-activated transcription, but not Tat-independent transcription, is squelched. Preincubation experiments with Tat or Tat-(1-48) and nuclear extracts show that the trans-activation region of Tat can interact with cellular coactivator(s) required for Tat trans-activation and that this interaction can occur in the absence of the human immunodeficiency virus long terminal repeat promoter. Furthermore, the putative coactivator(s) mediating trans-activation by Tat differ from those mediating trans-activation by the acidic activator VP16, as shown by reciprocal squelching experiments in vitro. Our results suggest that specific cellular coactivator(s) are required for mediating activated transcription by human immunodeficiency virus type 1 Tat.

A sensitive promoter assay based on the transcriptional activator Tat of the HIV-1 virus

We developed a sensitive vector system for the analysis of weak promoter activities. This promoter assay is based on the transcriptional activator protein, Tat, of human immunodeficiency virus type 1 (HIV-1). High-level expression of HIV requires activation in trans by Tat of the promoter in the long terminal repeat (LTR). Here we describe the construction of a promoterless pTat vector. Foreign promoter elements can be inserted upstream from the tat gene, and expression of Tat protein is measured in trans on a co-transfected LTR-CAT reporter plasmid. We show that this binary system is more sensitive than standard pCAT reporter assays.

Human immunodeficiency virus type 1 Rev-responsive element RNA binds to host cell-specific proteins

RNase protection-gel retention studies show human host cell-specific ribonucleoprotein complexes with human immunodeficiency virus type 1 Rev-responsive element (RRE) RNA. Nuclear proteins from rodent or murine cells appear to lack the ability to form these complexes. Human-mouse somatic cell hybrids retaining a single human chromosome, either 6 or 12, form the RRE-nuclear-protein complexes. One of the complexes requires the entire RRE RNA, while the other needs RRE RNA stem-loops 1 and 2 only. Two major proteins with molecular masses of 120 and 62 kDa specifically bind to RRE RNA. Rodent cells (CHO) either lack or contain small amounts of these RRE-binding proteins.

Susceptibility of human-mouse T cell hybrids to HIV-productive infection

Interspecies human x mouse cell hybrids were used to investigate the genetic basis of human permissivity to HTLV-IIIB infection. T cell hybrids between the mouse BW 51.47 T lymphoma line and normal, PHA-IL-2 activated, human peripheral mononuclear cells (PBMCs) were generated. These hybrids preferentially segregated human chromosomes, as assessed by phenotype and karyotype analysis. Viral integration occurred only in those hybrids expressing CD4+ at the cell surface. However, infectious progeny production was demonstrated only in two of the three CD4+ hybrids tested. By segregation analysis, we could correlate the absence of human chromosomes 1, 3, and 9 with the lack of infectious viral progeny.

TAR loop-dependent human immunodeficiency virus trans activation requires factors encoded on human chromosome 12

The trans-activator response region (TAR) RNA in the human immunodeficiency virus type 1 (HIV-1) and HIV-2 long terminal repeat forms stem-loop secondary structures in which the loop sequence is essential for trans activation. We investigated how the HIV trans-activation mechanism encoded on human chromosome 12 relates to the TAR RNA loop-dependent pathway. DNA transfection experiments showed that trans activation in human-hamster hybrid cells with the single human chromosome 12 and human T-cell lines was highly dependent on the native sequences of the HIV-1 TAR loop and the HIV-2 5' TAR loop. In nonhuman cell lines or hybrid cells without chromosome 12 that supported trans activation, the cellular mechanism was independent of the HIV-1 TAR loop and the response to mutations in the HIV-2 TAR loops differed from that found in human T-cell lines and human-hamster hybrid cells with chromosome 12. Our results suggest that the human chromosome 12 mechanism interacts directly with the TAR RNA loop or indirectly by regulating TAR RNA-binding proteins.

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.

The blocks to human immunodeficiency virus type 1 Tat and Rev functions in mouse cell lines are independent

Rodent cells present two blocks precluding the expression of the human immunodeficiency virus type 1 (HIV-1) genome. First, the viral protein Tat is only poorly active in these cells. Second, when the HIV-1 provirus is integrated in the genome of mouse cells, it electively fails to express the viral structural proteins, indicating a block to Rev action. Both defects can be complemented by fusion of the infected mouse cells with uninfected human cells. Because the production of high levels of Rev is dependent on Tat-mediated transactivation and because both Tat and Rev bind the viral transcript, it has been hypothesized that the two blocks found in rodent cells might be linked. In the present work, we demonstrate that overexpression of Rev in mouse cell lines does not relieve their block in HIV-1 structural-gene expression. In addition, we show that this defect is also present in human-mouse cell hybrids which contain human chromosome 12 and support Tat function. On that basis, we conclude that the blocks to HIV-1 Tat and Rev action in mouse cell lines are independent and result from the absence of distinct cellular elements that are critical for HIV-1 gene expression.

The negative effect of human immunodeficiency virus type 1 Nef on cell surface CD4 expression is not species specific and requires the cytoplasmic domain of CD4

The nef gene product of human immunodeficiency virus type 1 has been shown to induce CD4 downregulation from the surface of human cells. To determine if this effect is species specific, we used a retroviral vector to transduce the human immunodeficiency virus type 1 nef gene into murine cells expressing human, chimpanzee, or murine CD4. Our results indicate that Nef induces cell surface downregulation of all three molecules. We also determined that Nef is functional in murine T cells and induces downregulation of both murine CD4 and CD8 (Ly-2) from the cell surface. In contrast, Nef does not downregulate cell surface expression of human CD8 in either murine or human cells. By using a mutant of human CD4 lacking its cytoplasmic domain and a human CD4/CD8 chimera, we determined that the cytoplasmic domain of CD4 is required for its downregulation by Nef. Transduction with a control vector had no effect on CD4 cell surface levels, indicating that retroviral transduction by itself has no significant effect on the cell surface levels of CD4. These results show that the observed downregulation of CD4 by Nef is independent of human-specific factors, is not species specific, and requires the cytoplasmic domain of CD4.

Pathogenesis of human immunodeficiency virus infection

The lentivirus human immunodeficiency virus (HIV) causes AIDS by interacting with a large number of different cells in the body and escaping the host immune response against it. HIV is transmitted primarily through blood and genital fluids and to newborn infants from infected mothers. The steps occurring in infection involve an interaction of HIV not only with the CD4 molecule on cells but also with other cellular receptors recently identified. Virus-cell fusion and HIV entry subsequently take place. Following virus infection, a variety of intracellular mechanisms determine the relative expression of viral regulatory and accessory genes leading to productive or latent infection. With CD4+ lymphocytes, HIV replication can cause syncytium formation and cell death; with other cells, such as macrophages, persistent infection can occur, creating reservoirs for the virus in many cells and tissues. HIV strains are highly heterogeneous, and certain biologic and serologic properties determined by specific genetic sequences can be linked to pathogenic pathways and resistance to the immune response. The host reaction against HIV, through neutralizing antibodies and particularly through strong cellular immune responses, can keep the virus suppressed for many years. Long-term survival appears to involve infection with a relatively low-virulence strain that remains sensitive to the immune response, particularly to control by CD8+ cell antiviral activity. Several therapeutic approaches have been attempted, and others are under investigation. Vaccine development has provided some encouraging results, but the observations indicate the major challenge of preventing infection by HIV. Ongoing research is necessary to find a solution to this devastating worldwide epidemic.

TAR-independent replication of human immunodeficiency virus type 1 in glial cells

The molecular mechanisms involved in the replication of human immunodeficiency virus type 1 (HIV-1) may differ in various cell types and with various exogenous stimuli. Astrocytic glial cells, which can support HIV-1 replication in cell cultures and may be infected in vivo, are demonstrated to provide a cellular milieu in which TAR mutant HIV-1 viruses may replicate. Using transfections of various TAR mutant HIV-1 proviral constructs, we demonstrate TAR-independent replication in unstimulated astrocytic cells. We further demonstrate, using viral constructs with mutations in the tat gene and in the nuclear factor kappa B (NF-kappa B)-binding sites (enhancer) of the HIV-1 long terminal repeat, that TAR-independent HIV-1 replication in astrocytic cells requires both intact NF-kappa B moiety-binding motifs in the HIV-1 long terminal repeat and Tat expression. We measured HIV-1 p24 antigen production, syncytium formation, and levels and patterns of viral RNA expression by Northern (RNA) blotting to characterize TAR-independent HIV-1 expression in astrocytic glial cells. This alternative regulatory pathway of TAR-independent, Tat-responsive viral production may be important in certain cell types for therapies which seek to perturb Tat-TAR binding as a strategy to interrupt the viral lytic cycle.

A novel transgenic mouse model for the in vivo evaluation of anti-human immunodeficiency virus type 1 drugs

We have developed a binary transgenic mouse system that allows easy in vivo evaluation of new anti-human immunodeficiency virus type 1 (HIV-1) drugs or therapies specifically designed to target the viral transactivator protein (TAT) or long terminal repeat (LTR) functions. This approach consists of a simple genetic cross between an "activator" transgenic mouse expressing the HIV-1-tat gene exclusively to T lymphocytes and a "target" transgenic mouse bearing a silent reporter gene whose expression is under the control of the HIV-1-LTR. As expected, most of the target transgenic animals did not express the reporter gene; on the contrary, all the double-transgenic mice bearing both the activator and target transgenes strongly expressed the TAT-induced reporter gene. The choice of a secreted human alpha 1-antitrypsin variant (alpha 1-AT) as reporter gene readily permits in a single animal the quantitative determination of the plasma level of alpha 1-AT protein before and after anti-LTR or anti-TAT treatments. Such mice may be valuable as new laboratory models for the in vivo evaluation of agents with potential anti-HIV-1 activity.

Human immunodeficiency virus type 1 Tat-mediated trans activation correlates with the phosphorylation state of a cellular TAR RNA stem-binding factor

Protein kinase C (PKC) is involved in the mitogenic stimulation of cell proliferation and has recently been reported to be essential for Tat-mediated trans activation. We have determined that RNA binding of a cellular factor which specifically interacts with the trans-activation response region (TAR) is blocked in cells depleted of PKC activity by chronic phorbol myristate acetate stimulation. We also show that nuclear extracts can be depleted of the cellular TAR-binding factor by in vitro treatment with purified protein phosphatase 2A. Furthermore, TAR RNA-binding activity can be partially restored to depleted nuclear extracts in vitro by addition of PKC. Chimeric constructs in which the Tat protein is artificially tethered to viral RNA show PKC independence for Tat-mediated trans activation. Specific mutations in the TAR RNA stem region which cause reduced binding of host cell factor in vitro also cause reduced Tat-mediated trans activation in vivo. Together, these results suggest that phosphorylation-dependent binding of a cellular cofactor to TAR RNA is an essential step in Tat-mediated trans activation. Deciphering the regulation of Tat-mediated trans activation by phosphorylation will be critical in fully understanding the regulation of human immunodeficiency virus type 1 activation.

Human chromosome 12 is required for optimal interactions between Tat and TAR of human immunodeficiency virus type 1 in rodent cells

Levels of trans activation of the human immunodeficiency virus type 1 long terminal repeat (HIV-1 LTR) by the virally encoded transactivator Tat show marked species-specific differences. For example, levels of transactivation observed in Chinese hamster ovary (CHO) rodent cells are 10-fold lower than those in human cells or in CHO cells that contain the human chromosome 12. Thus, the human chromosome 12 codes for a protein or proteins that are required for optimal Tat activity. Here, the function of these cellular proteins was analyzed by using a number of modified HIV-1 LTRs and Tats. Neither DNA-binding proteins that bind to the HIV-1 LTR nor proteins that interact with the activation domain of Tat could be implicated in this defect. However, since species-specific differences were no longer observed with hybrid proteins that contain the activation domain of Tat fused to heterologous RNA-binding proteins, optimal interactions between Tat and the trans-acting responsive RNA (TAR) must depend on this factor(s).

Transcellular activation of the human immunodeficiency virus type 1 long terminal repeat in cocultured lymphocytes

One of the unexplained aspects of the progression of AIDS is that immunological abnormalities are detectable before CD4+ T-helper cell depletion occurs (A.R. Gruters, F.G. Terpstra, R. De Jong, C.J.M. Van Noesel, R.A.W. Van Lier, and F. Miedema, Eur. J. Immunol. 20:1039-1044, 1990; F. Miedema, A.J. Chantal-Petit, F.G. Terpstra, J.K.M.E. Schattenkerk, F. de Wolf, B.J.M. Al, M. Roos, J.M.A. Lang, S.A. Danner, J. Goudsmit, and P.T.A. Schellekens, J. Clin. Invest. 82:1908-1914, 1988; G.M. Shearer, D.C. Bernstein, K.S. Tung, C.S. Via, R. Redfield, S.Z. Salahuddin, and R.C. Gallo, J. Immunol. 137:2514-2521, 1986). In this report, we describe a mechanism by which human immunodeficiency virus type 1 (HIV-1)-infected cells can influence neighboring HIV-1-infected T lymphocytes and uninfected T cells as well. We have examined the interaction of T-cell and macrophage cell lines that are transfected with HIV-1 DNA by using cocultured lymphocytes. The HIV-1 constructs we used lack a functional pol gene and therefore do not produce infectious virus. Cocultivation results in the transcellular activation of the HIV long terminal repeat in the cocultured T cells. This transcellular activation is evident in as little as 3 h of cocultivation, at ratios of HIV-expressing cells to target cells as low as 1:1,000, and is dependent on the Tat-responsive element. The demonstration that a small number of HIV-expressing cells can affect a large number of uninfected bystander cells in a short period of time suggests a mechanism by which global immune dysfunction can precede the high prevalence of infected cells.

Cytokine augmentation of HIV-1 LTR-driven gene expression in neural cells

The induction of human immunodeficiency virus type 1 (HIV-1) gene expression by cytokines was investigated in cells of central nervous system origin. These were human neuroblastoma, glioblastoma, and astrocytoma cell lines, a murine oligodendroglioma and primary murine astrocyte cultures. The cytokines used were tumor necrosis factor alpha (TNF alpha), interleukin-1 beta (IL-1 beta), IL-6, and interferons alpha and gamma (IFN alpha, gamma). Transient transfection of cells with a chloramphenicol acetyltransferase (CAT) reporter gene under the control of the HIV-1 long terminal repeat (LTR) showed significant augmentation following treatment by particular cytokines. TNF alpha was found to augment HIV LTR-directed CAT activity in all cell types. IL-1 beta also activated the HIV LTR reporter gene in glioblastoma, astrocytoma, and astrocyte cells. IL-6 enhanced HIV gene expression in one example only, the primary astrocyte cultures. The interferons generally suppressed expression from the LTR except IFN gamma which produced a twofold rise in the murine glial cells and IFN alpha augmenting expression in one neuroblastoma cell line. No synergy was observed between pairs of activating cytokines tested. The HIV tat gene product was found to be functional in all cells, cotransfection of a tat expression vector transactivating expression from the LTR, with varying degrees of efficiency. In some cell lines the combination of an activating cytokine and tat resulted in an enhancement above that obtained by cotransfection of tat alone. In others, the level of CAT activity did not significantly change. Analysis of nuclear extracts from cytokine-treated cells further implicated the involvement of NFKB in the induction of HIV-1 gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of HIV transcription by Tat

Recent studies suggest that the human immunodeficiency virus transactivator, Tat, increases expression of viral genes primarily by enhancing the efficiency of transcriptional elongation. The degree to which Tat influences elongation may depend on the rate of transcriptional initiation. Current models in which Tat interacts with the transcription complex suggest directions for future studies.

Binding of a host cell nuclear protein to the stem region of human immunodeficiency virus type 1 trans-activation-responsive RNA

Human immunodeficiency virus type 1 (HIV-1) transcription is regulated by both viral and host cell factors. Although the viral trans-activator protein, Tat, and its cis-responsive element, trans-activation-responsive (TAR) RNA, have been identified and characterized, the mechanism of HIV-1 transcriptional regulation has not been satisfactorily described. Whereas Tat is necessary to activate transcription, additional factors, derived from the host cell, are important in regulating HIV-1 transcription. To identify such host cell-specific factors, we used an RNase protection mobility shift assay and UV cross-linking to detect a 140-kDa HeLa cell nuclear protein that binds specifically to TAR RNA. By extensive mutational analysis, we determined that the binding of this protein is dependent on both the sequence and the structure of the TAR RNA stem region. Other groups have shown that the production of prematurely terminated transcripts from the HIV-1 promoter is also dependent on the sequence and structure of the TAR RNA stem. This correlation with our results suggests that the TAR RNA stem-binding protein is involved in the production of prematurely terminated transcripts from the HIV-1 promoter and in the regulation of HIV-1 gene expression.

HIV regulatory proteins: potential targets for therapeutic intervention

With the incidence of HIV infection on the rise worldwide, it is obvious that new approaches must be taken to halt the spread of disease. Unfortunately, this is no easy task; of all retroviruses studied to date HIV remains the most complex in terms of genomic organization, regulation of gene expression, and replication. However, as the mechanism of action of the unique viral regulatory proteins is deciphered, new windows of opportunity for attacking the virus like cycle are opened. The essential regulatory function served by both Tat and Rev transacting regulatory proteins makes them attractive targets for prophylactic and therapeutic intervention. This review will focus on our current understanding of Tat and Rev function.

The establishment of rodent cell lines persistently producing HIV-1

Animal cells differ in susceptibility to HIV-1 infection. To identify rodent cells which are permissive to HIV-1 replication, we transfected murine and rat cells with an infectious clone of HIV-1 and a vector containing the chloramphenicol acetyl transferase gene under the control of HIV-1 LTR. Three groups of transfectants were distinguished: (i) Cells which permit neither HIV-1 LTR activation nor viral protein expression; (ii) Cells which permit activation of the HIV-1 LTR but not HIV-1 protein expression; and (iii) Cells which are fully permissive to both HIV-1 LTR activation and virus production. The latter included rat embryonal fibroblastoid (Rat2) cells, which, in short-term transfection assays, produced titers of HIV-1 proteins similar to transfected T lymphoid cells. To establish persistently infected cells, Rat2 cells were stably transfected with a plasmid containing an infectious clone of HIV-1/N1T-A and a neo gene, yielding several G-418-resistant, HIV-1-producing cell cultures. Of these, Rat2/A1 and Rat2/A2 cell cultures expressed up to 60 ng HIV-1 p24 core antigen per 1 x 10(6) cells 3 days after cell subculture over a period of 3 months. Southern blot hybridization revealed that Rat2/A1 and Rat2/A2 carried one to two HIV-1 DNA copies per cell; no rearrangements or deletions in viral DNA were present. Restriction endonuclease analysis of HIV-1 DNA in Rat2/A2 cells suggested clonal expansion of cells containing integrated HIV-1 genome. Virus produced by the Rat2/A1 cells was infectious in human T cells. These data demonstrate that some rodent cells have no inherent restriction to persistent and efficient production of infectious HIV-1.

Human chromosome-dependent and -independent pathways for HIV-2 trans-activation

Human immunodeficiency virus (HIV types 1 and 2) replication is controlled by the interaction of viral-encoded regulatory proteins and host cellular proteins with the viral long terminal repeat (LTR). The presence of HIV-1 and HIV-2 trans-activator proteins, tat1 and tat2, respectively, greatly increases viral gene expression from their homologous LTRs. It is unclear if the cellular factors that support tat1-directed trans-activation of the HIV-1 LTR are the same for tat2 trans-activation of the HIV-2 LTR. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV-1 and HIV-2 tat-directed transactivation. DNA transfection experiments showed that the presence of human chromosome 12 in human-hamster hybrid clones was necessary for high-level tat-directed trans-activation of the HIV-1 and -2 LTR. Cross-trans-activation of the HIV-2 LTR by tat1 was found to be chromosome 12 independent. In addition, chromosome 12 did not support trans-activation of another human retrovirus (human T-cell leukemia virus type I). Our results suggest that HIV-1 and -2 have evolved to employ a cellular pathway(s) encoded on human chromosome 12 for supporting homologous tat-directed trans-activation. Trans-activation of the HIV-2 LTR by tat1 in chromosome 12-minus cells suggests that multiple cellular pathways can be recruited to trans-activate the HIV-2 LTR and that these pathways may have been important in an HIV-like progenitor virus.

Isolation of a cellular protein that binds to the human immunodeficiency virus Tat protein and can potentiate transactivation of the viral promoter

The human immunodeficiency virus type 1 Tat protein is a powerful transactivator of the viral long terminal repeat (LTR). We have identified a cellular protein that strongly binds to Tat and can complement Tat transactivation in rodent cells. The cellular protein of about 36 kDa was isolated from extracts of human cells by Tat peptide-affinity chromatography and can form a complex with Tat in vitro. Tat transactivation is inefficient in rodent cells microinjected or transfected with the reporter plasmid pHIV-LTRCAT plus the Tat-expressing plasmid pCV-1. Remarkably, coinjection of purified 36-kDa protein with pHIV-LTRCAT plus pCV-1 stimulated Tat transactivation 2.7- to 4.9-fold. Taken together, our findings suggest that the 36-kDa protein may be a transcription factor or modulator that is important for efficient Tat transactivation.

Construction of a replication-competent murine retrovirus vector expressing the human immunodeficiency virus type 1 tat transactivator protein

A replication-competent Akv murine leukemia virus-based vector encoding the human immunodeficiency virus tat cDNA under control of the simian virus 40 early promoter sequences was constructed. The simian virus 40 tat sequences were placed within the U3 region of the 3' long terminal repeat. The resulting virus, derived by transfection, replicated efficiently in mouse NIH 3T3 cells and maintained the tat cDNA insert. It has been suggested that Tat function requires the presence of a human-specific cofactor, which is absent in murine cells. However, infection of murine cells with the Akv virus encoding tat resulted in significant transactivation of a human immunodeficiency virus long terminal repeat-driven reporter gene, indicating that human cofactors are not always required for Tat function. The vector system described may be useful for introduction of foreign genes in vivo and in whole animals when virus spread is required for efficient infection and levels of gene expression.

Genetic regulation of human immunodeficiency virus

Human immunodeficiency virus (HIV) has a complex life cycle in which both cellular and virus-encoded factors participate to determine the level of virus production. Two of the viral genes, tat and rev, are essential for virus replication and encode novel trans-activators that interact specifically with their cognate RNA target elements. Elucidation of their mechanisms of action is likely to expand our knowledge of gene regulation at transcriptional and posttranscriptional levels in the eukaryotic cell. Several viral genes (vif, vpu, and vpr) facilitate virus infection and/or release and may play a role in target cell tropism and infection in vivo. The functions of yet other viral genes (nef, vpt) remain unclear. Recent data also suggest that the tat gene product may have a role in HIV pathogenesis that goes beyond trans-activating virus expression. It can potentially impact on uninfected cells as a diffusible molecule and alter the growth of different cell types.

Characterization of a human TAR RNA-binding protein that activates the HIV-1 LTR

Human immunodeficiency virus type 1 (HIV-1) gene expression is activated by Tat, a virally encoded protein. Tat trans-activation requires viral (trans-activation--responsive; TAR) RNA sequences located in the R region of the long terminal repeat (LTR). Existing evidence suggests that Tat probably cooperates with cellular factors that bind to TAR RNA in the overall trans-activation process. A HeLa complementary DNA was isolated and characterized that encodes a TAR RNA-binding protein (TRBP). TRBP activated the HIV-1 LTR and was synergistic with Tat function.

Cellular factors regulate transactivation of human immunodeficiency virus type 1

It is hypothesized that the immediate-early (IE) gene products of human cytomegalovirus (CMV) and the transactivator (TAT) of human immunodeficiency virus type 1 (HIV-1) regulate HIV-1 gene expression through mechanisms involving host cell factors. By using transient transfection assays with the gene for chloramphenicol acetyltransferase (CAT) under the transcriptional control of the HIV-1 long terminal repeat (LTR), we examined transactivation of the LTR by plasmids that express either the HIV-1 gene for TAT or human CMV IE. The ratio of the level of transactivation by CMV IE to the level of transactivation by TAT varied up to 1,000-fold between cell types. The difference in the activities of these transactivators in various cell types was not a consequence of differential expression of the transactivator gene. Analysis of RNA species initiated in the HIV-1 LTR supports the conclusion that cellular factors regulate the level of elongation of the transcription complex on the LTR. Furthermore, evidence that in some cell types the predominant mechanism of transactivation by HIV-1 TAT involves posttranscriptional processes is presented.