Subcellular localization of the product of the long open reading frame of human T-cell leukemia virus type I

Sequence requirements for nucleolar localization of human T cell leukemia virus type I pX protein, which regulates viral RNA processing

The posttranscriptional regulator (p27x-III) of human T cell leukemia virus type I (HTLV-I) is located predominantly in the cell nucleolus. A highly basic amino-terminal sequence (NH2-Met-Pro-Lys-Thr-Arg-Arg-Arg-Pro-Arg-Arg-Ser-Gln-Arg-Lys-Arg-Pro-Pro -Thr- Pro) in this protein, when fused to the amino termini of beta-galactosidase and p40x of HTLV-I, acts as an autonomous signal capable of directing the hybrid proteins to the cell nucleolus.

Subcellular localization of the human immunodeficiency virus trans-acting art gene product

The genome of the human immunodeficiency virus is distinguished from other animal retroviruses by the presence of several additional open reading frames. The protein product of one of these novel genes, which has been termed art or trs, is required for the expression of the virus structural genes but not for the expression of virus encoded regulatory proteins. Immunocytochemistry and subcellular fractionation demonstrate that the art protein is located predominantly in the nucleus. Therefore, any proposed mechanism for the function of art is likely to involve nuclear events.

Human T-cell leukemia virus types I and II exhibit different DNase I protection patterns

Human T-cell leukemia virus types I (HTLV-I) and II (HTLV-II) are human retroviruses which normally infect T-lymphoid cells. HTLV-I infection is associated with adult T-cell leukemia-lymphoma, and HTLV-II is associated with an indolent form of hairy-cell leukemia. To identify potential transcriptional regulatory elements of these two related human retroviruses, we performed DNase I footprinting of both the HTLV-I and HTLV-II long terminal repeats (LTRs) by using extracts prepared from uninfected T cells, HTLV-I and HTLV-II transformed T cells, and HeLa cells. Five regions of the HTLV-I LTR and three regions of the HTLV-II LTR showed protection by DNase I footprinting. All three of the 21-base-pair repeats previously shown to be important in HTLV transcriptional regulation were protected in the HTLV-I LTR, whereas only one of these repeats was protected in the HTLV-II LTR. Several regions exhibited altered protection in extracts prepared from lymphoid cells as compared with HeLa cells, but there were minimal differences in the protection patterns between HTLV-infected and uninfected lymphoid extracts. A number of HTLV-I and HTLV-II LTR fragments which contained regions showing protection in DNase I footprinting were able to function as inducible enhancer elements in transient CAT gene expression assays in the presence of the HTLV-II tat protein. The alterations in the pattern of the cellular proteins which bind to the HTLV-I and HTLV-II LTRs may in part be responsible for differences in the transcriptional regulation of these two related viruses.

Subnuclear localization of the trans-activating protein of human T-cell leukemia virus type I

Human T-cell leukemia virus type I is associated with human lymphoid malignancies. The p40xI protein encoded by the x gene of this virus is believed to play some role in virally mediated transformation. This gene is known to encode a transcriptional trans activator which previous studies have shown to be a nuclear protein. Further characterization of the intracellular kinetics of this protein showed that it migrated into the nucleus very soon after synthesis. Within the nucleus, p40xI was distributed almost equally between the nucleoplasm and the nuclear matrix. Given the proposed role of the nuclear matrix in RNA transcription, the association of p40xI with the matrix places it in an appropriate cellular compartment to exercise an effect on transcription.

Binding of host-cell factors to DNA sequences in the long terminal repeat of human T-cell leukemia virus type I: implications for viral gene expression

Efficient expression of human T-cell leukemia virus type I (HTLV-I) genes requires both host and viral proteins and is dependent on DNA sequences in the proviral long terminal repeats (LTRs). We have used DNase I-protection assays (footprinting) to construct a map of protein-DNA interactions over a 250-nucleotide region of the LTR upstream of the start site for viral RNA synthesis. We find that a host factor (host expression factor 1, or HEF-1) binds to the imperfect 21-nucleotide repeats that have previously been implicated in HTLV-I gene expression. HEF-1 binding activity is present in preparations from both lymphoid and nonlymphoid cell lines. However, the boundaries of the protected regions and the presence of a flanking DNase-hypersensitive site vary with cell type. Several regions of binding are detected in addition to the HEF-1 sites, including a complex group of sites 40-90 nucleotides upstream of the RNA start site. A comparison of HTLV-I-transformed T lymphocytes that do and do not express the viral trans-activating protein p40xI shows that none of the observed features of the DNase I footprint pattern correlate directly with the presence of this protein in the extract. These results suggest (i) that the primary recognition of promoter elements in the HTLV-I LTR involves specific interactions with host-cell proteins and (ii) that p40xI influences the activity of one or more of these proteins, rather than interacting directly with the DNA.

The tat gene of human T-lymphotropic virus type 1 induces mesenchymal tumors in transgenic mice

Human T-lymphotropic virus type 1 (HTLV-1) is a suspected causative agent of adult T-cell leukemia. One of the viral genes encodes a protein (tat) that not only results in transactivation of viral gene expression but may also regulate the expression of certain cellular genes that are important for cell growth. Transgenic mice that expressed the authentic tat protein under the control of the HTLV-1 long terminal repeat were generated, and cell types that are permissive for the viral promoter and the effects of the tat gene on these cells were studied. Three of eight founder mice with high levels of expression of the transgene in muscle were bred and then analyzed. All developed soft tissue tumors at multiple sites between 13 to 17 weeks of age. This phenotype was transmitted to nine of nine offspring that inherited the tat gene and were available for analysis. The remaining five founders expressed the transgene in the thymus, as well as in muscle. This second group of mice all exhibited extensive thymic depletion and growth retardation; in all of these mice, death occurred between 3 to 6 weeks of age before tumors became macroscopically visible. The tat gene under the control of the HTLV-1 regulatory region showed tissue-specific expression and the tat protein efficiently induced mesenchymal tumors. The data establish tat as an oncogenic protein and HTLV-1 as a transforming virus.

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

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

Expression of a cDNA clone corresponding to the long open reading frame (XBL-I) of the bovine leukemia virus

Nucleotide sequence analysis of a cDNA clone corresponding to the XBL-I open reading-frame of bovine leukemia virus (BLV) revealed that the AUG initiation codon was located 44 bases downstream from that of the env gene and was part of the p34x mRNA splice donor. . .ATGG/GTAA at the end of the pol gene sequence. RNA from this clone was synthesized in vitro by the SP6 RNA polymerase and translated into a 34,000 mol wt protein in rabbit reticulocyte lysates. The protein (p34x) is recognized in Western blots by most sera of BLV-infected sheep and tumor-bearing cattle, by an anti-synthetic peptide rabbit serum, and by the serum of a rabbit immunized by XBL-I RNA programmed reticulocyte lysates. Both sera react with a 34,000 mol wt protein present in nuclei of BLV-infected cells.

Human protein binding to DNA sequences surrounding the human T-cell lymphotropic virus type-I long terminal repeat polyadenylation site

The long terminal repeats (LTRs) of RNA tumor viruses, including human T-cell lymphotropic virus type I (HTLV-I), contain the control elements for expression of viral genes. Sequence-specific LTR-DNA-binding proteins could regulate viral functions. To search for such proteins we have used an in vitro non-denaturing polyacrylamide gel assay, with restriction fragments of the HTLV-I LTR and nuclear protein extracts from several HTLV-I-infected cell lines and an uninfected T-cell line, H9. Four DNA-binding activities were observed, including non-specific DNA-binding activity and at least two activities (forms I and II) which bind specifically to a HinfI restriction fragment from nucleotides +181 to +334 relative to the transcription start site. DNA-binding activities I and II were partially resolved by ion-exchange chromatography and mapped by protection experiments to two 10-20-bp blocks surrounding the polyadenylation site at +221. Of the cell lines tested, form II was abundantly found in C10/MJ, and forms I and IV were also found in C91/PL, C81-66-45, MT2 and H9 cells.

Bovine leukemia virus transcription is controlled by a virus-encoded trans-acting factor and by cis-acting response elements

Bovine leukemia virus (BLV) gene expression is exquisitely regulated at multiple levels, including a transcriptional control effected by virus-encoded trans-acting factors and cis-acting target sequences. Like the human T-cell leukemia viruses type I and type II, but unlike other RNA tumor viruses, BLV contains several open reading frames at the 3' end of its genome. A subgenomic mRNA which encodes two overlapping reading frames from this region could produce proteins of 38 and 18 kilodaltons (kDa). A series of cis-trans experiments using transfected virus gene constructs in different combinations revealed that expression of the 38-kDa protein was both necessary and sufficient to activate, in trans, the BLV promoter. This activation was specific for the BLV long terminal repeat, as a variety of related retroviral promoters were not responsive to the expression of the 38-kDa protein p38(XBL). Deletion analysis and construction of chimeric promoters identified a 75-base-pair long terminal repeat region which functions like a p38(XBL)-dependent enhancer element.

Activation of interleukin 2 and interleukin 2 receptor (Tac) promoter expression by the trans-activator (tat) gene product of human T-cell leukemia virus, type I

Cotransfection of cDNA encoding the trans-activator gene product of human T-cell leukemia virus, type I (HTLV-I) (tat-I), which acts in trans to augment viral gene expression, has revealed strong regulatory effects of this viral protein on the inducible cellular promoters governing human interleukin 2 (IL-2) and IL-2 receptor (Tac) gene expression. The tat-I protein stimulates a 3- to 6-fold increase in IL-2 receptor (Tac) promoter activity in transfected Jurkat T cells, but not in the natural killer-like YT cell line, as measured by changes in the expression of the chloramphenicol acetyltransferase (CAT; EC 2.3.1.28) reporter gene linked to this promoter. In contrast, tat-I alone has little or no effect on IL-2 promoter activity in Jurkat T cells but markedly synergizes with other mitogenic stimuli (phytohemagglutinin, phorbol 12-myristate 13-acetate, or the OKT3 monoclonal antibody), which alone are ineffective. The tat-I protein also partially circumvents the pronounced inhibitory effects of cyclosporin A on the IL-2 promoter. Other cellular and viral promoters are unaffected by the tat-I gene product, either alone or in combination with other mitogens. The specific effects of the tat-I gene product on the IL-2 and IL-2 receptor (Tac) promoters suggest the possibility of an autocrine or paracrine mechanism of T-cell growth as an early event in HTLV-I-mediated leukemogenesis.

Multiple sequence elements are required for regulation of human T-cell leukemia virus gene expression

The U3 region of the long terminal repeat (LTR) of human T-cell leukemia virus type I (HTLV-I) contains sequences that respond to the trans-activating transcription (tat) factor encoded by the pX region of the provirus. Results presented here show that there are multiple tat-responsive sequences within the LTR and that a single 21-nucleotide sequence, which is repeated three times within the U3 region, is sufficient to determine the response to the trans-activator. This sequence is capable of conferring a tat-responsive phenotype upon the HTLV-I and simian virus 40 promoters, independent of orientation. Sequences required for efficient HTLV-I LTR-directed gene expression are also located 3' to the site of RNA initiation, within the R and U5 regions of the LTR.

Evidence for aberrant activation of the interleukin-2 autocrine loop by HTLV-1-encoded p40x and T3/Ti complex triggering

In this study we provide evidence that distinct DNA sequences within the 5'-flanking regions of the genes for interleukin-2 (IL-2) and its receptor (IL-2R) are involved in human T-cell-specific activation of transcription by p40x, a product of human T cell leukemia virus type I (HTLV-1). The same DNA sequences appear to be responsible for induction of the genes in a T cell line, Jurkat, by mitogens. Although the IL-2 gene sequences are activated by p40x with much lower efficiency than the IL-2R gene sequences, they are synergistically activated by the p40x expression and subsequent extracellular stimulation by Concanavalin-A or anti-T3. We propose a model for two-step activation of the IL-2 autocrine loop in ATL development.

Plant-derived diterpene esters enhance HTLV-I-induced colony formation of lymphocytes in co-culture

The addition to culture dishes of 10-50 ng/ml of the essential diterpene ester of Sapium sebiferum, 12-O-hexadecanoylphorbol-13-acetate (HPA), increased colony formation of normal peripheral blood lymphocytes co-cultured with gamma-irradiated HTLV-I-producing HUT 102 cells. The cells in the stimulated colonies showed an approximately 3-fold increase in the expression of interleukin-2 (IL-2) receptors and a 1.5- to 2.0-fold increase in human T-lymphotropic virus type- I (HTLV-I) p19-positive cells. This biological potency was analogous to that induced by the most potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and stronger than that of 12-O-hexadecanoyl-16-hydroxyphorbol-13-acetate (HHPA) isolated from Aleurites fordii.

A sequence homology between the pX genes of HTLV-I/II and the murine IL-3 gene

Searching the protein sequence database for amino acid sequences homologous to the x-lor sequence in the pX region of human T-cell leukemia virus types I and II (HTLV-I/II), we found that there is a region of 38 amino acids where the murine interleukin 3 (IL-3) sequence has a 40% homology with the x-lor sequence. A statistical analysis shows that this homology is highly significant with a probability of 1.57 X 10(-10). The biological implication of this homology is discussed.

Dynamic and nonspecific dispersal of human T-cell leukemia/lymphoma virus type-I integration in cultured lymphoma cells

The progression of HTLV-I proviral integration over a 3-year period of in vitro culture was examined in two human lymphoma lines, Hut 102 and MJ. Using specific HTLV-I molecular clones and a Southern analysis at different cell passages, Hut 102 increased from 2 to 19 integrated proviral integrations while MJ increased to at least 25 different integrations by passage 43. During the progress of increased superinfection and novel integration in vitro some of the previous proviral integrations were lost from the cultures. The 19 integrations of late passage Hut 102 cells were shown to be dispersed to 19 different human chromosomes by analysis of 34 distinct rodent X Hut 102 somatic cell hybrids which segregated human chromosomes (and included proviral integrations) in different combinations. The two primary integrations in Hut 102 were located on human chromosomes 4 and 20, respectively. A similar pattern of nonspecific integration was observed in somatic cell hybrid analysis of the 25 proviral integrations of MJ. The dynamic infection-reintegration process in vitro revealed in these studies may confuse experimental verification of potential cis acting functions of HTLV-I in the as yet poorly understood mechanism of neoplastic transformation.

Expression of the complete human T-cell leukemia virus type I pX coding sequence as a functional protein in Escherichia coli

Human T-cell leukemia virus type I (HTLV-I), a virus associated with adult T-cell leukemia, contains a long open reading frame (LOR) in the 3' end of its genome between the env region and the 3' long terminal repeat (LTR). This open reading frame encodes a 40-kDa protein (designated p40x) that has been implicated as a positive control element for transcription from the HTLV-I LTR in a phenomenon known as trans-activation. We now report the expression of the complete p40x coding sequence as a 40-kDa protein in Escherichia coli. The p40x protein produced in bacteria is shown, using the protoplast fusion technique, to possess biological activity by its ability to trans-activate a HTLV-I LTR-chloramphenicol acetyltransferase plasmid that is stably integrated into the genome of mouse L cells. This stimulatory activity could be detected within 2 hr after fusion, suggesting the possibility of a direct role for p40x in trans-activation of the HTLV-I LTR. The production of p40x in large quantities in E. coli, together with the rapid protoplast fusion assay for its biological activity, should facilitate the analysis of p40x mutants and the elucidation of the molecular mechanism of trans-activation.

Human T-cell leukemia virus-associated nuclear antigen (HTLV-NA)

Replication-competent retroviruses are not known to encode or induce nuclear antigens that are immunogenic in their natural hosts. We describe here the detection of a human T-lymphotropic virus (type I and type II) associated nuclear antigen (HTLV-NA) by an anticomplement immunofluorescence assay. Antibody to HTLV-NA is detected in 18 of 68 (26%) HTLV-I seropositives.

Transactivation induced by human T-lymphotropic virus type III (HTLV III) maps to a viral sequence encoding 58 amino acids and lacks tissue specificity

The acquired immune deficiency syndrome (AIDS) retrovirus, HTLV-III/LAV, encodes a transacting factor which directly or indirectly stimulates the expression of genes linked to its LTR. To further dissect this phenomenon, we have cotransfected a biologically active molecular clone of HTLV-III and a recombinant plasmid containing an indicator gene, the bacterial gene for chloramphenicol acetyltransferase (CAT), under the control of the HTLV-III LTR. Amplified CAT activity was detected in both lymphoid cells and fibroblasts from a number of species in the presence of the proviral DNA. Deletion experiments confirm the previous assignment of the gene required for transactivation to a region immediately 5' to the envelope gene, and further narrow down the critical functional domain to a coding sequence of 58 codons.

Identification of the gene responsible for human T-cell leukaemia virus transcriptional regulation

Human T-cell leukaemia viruses (HTLVs) have genomic organization distinct from that of other replication-competent retroviruses, possessing four genes, gag, pol, env and chi. The unique fourth gene, chi (also referred to as lor), is located between env and the 3' long terminal repeat (LTR), encoding a protein of relative molecular mass 40,000 for HTLV-I and 37,000 for HTLV-II, located in the nucleus of infected cells. HTLV-I is the causative agent of adult T-cell leukaemia (ATL), a T-lymphocyte malignancy, while HTLV-II has been found associated with a T-cell variant of hairy cell leukaemia. Both viruses immortalize T cells in vitro. However, the mechanism of cellular transformation induced by HTLV is not known as there seems to be no common site of provirus integration in primary ATL cells and the virus contains no classical oncogene sequences. These observations have provoked speculation that the unique and strongly conserved chi protein (85% amino-acid homology between HTLV-I and -II) is involved in HTLV leukaemogenesis. Recent mutagenesis experiments in our laboratory have shown that the chi gene is essential for HTLV replication. It has also has been shown that the LTRs of HTLV and the related bovine leukaemia virus (BLV) are activated in trans in virus-infected cells, and, although such experiments did not directly demonstrate a role for the chi protein in transcriptional activation, it has been suggested that the chi protein is responsible for the transcriptional activation of the LTR and may be involved in cellular transformation. We have now developed a transient co-transfection assay which demonstrates that transcriptional activation of the HTLV LTR is mediated solely by the chi protein and that no other virus genes are required.

Identification and some biochemical properties of the major XBL gene product of bovine leukemia virus

Using a rabbit antiserum directed against a synthetic oligopeptide whose sequence was deduced from the nucleotide sequence of the XBL gene of bovine leukemia virus, we detected a 38-kDa protein in virus-producing cell lines. In vitro translation of hybrid-selected RNA unequivocally demonstrates that this protein, designated p38(XBL), is indeed encoded by the XBL gene. Unlike the other virus-encoded proteins, however, p38(XBL) resides within the cells without being incorporated into virions. It undergoes no gross post-translational modifications and has a relatively short half-life (5-6 hr) in vivo. Furthermore, cell fractionation combined with pulse-chase experiment reveals that a significant fraction (more than half) of the p38(XBL) localizes to the nucleus of the infected cell after synthesis. We conclude that the XBL gene of bovine leukemia virus is a functional gene encoding a nonvirion protein p38(XBL), which possibly functions within the nucleus of the infected cell to regulate viral or cellular gene expression. p38(XBL) is presumably translated from a doubly spliced, bicistronic mRNA that has the capability to encode another small polypeptide in a different reading frame.

Association of the pX gene product of human T-cell leukemia virus type-I with nucleus

Human T-cell leukemia virus type I (HTLV-I) contains a unique gene pX coding for p40 chi, and this protein was suggested to activate the transcription from the LTR of HTLV. By a similar mechanism, this viral function might be involved in immortalization of T-cells and leukemogenesis in adult T-cell leukemia induced by HTLV-I. In this communication, a part of the p40 chi was found to be tightly associated with nuclei in infected cell lines by subcellular fractionation and immunofluorescence staining.

p27x-III and p21x-III, proteins encoded by the pX sequence of human T-cell leukemia virus type I

Human T-cell leukemia virus type I (HTLV-I) is an etiological agent of adult T-cell leukemia and has a unique sequence, pX, that contains four possible open reading frames, I-IV. p40x was previously identified as the gene product of frame IV (x-lor) and was suggested to mediate transcriptional trans-activation of the viral long terminal repeats. We have identified two pX gene products, p27x-III and p21x-III, encoded by frame III, which mostly overlapped frame IV. These proteins were detected with rabbit antiserum against the synthetic peptide predicted from the 3' end of frame III. p27x-III is phosphorylated in cultured cells, and the phosphoprotein (pp27x-III) is localized in nuclei; some pp27x-III was tightly bound to nuclear components. p27x-III was detected in a number of cell lines that express other viral antigens, including a cell line previously reported to express only p40x as a viral protein. The function(s) of p27x-III and p21x-III is not known, but the tight binding of pp27x-III to nuclear components suggests that it is associated with regulation of viral gene expression.

Functional relation between HTLV-II x and adenovirus E1A proteins in transcriptional activation

The mechanism of cellular transformation by the human T-cell leukemia viruses (HTLV) is thought to involve a novel gene known as the x gene. This gene is essential for HTLV replication and acts by enhancing transcription from the HTLV long terminal repeat. The HTLV x gene product may also cause aberrant transcription of normal cellular genes, resulting in transformation of the infected cells. Although there is no evidence as yet for such a mechanism, it was shown that the HTLV-II x gene product can activate transcription from adenovirus E1A-dependent early promoters and therefore has the potential to activate cellular genes. It was also shown that the adenovirus and herpes pseudorabies immediate early proteins activate expression from the HTLV-I and HTLV-II long terminal repeats, though at lower levels than with the x gene product. These findings indicate possible common mechanisms of action for transcription-regulatory genes of distinct viruses.

Infection of human T-lymphotropic virus type-I (HTLV-I)-bearing MT-4 cells with HTLV-III (AIDS virus): chronological studies of early events

Early events in the infection of the human T-lymphotropic virus type-I (HTLV-I)-positive MT-4 cell line by the acquired immune deficiency syndrome (AIDS) retrovirus HTLV-III were investigated. The virus was adsorbed completely to the cells within 60 min incubation after inoculation of the virus. Then, infected MT-4 cells started to produce the HTLV-III-specific antigens between 12 and 24 hr postinfection. Synthesis of the viral antigens consisting of 120K, 46K, 24K, and 17K polypeptides was suppressed by the treatment of the virus-infected MT-4 cells with cytosine arabinoside (Ara-C) or by the treatment of the virus with anti-HTLV-III-positive sera. The progeny of the virus released from the infected MT-4 cells was titrated by a newly developed plaque-forming assay method and reverse transcriptase activity. The maximum activity of HTLV-III (3 X 10(5) PFU/ml) was observed on Days 4 and 5 p.i. Most of the viral activities in this preparation were ascribed to HTLV-III, and not to HTLV-I. No phenotypic mixing between HTLV-III and HTLV-I was discerned, although MT-4 cells were HTLV-I-producer cell line. Thus, HTLV-III-infected MT-4 cells are thought to be useful in further study of the interaction between host cells and the virus, and appear to be a good viral source for the analysis of the virus.

Selective cytotoxicity of AIDS virus infection towards HTLV-I-transformed cell lines

Previously, we reported that cells of the human T-cell lymphotropic virus type I (HTLV-I)-transformed lines MT-2 and MT-4 were extensively killed by infection with AIDS retrovirus HTLV-III. We have investigated this phenomenon more systematically using light and electron microscopy as well as immunofluorescence. The cell lines used in the present studies included 14 of those carrying not only human HTLV-I but also related simian agents and 6 HTLV-I-negative T- and B-cell lines. The results showed that the cytocidal effects occurred in the HTLV-I-transformed cell lines exclusively and were not present in further subcultures. In these cell lines the cytotoxic response was closely correlated with the induction of HTLV-III antigens after virus infection. However, cells of 6 HTLV-I-free lines were not killed to a marked extent by HTLV-III and were passaged as continuous producers of AIDS virus. Only 2 cell lines were resistant to the cytocidal effect of HTLV-III among 14 HTLV-I carrying cell lines. They were also resistant to the replication of infected HTLV-III. This AIDS virus-specific cytotoxic effect observed in HTLV-I-transformed cell lines did not appear to be associated with gene expression of the gag and pXs region of HTLV-I genomes. This result may indicate that HTLV-III specifically interferes with some steps of HTLV-I transformation.

Location of cis-acting regulatory sequences in the human T-cell leukemia virus type I long terminal repeat

The location of cis-acting regulatory regions within the long terminal repeat (LTR) of the human T-cell leukemia virus type I (HTLV-1) was determined. The sequences present between nucleotides -350 and -55 (cap site +1) contain an enhancer element that is active in lymphoid and nonlymphoid cell lines. The sequences located near the "TATA" and RNA initiation sites contain a promoter, the activity of which can be augmented by homologous and heterologous enhancer elements. A region responsive to trans-acting transcription factors present in HTLV-I- and HTLV type II-infected cells is located between nucleotides -159 and +315. HTLV-I LTR deletion mutants respond in a similar manner both to the trans-acting factors present in infected cells and to the tat protein encoded by the x-lor region of the genome, thus providing further evidence that the tat protein mediates transcriptional trans-activation of the LTR in HTLV-infected cells.

trans-Activation of the human T-cell leukemia virus long terminal repeat correlates with expression of the x-lor protein

Cell lines established directly from adult T-cell leukemia-lymphoma patients or immortalized by human T-cell leukemia virus type I (HTLV-I) in vitro that do not produce complete HTLV virions were characterized both for the content of viral proteins and for the presence of trans-acting factors activating gene expression under the control of the HTLV long terminal repeat. The expression of the 42-kilodalton HTLV x-lor product correlated with trans-activation of the long terminal repeat. The implications of this study for understanding the role of the HTLV x-lor product in the initiation and maintenance of T-lymphocyte transformation are discussed.

The pX protein of HTLV-I is a transcriptional activator of its long terminal repeats

Expression of the pX protein of human T-cell leukemia virus type I (HTLV-I) in animal cells demonstrates that this protein is a specific transcriptional activator of the long terminal repeats (LTR) of HTLV-I. Several other promoters are not affected by pX. No lymphocyte-specific factors are required for this activation. pX can be detected in the nucleus of transfected monkey kidney cells (line CV1) by indirect immunofluorescence. These results indicate that the pX protein is essential for the replication cycle of the virus and that it may be directly involved in the immortalization of human lymphocytes by HTLV-I.

The x gene is essential for HTLV replication

The human T-cell leukemia viruses (HTLV) are associated with T-cell malignancies in man and will transform normal human T cells in vitro. The mechanism of malignant transformation by HTLV is unknown but appears to be distinct from that of other classes of retroviruses, which induce malignant transformation through viral or cellular oncogenes. Recently a new gene, termed x, was identified in HTLV. This gene has been hypothesized to be the transforming gene of HTLV because of its conservation within the HTLV class of retroviruses. By in vitro mutagenesis of the HTLV-II x gene, it is now demonstrated that the presence of a functional x gene product is necessary for efficient HTLV transcription. Therefore, these studies provide direct evidence for an important function of the x gene in HTLV replication. The functional analogies between the x gene and transcriptional regulatory genes of some DNA viruses suggest that these viruses share similar mechanisms for cellular transformation.

Trans-activator gene of human T-lymphotropic virus type III (HTLV-III)

Human T-lymphotropic virus type III (HTLV-III) encodes a trans-acting factor that activates the expression of genes linked to the HTLV-III long terminal repeat. By functional mapping of complementary DNA transcripts of viral messenger RNA's the major functional domain of the gene encoding this factor was localized to a region immediately before the env gene of the virus, a region previously thought to be noncoding. This newly identified gene consists of three exons, and its transcription into messenger RNA involves two splicing events bringing together sequences from the 5' part (287 base pairs), middle (268 base pairs), and 3'part (1258 base pairs) of the HTLV-III genome. A similar messenger RNA with a truncated second exon (70 base pairs) does not encode a trans-acting function. It is proposed that this second messenger RNA is the transcript of a gene (3'-orf) located after the env gene. Messenger RNA's were also identified for the env and gag-pol genes of HTLV-III.

Location of the trans-activating region on the genome of human T-cell lymphotropic virus type III

The retrovirus involved in acquired immune deficiency syndrome (HTLV-III/LAV) contains a region that is necessary for stimulation of gene expression directed by the viral long terminal repeat. This region is located between nucleotides 5365 and 5607, immediately 5' to the envelope gene. A doubly-spliced message containing this region could encode an 86-amino acid protein with structural features similar to those of nucleic acid-binding proteins.

Studies of the putative transforming protein of the type I human T-cell leukemia virus

The putative transforming protein of the type I human T-cell leukemia virus (HTLV-1) is a 40-kilodalton protein encoded by the X region and is termed p40XI. On the basis of both subcellular fractionation techniques and immunocytochemical analysis, it is now shown that p40XI is a nuclear protein with a relatively short half-life (120 minutes). It is synthesized de novo in considerable quantities in a human T-cell line infected with and transformed by the virus in vitro, and it is not packaged in detectable amounts in the extracellular virus.

A transcriptional activator protein encoded by the x-lor region of the human T-cell leukemia virus

Human T-cell leukemia viruses type I and II (HTLV-I and -II) exhibit several features characteristic of this retroviral family: the presence of an x-lor gene encoding a nuclear protein, transformation properties suggesting the involvement of a virus-associated trans-acting factor, and transcriptional trans-activation of the long terminal repeat (LTR) in infected cells. In the study described here the HTL x-lor products, in the absence of other viral proteins, were able to activate gene expression in trans directed by HTLV LTR. The regulation of the expression of particular genes in trans by HTLV x-lor products suggests that they play a role in viral replication and possibly in transformation of T lymphocytes.