Proviral sequences that restrict retroviral expression in mouse embryonal carcinoma cells

Differential control of retrovirus silencing in embryonic cells by proteasomal regulation of the ZFP809 retroviral repressor

Replication of the murine leukemia viruses is strongly suppressed in mouse embryonic stem (ES) cells. Proviral DNAs are formed normally but are then silenced by a large complex bound to DNA by the ES cell-specific zinc-finger protein ZFP809. We show here that ZFP809 expression is not regulated by transcription but rather by protein turnover: ZFP809 protein is stable in embryonic cells but highly unstable in differentiated cells. The protein is heavily modified by the accumulation of polyubiquitin chains in differentiated cells and stabilized by the proteasome inhibitor MG132. A short sequence of amino acids at the C terminus of ZFP809, including a single lysine residue (K391), is required for the rapid turnover of the protein. The silencing cofactor TRIM28 was found to promote the degradation of ZFP809 in differentiated cells. These findings suggest that the stem cell state is established not only by an unusual transcriptional profile but also by unusual regulation of protein levels through the proteasomal degradation pathway.

Transcriptional Silencing of Moloney Murine Leukemia Virus in Human Embryonic Carcinoma Cells

Embryonic carcinoma (EC) cells are malignant counterparts of embryonic stem (ES) cells and serve as useful models for investigating cellular differentiation and human embryogenesis. Though the susceptibility of murine EC cells to retroviral infection has been extensively analyzed, few studies of retrovirus infection of human EC cells have been performed. We tested the susceptibility of human EC cells to transduction by retroviral vectors derived from three different retroviral genera. We show that human EC cells efficiently express reporter genes delivered by vectors based on human immunodeficiency virus type 1 (HIV-1) and Mason-Pfizer monkey virus (M-PMV) but not Moloney murine leukemia virus (MLV). In human EC cells, MLV integration occurs normally, but no viral gene expression is observed. The block to MLV expression of MLV genomes is relieved upon cellular differentiation. The lack of gene expression is correlated with transcriptional silencing of the MLV promoter through the deposition of repressive histone marks as well as DNA methylation. Moreover, depletion of SETDB1, a histone methyltransferase, resulted in a loss of transcriptional silencing and upregulation of MLV gene expression. Finally, we provide evidence showing that the lack of MLV gene expression may be attributed in part to the lack of MLV enhancer function in human EC cells.

IMPORTANCE

Human embryonic carcinoma (EC) cells are shown to restrict the expression of murine leukemia virus genomes but not retroviral genomes of the lentiviral or betaretroviral families. The block occurs at the level of transcription and is accompanied by the deposition of repressive histone marks and methylation of the integrated proviral DNA. The host machinery required for silencing in human EC cells is distinct from that in murine EC cell lines: the histone methyltransferase SETDB1 is required, but the widely utilized corepressor TRIM28/Kap1 is not. A transcriptional enhancer element from the Mason-Pfizer monkey virus can override the silencing and promote transcription of chimeric proviral DNAs. The findings reveal novel features of human EC gene regulation not present in their murine counterparts.

Spotting the enemy within: Targeted silencing of foreign DNA in mammalian genomes by the Krüppel-associated box zinc finger protein family

Tandem C2H2-type zinc finger proteins (ZFPs) constitute the largest transcription factor family in animals. Tandem-ZFPs bind DNA in a sequence-specific manner through arrays of multiple zinc finger domains that allow high flexibility and specificity in target recognition. In tetrapods, a large proportion of tandem-ZFPs contain Krüppel-associated-box (KRAB) repression domains, which are able to induce epigenetic silencing through the KAP1 corepressor. The KRAB-ZFP family continuously amplified in tetrapods through segmental gene duplications, often accompanied by deletions, duplications, and mutations of the zinc finger domains. As a result, tetrapod genomes contain unique sets of KRAB-ZFP genes, consisting of ancient and recently evolved family members. Although several hundred human and mouse KRAB-ZFPs have been identified or predicted, the biological functions of most KRAB-ZFP family members have gone unexplored. Furthermore, the evolutionary forces driving the extraordinary KRAB-ZFP expansion and diversification have remained mysterious for decades. In this review, we highlight recent studies that associate KRAB-ZFPs with the repression of parasitic DNA elements in the mammalian germ line and discuss the hypothesis that the KRAB-ZFP family primarily evolved as an adaptive genomic surveillance system against foreign DNA. Finally, we comment on the computational, genetic, and biochemical challenges of studying KRAB-ZFPs and attempt to predict how these challenges may be soon overcome.

EBP1, a novel host factor involved in primer binding site-dependent restriction of moloney murine leukemia virus in embryonic cells

Mouse embryonic cells are unable to support the replication of Moloney murine leukemia virus (MLV). The integrated viral DNA is transcriptionally silenced, largely due to binding of host transcriptional repressors to the primer binding site (PBS) of the provirus. We have previously shown that a PBS DNA-binding repressor complex contains ZFP809 and TRIM28. Here, we identified ErbB3-binding protein 1 (EBP1) to be a novel component of the ZFP809-TRIM28 silencing complex and show that EBP1 depletion reduces PBS-mediated retroviral silencing.

The murine stem cell virus promoter drives correlated transgene expression in the leukocytes and cerebellar Purkinje cells of transgenic mice

The murine stem cell virus (MSCV) promoter exhibits activity in mouse hematopoietic cells and embryonic stem cells. We generated transgenic mice that expressed enhanced green fluorescent protein (GFP) under the control of the MSCV promoter. We obtained 12 transgenic founder mice through 2 independent experiments and found that the bodies of 9 of the founder neonates emitted different levels of GFP fluorescence. Flow cytometric analysis of circulating leukocytes revealed that the frequency of GFP-labeled leukocytes among white blood cells ranged from 1.6% to 47.5% across the 12 transgenic mice. The bodies of 9 founder transgenic mice showed various levels of GFP expression. GFP fluorescence was consistently observed in the cerebellum, with faint or almost no fluorescence in other brain regions. In the cerebellum, 10 founders exhibited GFP expression in Purkinje cells at frequencies of 3% to 76%. Of these, 4 mice showed Purkinje cell-specific expression, while 4 and 2 mice expressed GFP in the Bergmann glia and endothelial cells, respectively. The intensity of the GFP fluorescence in the body was relative to the proportion of GFP-positive leukocytes. Moreover, the frequency of the GFP-expressing leukocytes was significantly correlated with the frequency of GFP-expressing Purkinje cells. These results suggest that the MSCV promoter is useful for preferentially expressing a transgene in Purkinje cells. In addition, the proportion of transduced leukocytes in the peripheral circulation reflects the expression level of the transgene in Purkinje cells, which can be used as a way to monitor transgene expression properties in the cerebellum without invasive techniques.

Epigenetic silencing of engineered L1 retrotransposition events in human embryonic carcinoma cells

Long INterspersed Element-1 (LINE-1 or L1) retrotransposition continues to impact human genome evolution1,2. L1s can retrotranspose in the germline, during early development, and in select somatic cells3,4,5,6,7,8; however, the host response to L1 retrotransposition remains largely unexplored. Here, we show that reporter genes introduced into the genome of various human embryonic carcinoma-derived cell lines (ECs) by L1 retrotransposition are rapidly and efficiently silenced either during or immediately after their integration. Treating ECs with histone deacetylase inhibitors (IHDACs) rapidly reverses this silencing, and chromatin immunoprecipitation (ChIP) experiments revealed that reactivation of the reporter gene was correlated with changes in chromatin status at the L1 integration site. Under our assay conditions, rapid silencing also was observed when reporter genes were delivered into ECs by mouse L1s and a zebrafish LINE-2 element, but not when similar reporter genes were delivered into ECs by Moloney murine leukemia virus (MMLV) or human immunodeficiency virus (HIV), suggesting these integration events are silenced by distinct mechanisms. Finally, we demonstrate that subjecting ECs to culture conditions that promote differentiation attenuates the silencing of reporter genes delivered by L1 retrotransposition, but that differentiation, per se, is not sufficient to reactivate previously silenced reporter genes. Thus, our data suggest that ECs differ from many differentiated cells in their ability to silence reporter genes delivered by L1 retrotransposition.

Host restriction factors blocking retroviral replication

Retroviruses are highly successful intracellular parasites, and as such they are found in nearly all branches of life. Some are relatively benign, but many are highly pathogenic and can cause either acute or chronic diseases. Therefore, there is tremendous selective pressure on the host to prevent retroviral replication, and for this reason cells have evolved a variety of restriction factors that act to inhibit or block the viruses. This review is a survey of the best-characterized restriction factors capable of inhibiting retroviral replication and aims to highlight the diversity of strategies used for this task.

TRIM28 mediates primer binding site-targeted silencing of Lys1,2 tRNA-utilizing retroviruses in embryonic cells

Murine leukemia viruses (MLVs) and related retroelements are potently restricted in embryonic cells by postintegration transcriptional silencing, likely to protect the germ line from insertional mutagenesis. This silencing is in large part attributable to the presence of a nuclear repression complex, which targets a sequence element of the proviral DNA, the repressor-binding site. The repressor-binding site closely overlaps the tRNA primer binding site, a highly conserved sequence essential for virus replication and defining the site of initiation of DNA synthesis during reverse transcription. We have recently demonstrated that the cellular corepressor TRIM28 is recruited to the proline tRNA primer-binding site used by many MLVs and is required to mediate this silencing. Here, we show that TRIM28 is also required for the restriction of retroviruses using a completely distinct tRNA for the priming of their DNA synthesis, namely Lys-1,2 tRNA. These results generalize the role of TRIM28 in retroviral restriction and suggest that this system has evolved to restrict multiple retroviruses.

The Moloney murine leukemia virus repressor binding site represses expression in murine and human hematopoietic stem cells

The Moloney murine leukemia virus (MLV) repressor binding site (RBS) is a major determinant of restricted expression of MLV in undifferentiated mouse embryonic stem (ES) cells and mouse embryonal carcinoma (EC) lines. We show here that the RBS repressed expression when placed outside of its normal MLV genome context in a self-inactivating (SIN) lentiviral vector. In the lentiviral vector genome context, the RBS repressed expression of a modified MLV long terminal repeat (MNDU3) promoter, a simian virus 40 promoter, and three cellular promoters: ubiquitin C, mPGK, and hEF-1a. In addition to repressing expression in undifferentiated ES and EC cell lines, we show that the RBS substantially repressed expression in primary mouse embryonic fibroblasts, primary mouse bone marrow stromal cells, whole mouse bone marrow and its differentiated progeny after bone marrow transplant, and several mouse hematopoietic cell lines. Using an electrophoretic mobility shift assay, we show that binding factor A, the trans-acting factor proposed to convey repression by its interaction with the RBS, is present in the nuclear extracts of all mouse cells we analyzed where expression was repressed by the RBS. In addition, we show that the RBS partially repressed expression in the human hematopoietic cell line DU.528 and primary human CD34(+) CD38(-) hematopoietic cells isolated from umbilical cord blood. These findings suggest that retroviral vectors carrying the RBS are subjected to high rates of repression in murine and human cells and that MLV vectors with primer binding site substitutions that remove the RBS may yield more-effective gene expression.

Lack of shielding of primer binding site silencer-mediated repression of an internal promoter in a retrovirus vector by the putative insulators scs, BEAD-1, and HS4

A major determinant for transcriptional incompetence of murine leukemia virus (MLV) and MLV-derived vectors in embryonal cells is located at the proline primer binding site (PBS). The mechanism of silencing is unknown, yet the effect is capable of spreading to adjacent promoters. Based on a retroviral vector containing an internal promoter and the escape mutant B2 PBS with expressional capacity in embryonal cells, we have developed an assay to test the ability of putative insulators to shield the silencer at the PBS. Since the B2 PBS reverts to the wild-type PBS at high frequency, a shielding ability of a putative insulator can be assessed from the ratio of expressing B2 PBS to proline PBS proviruses in the target embryonal carcinoma cell population as measured by primer extension. Our results show that none of the possible insulators, scs, BEAD-1, or HS4, is able to shield an internal promoter from the repressive effect of the silencer at the PBS region when inserted between the silencer and the promoter.

Consistent, persistent expression from modified retroviral vectors in murine hematopoietic stem cells

Retroviral vectors based on the Moloney murine leukemia virus (MoMuLV) have shown inconsistent levels and duration of expression as well as a propensity for the acquisition of de novo methylation in vivo. MoMuLV-based vectors are known to contain sequences that are capable of suppressing or preventing expression from the long terminal repeat. Previously, we constructed a series of modified retroviral vectors and showed that they function significantly better than MoMuLV-based vectors in vitro. To test the efficacy of the modified vectors in hematopoietic stem cells in vivo, we examined gene expression and proviral methylation in differentiated hematopoietic colonies formed in the spleens of mice after serial transplantation with transduced bone marrow (2 degreesCFU-S). We found a significant increase in the frequency of expression with our modified vectors (>90% expression in vector DNA containing 2 degreesCFU-S) over the frequency observed with the standard MoMuLV-based vector (28% expression in vector containing 2 degreesCFU-S). Expression from the modified vectors was highly consistent, with expression in >50% of the vector-containing 2 degreesCFU-S from all 20 transplant recipients analyzed, whereas expression from the standard MoMuLV-based vector was inconsistent, with expression in 0-10% of the vector containing 2 degreesCFU-S from 8 recipients and expression in >50% of the vector-containing 2 degreesCFU-S from 4 other recipients. In addition, we established that the modified vectors had a lower level of DNA methylation than the control vector. These findings represent significant advances in the development and evaluation of effective retroviral vectors for application in vivo.

Transcription factor binding sites downstream of the human immunodeficiency virus type 1 transcription start site are important for virus infectivity

When transcriptionally active, the human immunodeficiency virus (HIV) promoter contains a nucleosome-free region encompassing both the promoter/enhancer region and a large region (255 nucleotides [nt]) downstream of the transcription start site. We have previously identified new binding sites for transcription factors downstream of the transcription start site (nt 465 to 720): three AP-1 sites (I, II, and III), an AP3-like motif (AP3-L), a downstream binding factor (DBF) site, and juxtaposed Sp1 sites. Here, we show that the DBF site is an interferon-responsive factor (IRF) binding site and that the AP3-L motif binds the T-cell-specific factor NF-AT. Mutations that abolish the binding of each factor to its cognate site are introduced in an infectious HIV-1 molecular clone to study their effect on HIV-1 transcription and replication. Individual mutation of the DBF or AP3-L site as well as the double mutation AP-1(III)/AP3-L did not affect HIV-1 replication compared to that of the wild-type virus. In contrast, proviruses carrying mutations in the Sp1 sites were totally defective in terms of replication. Virus production occurred with slightly delayed kinetics for viruses containing combined mutations in the AP-1(III), AP3-L, and DBF sites and in the AP3-L and DBF-sites, whereas viruses mutated in the AP-1(I,II,III) and AP3-L sites and in the AP-1(I,II,III), AP3-L, and DBF sites exhibited a severely defective replicative phenotype. No RNA-packaging defect could be measured for any of the mutant viruses as determined by quantification of their HIV genomic RNA. Measurement of the transcriptional activity of the HIV-1 promoter after transient transfection of the HIV-1 provirus DNA or of long terminal repeat-luciferase constructs showed a positive correlation between the transcriptional and the replication defects for most mutants.

Transfer of single gene-containing long terminal repeats into the genome of mammalian cells by a retroviral vector carrying the cre gene and the loxP site

Retroviral vectors contain viral cis-acting elements to achieve the packaging, reverse transcription, integration, and expression of the retroviral genomic nucleic acid sequence. However, these elements are not useful in the integrated provirus and can be the cause of problems. We have developed a vector which eliminates the majority of these viral elements. This vector, a long terminal repeat (LTR) enhancer-deleted vectors, exploits the Cre-lox recombination system of the P1 bacteriophage. The Cre-lox system is neutral for eukaryotic cells. The 32-nucleotide loxP site is inserted within the U3 of the 3' LTR along with with the gene to be transduced (in place of the viral enhancers). Following the LTR-mediated loxP duplication, the LTRs can be recombined by the Cre enzyme. The structure of the resulting provirus in the host genome corresponds to a single LTR (deleted of the viral enhancers) carrying a single copy of the gene to be transduced. If the Cre expression unit is furnished after the integration of a loxP-containing virus, the efficiency of the recombination is not absolute. If the Cre expression unit is inserted between the two LTRs, only single LTR proviral structures are found following infection by the retroviral vector.

Multiple modifications in cis elements of the long terminal repeat of retroviral vectors lead to increased expression and decreased DNA methylation in embryonic carcinoma cells

Infection by murine retroviruses in embryonic carcinoma (EC) and embryonic stem cells is highly restricted. The transcriptional unit of the Moloney murine leukemic virus (MoMuLV) long terminal repeat (LTR) is inactive in EC and embryonic stem cells in association with increased proviral methylation. In this study, expression in F9 EC cells was achieved from novel retroviral vectors containing three modifications in the MoMuLV-based retroviral vector: presence of the myeloproliferative sarcoma virus LTR, substitution of the primer binding site, and either deletion of a negative control region at the 5' end of the LTR or insertion of a demethylating sequence. We conclude that inhibition of expression from the MoMuLV LTR in EC cells is mediated through the additive effects of multiple cis-acting elements affecting the state of methylation of the provirus.

Stem cell factor binding to retrovirus primer binding site silencers

Using modified nuclear lysis and binding conditions, we have examined the binding of an embryonal carcinoma (EC) cell factor, binding factor A, to a stem cell-specific silencer which acts at the DNA level and overlaps the Moloney murine leukemia virus (M-MuLV) proline primer binding site (PBS). Following our protocol, we found that in vitro binding of factor A correlated with the in vivo activity of the M-MuLV silencer. Factor A bound specifically to the wild-type silencer element at room temperature and 30 degrees C, but not at 4 degrees C, and bound 10-fold better to the full-length silencer than to a minimal silencer core element. The factor was enriched in nuclear compared with cytosolic extracts and in undifferentiated EC cells compared with differentiated cells in which the silencer is nonfunctional. Salt and ion requirements for factor A binding were investigated, and partial purification steps indicated the factor to be a heparin-Sepharose-binding moiety of greater than 100 kDa. To examine possible relationships between silencer and PBS activities, sequences representing phenylalanine, isoleucine, lysine-1,2, lysine-3, methionine, and tryptophan PBS DNA fragments were tested in vivo for stem cell-specific repression of M-MuLV expression and in vitro in DNA binding assays. Of these PBS elements, only the lysine-1,2 PBS DNA fragment showed consistently high levels of repression. Interestingly, the lysine-1,2 PBS DNA fragment also formed a complex with an EC cell factor with characteristics similar to those of factor A. However, the two factors did not cross-compete in binding studies, suggesting that they may be different but related factors. Our results suggest that expression of Mason-Pfizer monkey virus, visna virus, and spumavirus, which use the lysine-1,2 PBS, may be inhibited in undifferentiated stem cells.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

Retrovirus-mediated insertion of expressed and non-expressed genes at identical chromosomal locations

During retrovirus replication, a cellularly derived tRNA is annealed to the viral RNA at the primer binding site (PBS) to prime reverse transcription, and both the tRNA and the PBS become copied and matched together on complementary proviral DNA strands prior to integration. Using a viral PBS single base pair mutant which affects provirus expression in undifferentiated cells, we show that reversion to wild type (wt) occurs at a frequency of approximately 50%. Daughter cell lines containing wt or mutant proviruses at identical chromosomal sites have been isolated, supporting a model where an integrated PBS-mismatched provirus was copied before mismatch correction could occur. Virus expression in daughter cells containing the mutant provirus was 100-fold higher than in cells bearing the wt counterpart. Additionally, proviral 5' DNA and cellular 5' flanking DNA became methylated in daughter cells containing wt but not mutant integrants. These results strongly support the current model of retrovirus reverse transcription, and indicate that the wt PBS region contains an element which suppresses virus expression and directs the methylation of viral and neighboring cellular DNA.

Transcriptional interaction between retroviral long terminal repeats (LTRs): mechanism of 5' LTR suppression and 3' LTR promoter activation of c-myc in avian B-cell lymphomas

Chicken syncytial viruses induce bursal lymphomas by integrating into the c-myc locus and activating myc expression by 3' long terminal repeat (LTR) promoter insertion. In contrast to wild-type proviruses, in which transcription initiates predominantly in the 5'LTR, these myc-associated proviruses exhibit a predominance of transcription from the 3' LTR and little transcription from the 5' LTR. Most of these proviruses contain deletions within the 5' end of their genome that spare the 5' LTR. We report the identification of a 0.3-kb viral leader sequence that modulates 5' and 3' LTR transcriptional activities. In the presence of this sequence, transcription from the 5' LTR predominates, but in its absence, the 3' LTR promoter becomes activated, resulting in a high level of myc expression. This viral sequence does not behave like a classical enhancer; it activates transcription only when located downstream from the promoter and in the sense orientation. In this regard, it resembles the recently described human immunodeficiency virus RNA enhancer. This study suggests that retroviruses contain internal sequences which directionally activate the 5' LTR promoter to facilitate transcription of the viral genome and that deletion of these sequences is one step in the activation of the 3' LTR of myc-associated proviruses in avian bursal lymphomas.

DNase I-hypersensitive sites are associated with both long terminal repeats and with the intragenic enhancer of integrated human immunodeficiency virus type 1

After reverse transcription and integration of the genome of human immunodeficiency virus (HIV) in a target cell, the viral DNA becomes packaged into chromatin. Regions of chromatin associated with regulatory functions in eukaryotes can generally be distinguished from the bulk of chromatin by an increased accessibility of the DNA to nucleases (nuclease-hypersensitive sites). In this report, the chromatin structure of the complete HIV-1 genome has been analyzed in three chronically infected cell lines of monocyte/macrophage and lymphoid origins. Digestion of purified nuclei from these cells with DNase I followed by restriction digestion and Southern blotting identified several DNase I-hypersensitive regions throughout the viral genome. Two constitutive sites were associated with the U3 region of the 5' long terminal repeat (LTR) in which the viral promoter and enhancer are located. An additional site in the R region of the 5' LTR was present only after activation of viral transcription by phorbol ester or tumor necrosis factor alpha. A fourth site was identified in all cell lines downstream of the 5' LTR (nucleotides [nt] 656 to 720), and the band corresponding to this site decreased in intensity upon activation of transcription. In the 3' LTR, a constitutive hypersensitive site was identified in all cell lines (nt 9322 to 9489). A major site (nt 4534 to 4733) was present only in a cell line of macrophage/monocyte origin in a region of the genome in which an intragenic enhancer was recently identified (E. Verdin, N. Becker, F. Bex, L. Droogmans, and A. Burny, Proc. Natl. Acad. Sci. USA 87:4874-4878, 1990). This study defines regions of the HIV genome associated with an open chromatin configuration and points to the potential regulatory role of these elements in the HIV life cycle.

Retrovirus-induced insertional mutagenesis: mechanism of collagen mutation in Mov13 mice

The Mov13 mouse strain carries a mutation in the alpha 1(I) procollagen gene which is due to the insertion of a Moloney murine leukemia provirus into the first intron. This insertion results in the de novo methylation of the provirus and flanking DNA, the alteration of chromatin structure, and the transcriptional inactivity of the collagen promoter. To address the mechanism of mutagenesis, we reintroduced a cloned and therefore demethylated version of the Mov13 mutant allele into mouse fibroblasts. The transfected gene was not transcribed, indicating that the transcriptional defect was not due to the hypermethylation. Rather, this result strongly suggests that the mutation is due to the displacement or disruption of cis-acting regulatory DNA sequences within the first intron. We also constructed a Mov13 variant allele containing a single long terminal repeat instead of the whole provirus. This construct also failed to express mRNA, indicating that the Mov13 mutation does not revert by provirus excision as has been observed for other retrovirus-induced mutations.

Retrovirus-induced insertional mutagenesis: mechanism of collagen mutation in Mov13 mice

The Mov13 mouse strain carries a mutation in the alpha 1(I) procollagen gene which is due to the insertion of a Moloney murine leukemia provirus into the first intron. This insertion results in the de novo methylation of the provirus and flanking DNA, the alteration of chromatin structure, and the transcriptional inactivity of the collagen promoter. To address the mechanism of mutagenesis, we reintroduced a cloned and therefore demethylated version of the Mov13 mutant allele into mouse fibroblasts. The transfected gene was not transcribed, indicating that the transcriptional defect was not due to the hypermethylation. Rather, this result strongly suggests that the mutation is due to the displacement or disruption of cis-acting regulatory DNA sequences within the first intron. We also constructed a Mov13 variant allele containing a single long terminal repeat instead of the whole provirus. This construct also failed to express mRNA, indicating that the Mov13 mutation does not revert by provirus excision as has been observed for other retrovirus-induced mutations.

Analysis of the binding proteins and activity of the long terminal repeat of Moloney murine leukemia virus during differentiation of mouse embryonal carcinoma cells

Mouse embryonal carcinoma (EC) cell lines were established which carry the stably integrated chloramphenicol acetyltransferase (CAT) gene under the control of the transcriptional elements of the long terminal repeat (LTR) of Moloney murine leukemia virus. The activity of three elements of the stably integrated LTR was analyzed in undifferentiated EC cells (stable CAT assay). Results of the study are summarized as follows. (i) In the stable assay, the promoter region of the LTR was inactive in undifferentiated ECA2 and F9 cells, and the level of the activity was 10(-4) of that in NIH 3T3 cells. (ii) In contrast to the results of the transient assay, the enhancer was active in undifferentiated ECA2 cells and in F9 cells. It activated CAT activity more than 60-fold and about 8-fold in ECA2 cells and F9 cells, respectively. (iii) Suppression by ELP, the embryonal LTR-binding protein, was more pronounced in the stable assay than in the transient assay. These data suggest that, when compared with NIH 3T3 cells, a major factor for the inactivity of the LTR in EC cells is the inefficiency of the promoter in this assay. Transcriptional activity of the LTR was analyzed during the differentiation of EC cells. In the case of ECA2 cells, the magnitude of activation by the enhancer did not change during differentiation. The activity of the promoter increased about 10-fold, and the suppression by ELP became negligible 4 days after the induction of differentiation. Upon differentiation of F9 cells, the activity of the enhancer increased more than 300-fold, but the promoter remained inactive. The pattern of LTR-binding proteins also varied during the differentiation of EC cells. Our present data suggest that the activity of LTR elements as assayed by the stable assay differs from the activity as assayed by the transient assay. It also indicates that the activity of these elements exhibits cell-type-specific changes during the differentiation of EC cells.

Promoter interactions in retrovirus vectors introduced into fibroblasts and embryonic stem cells

The activity of the Moloney murine leukemia virus promoter is restricted in mouse embryonic stem cells. Gene expression with retrovirus vectors can be achieved in these cells if internal promoters are used. To address the possible influence of the viral enhancer sequences on expression from the internal promoter, we have constructed high-titer, self-inactivating retrovirus vectors which delete viral regulatory sequences upon integration in the host genome. We show that deleting most of the viral enhancer sequences has no significant effect on viral titer. This enhancer deletion leads to either an increase or a decrease in the amount of RNA transcribed from the internal promoter, but no consistent change can be found with any type of vector. The same changes in expression from the internal promoter observed in embryonic stem cells are also observed in 3T3 fibroblast cells, in which the viral promoter is active. These results indicate that viral regulatory elements influence expression from an internal promoter independently of expression from the virus promoter.

A stem cell-specific silencer in the primer-binding site of a retrovirus

Retrovirus expression in embryonal carcinoma (EC) cells is blocked at a postintegration stage of the viral life cycle, in part because of the inadequate function of the viral long terminal repeat promoter in this cell type. However, selection for retrovirus expression in EC cells has identified mutations in Moloney murine leukemia virus (M-MuLV) located in the tRNA primer-binding site (PBS) region which relieve the EC cell-specific repression. We have found that exchanging the M-MuLV proline PBS for a glutamine one in a recombinant virus permits expression in EC cells. By using the recombinant virus as a backbone, the EC cell-specific repressor-binding site (RBS) element has been mapped to M-MuLV nucleotides 147 to 174. The RBS does not require precise positioning downstream of the M-MuLV promoter and can function in either orientation and in an intron, indicating that the regulatory effect is probably at the DNA, rather than RNA, level. We also show that the RBS element can repress heterologous promoters from an upstream position. Our results indicate that the RBS acts as a silencer that its inhibitory effect is mediated by a trans-acting factor, and that the mechanism of action is probably at the level of transcription. Through in vitro binding assays we have identified a binding factor which specifically recognizes the wild-type RBS sequence (binding factor A). The binding characteristics of factor A suggest that it is a stem cell repressor which acts at the M-MuLV RBS. Our DNA-binding assays also have identified a unique binding factor (binding factor Hp) which specifically recognizes a hemimethylated form of the wild-type RBS. This factor may play a role in methylation mediated control of retrovirus expression in EC cells.

A stem cell-specific silencer in the primer-binding site of a retrovirus

Retrovirus expression in embryonal carcinoma (EC) cells is blocked at a postintegration stage of the viral life cycle, in part because of the inadequate function of the viral long terminal repeat promoter in this cell type. However, selection for retrovirus expression in EC cells has identified mutations in Moloney murine leukemia virus (M-MuLV) located in the tRNA primer-binding site (PBS) region which relieve the EC cell-specific repression. We have found that exchanging the M-MuLV proline PBS for a glutamine one in a recombinant virus permits expression in EC cells. By using the recombinant virus as a backbone, the EC cell-specific repressor-binding site (RBS) element has been mapped to M-MuLV nucleotides 147 to 174. The RBS does not require precise positioning downstream of the M-MuLV promoter and can function in either orientation and in an intron, indicating that the regulatory effect is probably at the DNA, rather than RNA, level. We also show that the RBS element can repress heterologous promoters from an upstream position. Our results indicate that the RBS acts as a silencer that its inhibitory effect is mediated by a trans-acting factor, and that the mechanism of action is probably at the level of transcription. Through in vitro binding assays we have identified a binding factor which specifically recognizes the wild-type RBS sequence (binding factor A). The binding characteristics of factor A suggest that it is a stem cell repressor which acts at the M-MuLV RBS. Our DNA-binding assays also have identified a unique binding factor (binding factor Hp) which specifically recognizes a hemimethylated form of the wild-type RBS. This factor may play a role in methylation mediated control of retrovirus expression in EC cells.

Inactivation of the Moloney murine leukemia virus long terminal repeat in murine fibroblast cell lines is associated with methylation and dependent on its chromosomal position

The expression of a retroviral vector with the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) promoter after integration into the genome of murine fibroblast cell lines was monitored with the Escherichia coli-derived beta-galactosidase (beta-gal) gene as the reporter. Monoclonal cell lines derived after retroviral infection exhibited a marked heterogeneity in their expression of the reporter gene. We studied two monoclonal cell lines with a single unrearranged copy of the vector provirus integrated into their genome. The first, BB10, expressed the marker enzyme in only 8% of its cell population, whereas in the second, BB16, beta-gal expression could be detected in over 98% of the cells. Treatment of BB10 with the DNA-demethylating agent 5-azacytidine raised the number of beta-gal-positive cells to over 60%. Transfection experiments showed that the Mo-MuLV LTR promoter-enhancer is potentially fully functional in both the BB10 and BB16 cell lines. The inactivated provirus from BB10 cells was cloned and subsequently used to generate retrovirus stocks. The promoter-enhancer activity of its LTR after infection with these BB10-derived viruses showed a variation similar to that of the original virus stocks. Our data showed that (1) inactivation of the Mo-MuLV LTR is a frequent event in murine fibroblast cell lines, (2) inactivation is associated with de novo methylation of cytidine residues, (3) the frequency of inactivation of the provirus must be determined by its chromosomal position, (4) the process of methylation of sequences within the LTR is not necessarily the same as the transcription-repression mechanism that is operating in undifferentiated embryonal carcinoma cells.

Determinants of retrovirus gene expression in embryonal carcinoma cells

The expression of Moloney murine leukemia virus is restricted in embryonal carcinoma (EC) cells. To characterize specific mutations necessary for expression of retroviruses in EC cells, we analyzed the expression of retrovirus mutants and recombinants thereof in EC cell lines F9 and PCC4. DNA sequence comparison and functional studies allowed us to define three point mutations in the enhancer region of the viral mutants at positions -345, -326, and -166 and two point mutations within the 5'-untranslated region of the viral genome at positions +164 and +165 that were essential for retrovirus expression in EC cells. DNA fragments derived from either the wild type or mutant viruses were used to search for sequence-specific DNA-binding factors in nuclear extracts from undifferentiated PCC4 cells. A cellular factor was found to bind strongly to sequences within the enhancer region (-354 to -306) of wild-type viruses but only weakly to sequences derived from mutant viruses. This factor was named ECF-I (for EC cell factor I). Retroviral expression in EC cells correlates with decreased binding affinity for ECF-I.

Embryonic stem cell virus, a recombinant murine retrovirus with expression in embryonic stem cells

The expression of Moloney murine leukemia virus and vectors derived from it is restricted in undifferentiated mouse embryonal carcinoma and embryonal stem (ES) cells. We have developed a retroviral vector, the murine embryonic stem cell virus (MESV), that is active in embryonal carcinoma and ES cells. MESV was derived from a retroviral mutant [PCC4-cell-passaged myeloproliferative sarcoma virus (PCMV)] expressed in embryonal carcinoma cells but not in ES cells. The enhancer region of PCMV was shown to be functional in both cell types, but sequences within the 5' untranslated region of PCMV were found to restrict viral expression in ES cells. Replacement of this region by related sequences obtained from the dl-587rev retrovirus results in MESV, a modified PCMV virus that confers G418 resistance to fibroblasts and ES cells with similar efficiencies. Expression of MESV in ES cells is mediated by transcriptional regulatory elements within the 5' long terminal repeat of the viral genome.

Evidence for a stem cell-specific repressor of Moloney murine leukemia virus expression in embryonal carcinoma cells

A negative regulatory element (NRE) spanning the tRNA primer-binding site (PBS) of Moloney murine leukemia virus (M-MuLV) mediates repression of M-MuLV expression specifically in embryonal carcinoma (EC) cells. We precisely defined the element by base-pair mutagenesis to an 18-base-pair segment of the tRNA PBS and showed that the element also restricted expression when moved upstream of the long terminal repeat. A DNA-binding activity specific for the M-MuLV NRE was detected in vitro by using crude EC nuclear extracts in exonuclease III protection assays. Binding was strongly correlated with repression in EC cells. Mutations within the NRE that relieved repression disrupted binding activity. Also, nuclear extracts prepared from permissive, differentiated EC cell cultures showed reduced binding activity for the NRE. These results indicate the presence of a stem cell-specific repressor that extinguishes M-MuLV expression via the NRE at the tRNA PBS.

Evidence for a stem cell-specific repressor of Moloney murine leukemia virus expression in embryonal carcinoma cells

A negative regulatory element (NRE) spanning the tRNA primer-binding site (PBS) of Moloney murine leukemia virus (M-MuLV) mediates repression of M-MuLV expression specifically in embryonal carcinoma (EC) cells. We precisely defined the element by base-pair mutagenesis to an 18-base-pair segment of the tRNA PBS and showed that the element also restricted expression when moved upstream of the long terminal repeat. A DNA-binding activity specific for the M-MuLV NRE was detected in vitro by using crude EC nuclear extracts in exonuclease III protection assays. Binding was strongly correlated with repression in EC cells. Mutations within the NRE that relieved repression disrupted binding activity. Also, nuclear extracts prepared from permissive, differentiated EC cell cultures showed reduced binding activity for the NRE. These results indicate the presence of a stem cell-specific repressor that extinguishes M-MuLV expression via the NRE at the tRNA PBS.

Mechanism of suppression of the long terminal repeat of Moloney leukemia virus in mouse embryonal carcinoma cells

Sequence-specific DNA-binding proteins that bind to the long terminal repeat (LTR) of Moloney leukemia virus in undifferentiated and differentiated mouse embryonal carcinoma (EC) cells were identified by gel retardation assay. The proteins that bind to the CCAAT box were present in both undifferentiated and differentiated EC cells. The amounts and the number of species of the proteins that bind to the enhancer and the GC-rich region were far lower in undifferentiated EC cells than in the differentiated counterparts. These proteins were supposed to be transcriptional activators. Proteins that bind upstream of the enhancer, namely, the -352 to -346 region and the -407 to -404 region, were identified. These proteins were designated the embryonic LTR-binding protein (ELP) and the LTR-binding protein, respectively. The ELP was present only in undifferentiated EC cell lines. The LTR-binding protein was detected in all cell lines tested. The mechanism of suppression of the LTR was investigated by the chloramphenicol acetyltransferase assay. The enhancer and the GC-rich region of the LTR functioned poorly in undifferentiated cells. When eight copies of ELP-binding sequences were inserted upstream of the enhancer region, expression of the chloramphenicol acetyltransferase gene was reduced about threefold in ECA2 cells. From these data, we concluded that two mechanisms, the shortage of activator proteins and the presence of a negative regulatory protein (ELP), are involved in the suppression of the LTR in undifferentiated EC cells.

Induced expression from the Moloney murine leukemia virus long terminal repeat during differentiation of human myeloid cells is mediated through its transcriptional enhancer

Transcription from the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) is inhibited in murine stem cells and induced during maturation of these cells. We have investigated whether alterations in the activity of this viral regulatory element also occur during differentiation of human myeloid leukemia cells. The Mo-MuLV LTR and the simian virus 40 (SV40) early promoter were introduced into HL-60 promyelocytes on Epstein-Barr virus-derived chloramphenicol acetyltransferase expression vectors. When these cells were induced to terminally differentiate, transcription from the Mo-MuLV LTR was induced approximately 10-fold. Expression from the SV40 promoter remained constant during differentiation of these cells. Replacing the SV40 transcriptional enhancer with the Mo-MuLV LTR transcriptional enhancer rendered the SV40 promoter inducible during differentiation. We conclude that sequences within the transcriptional enhancer of the Mo-MuLV LTR contain cis-acting elements responsible for induction of gene expression during differentiation of human myeloid cells.

Mechanism of suppression of the long terminal repeat of Moloney leukemia virus in mouse embryonal carcinoma cells

Sequence-specific DNA-binding proteins that bind to the long terminal repeat (LTR) of Moloney leukemia virus in undifferentiated and differentiated mouse embryonal carcinoma (EC) cells were identified by gel retardation assay. The proteins that bind to the CCAAT box were present in both undifferentiated and differentiated EC cells. The amounts and the number of species of the proteins that bind to the enhancer and the GC-rich region were far lower in undifferentiated EC cells than in the differentiated counterparts. These proteins were supposed to be transcriptional activators. Proteins that bind upstream of the enhancer, namely, the -352 to -346 region and the -407 to -404 region, were identified. These proteins were designated the embryonic LTR-binding protein (ELP) and the LTR-binding protein, respectively. The ELP was present only in undifferentiated EC cell lines. The LTR-binding protein was detected in all cell lines tested. The mechanism of suppression of the LTR was investigated by the chloramphenicol acetyltransferase assay. The enhancer and the GC-rich region of the LTR functioned poorly in undifferentiated cells. When eight copies of ELP-binding sequences were inserted upstream of the enhancer region, expression of the chloramphenicol acetyltransferase gene was reduced about threefold in ECA2 cells. From these data, we concluded that two mechanisms, the shortage of activator proteins and the presence of a negative regulatory protein (ELP), are involved in the suppression of the LTR in undifferentiated EC cells.

Induced expression from the Moloney murine leukemia virus long terminal repeat during differentiation of human myeloid cells is mediated through its transcriptional enhancer

Transcription from the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) is inhibited in murine stem cells and induced during maturation of these cells. We have investigated whether alterations in the activity of this viral regulatory element also occur during differentiation of human myeloid leukemia cells. The Mo-MuLV LTR and the simian virus 40 (SV40) early promoter were introduced into HL-60 promyelocytes on Epstein-Barr virus-derived chloramphenicol acetyltransferase expression vectors. When these cells were induced to terminally differentiate, transcription from the Mo-MuLV LTR was induced approximately 10-fold. Expression from the SV40 promoter remained constant during differentiation of these cells. Replacing the SV40 transcriptional enhancer with the Mo-MuLV LTR transcriptional enhancer rendered the SV40 promoter inducible during differentiation. We conclude that sequences within the transcriptional enhancer of the Mo-MuLV LTR contain cis-acting elements responsible for induction of gene expression during differentiation of human myeloid cells.

Negative regulatory element associated with potentially functional promoter and enhancer elements in the long terminal repeats of endogenous murine leukemia virus-related proviral sequences

Three series of recombinant DNA clones were constructed, with the bacterial chloramphenicol acetyltransferase (CAT) gene as a quantitative indicator, to examine the activities of promoter and enhancer sequence elements in the 5' long terminal repeat (LTR) of murine leukemia virus (MuLV)-related proviral sequences isolated from the mouse genome. Transient CAT expression was determined in mouse NIH 3T3, human HT1080, and mink CCL64 cultured cells transfected with the LTR-CAT constructs. The 700-base-pair (bp) LTRs of three polytropic MuLV-related proviral clones and the 750-bp LTRs of four modified polytropic proviral clones, in complete structures either with or without the adjacent downstream sequences, all showed very little or negligible activities for CAT expression, while ecotropic MuLV LTRs were highly active. The MuLV-related LTRs were divided into three portions and examined separately. The 3' portion of the MuLV-related LTRs that contains the CCAAC and TATAA boxes was found to be a functional promoter, being about one-half to one-third as active as the corresponding portion of ecotropic MuLV LTRs. A MboI-Bg/II fragment, representing the distinct 190- to 200-bp inserted segment in the middle, was found to be a potential enhancer, especially when examined in combination with the simian virus 40 promoter in CCL64 cells. A PstI-MboI fragment of the 5' portion, which contains the protein-binding motifs of the enhancer segment as well as the upstream LTR sequences, showed moderate enhancer activities in CCL6 cells but was virtually inactive in NIH 3T3 cells and HT1080 cells; addition of this fragment to the ecotropic LTR-CAT constructs depressed CAT expression. Further analyses using chimeric LTR constructs located the presence of a strong negative regulatory element within the region containing the 5' portion of the enhancer and the immediate upstream sequences in the MuLV-related LTRs.

Two blocks in Moloney murine leukemia virus expression in undifferentiated F9 embryonal carcinoma cells as determined by transient expression assays

Transient expression assays were used to investigate the restriction of Moloney murine leukemia virus (MoMuLV) expression in undifferentiated mouse F9 embryonal carcinoma (EC) cells. We previously reported that the MoMuLV long terminal repeat (LTR) is inactive in undifferentiated F9EC cells due to inactivity of the tandemly repeated MoMuLV transcriptional enhancers. Others suggested that the inactivity was due to the presence of negative regulatory elements that interact with the MoMuLV tandem repeats. Two heterologous enhancer sequences that are active in undifferentiated F9 EC cells were inserted into the MoMuLV LTR: the B enhancers from the F101 variant of polyomavirus and a cellular enhancer sequence isolated from EC cells that we previously identified. The chimeric LTRs were then fused to the bacterial chloramphenicol acetyltransferase gene and tested for expression by transfection into F9 EC or NIH 3T3 cells. Insertion of these enhancers either upstream or downstream of the MoMuLV tandem repeats resulted in transcriptionally active LTRs in undifferentiated EC cells, which did not support the existence of negative regulatory elements interacting with the tandem repeats. In our previous MoMuLV enhancer deletion constructs, the GC-rich sequences downstream from the tandem repeats were also deleted, which might have contributed to the inactivity in EC cells. However, restoration of the GC-rich sequences did not yield an active LTR. The experiments also suggested that the EC cellular enhancer was preferentially active in undifferentiated EC cells and inactive in NIH 3T3 cells. The possibility of negative regulatory sequences in the vicinity of the MoMuLV primer-binding site was tested by inserting MoMuLV sequences from +30 to +419 base pairs into the LTR-chloramphenicol acetyltransferase gene constructs downstream of the transcriptional start site. Transient expression assays confirmed that these sequences reduced expression from functional LTRs in undifferentiated F9 EC cells but reduced expression significantly less in NIH 3T3 cells. Moreover, equivalent sequences from myeloproliferative sarcoma virus did not exhibit this effect. These results supported restriction of MoMuLV expression in undifferentiated F9 EC cells at two levels, inactivity of the MoMuLV enhancers and interaction of negative regulatory factors in the vicinity of the primer-binding site.

Negative regulation of retrovirus expression in embryonal carcinoma cells mediated by an intragenic domain

An intragenic region spanning the tRNA primer binding site of a Moloney murine leukemia virus recombinant retrovirus was found to restrict expression specifically in embryonal carcinoma (EC) cells. When the inhibitory domain was present, the levels of steady-state RNA synthesized from integrated recombinant templates in stable cotransformation assays were reduced 20-fold in EC cells but not in C2 myoblast cells. Transient-cotransfection assays showed that repression of a template containing the EC-specific inhibitory component was relieved by an excess of specific competitor DNA. In addition, repression mediated by the inhibitory component was orientation independent. This evidence demonstrates the presence of a saturable, trans-acting negative regulatory factor(s) in EC cells and suggests that the interaction of the factor(s) with the intragenic inhibitory component occurs at the DNA level.

Comparison of expression in hemopoietic cells by retroviral vectors carrying two genes

In order to identify factors that influence expression by retroviral vectors in hemopoietic cells, we have compared viral RNA levels in cells infected with several different recombinant viruses. All of the vectors tested carry the neomycin resistance gene and provide for the insertion of a second gene which, in these studies, comprised sequences from the myc or myb oncogenes or the gene encoding granulocyte-macrophage colony-stimulating factor. The vectors utilize two different strategies for the coexpression of the two genes: alternate splicing and the use of a separate internal promoter. We found that expression in hemopoietic cells could be increased by substituting sequences from the myeloproliferative sarcoma virus long terminal repeat for those of the Moloney murine leukemia virus long terminal repeat. However, none of the vectors examined was able to express a second gene at levels equivalent to those achieved by the parental vectors carrying only the neomycin resistance gene. The reasons for this varied with the different vectors and included inefficient splicing and/or a reduction in the level of unspliced transcripts upon insertion of a second gene. Although the basis of the latter phenomenon is not clear, it is probably related to the position--near the 5' long terminal repeat--at which the second gene was inserted, since insertion of the same genes near the 3' end of another vector had no effect on viral RNA levels. In an attempt to circumvent some of these problems, we constructed a vector that employs an internal beta-actin promoter. Although this vector could express granulocyte-macrophage colony-stimulating factor sequences in a responsive hemopoietic cell line, the level of granulocyte-macrophage colony-stimulating factor produced was disappointingly low. The results from these studies suggest approaches to the design of improved vectors for effective expression of genes in hemopoietic cells.