The HMG protein T160 colocalizes with DNA replication foci and is down-regulated during cell differentiation

The high mobility group protein T160, the murine homolog of the human structure-specific recognition protein 1, was first supposed to be involved in the process of V-(D)-J recombination, since it could bind to recombination signal sequence probes. We have recently cloned T160 by using an unrelated DNA probe and shown that it binds to either cruciform or linear DNA with no sequence specificity. In this work, we performed a detailed analysis of T160 expression and immunolocalization. We show that T160 is a phosphoprotein broadly conserved from yeast to mammals, with a high level of expression in all the cell lines tested and in tissues containing a high degree of proliferating cells. Indirect immunofluorescence analysis by confocal laser microscopy revealed that T160 distribution in the cell nucleus is not uniform, and focus-like staining was observed. Cell cycle studies by BrdU incorporation suggest that the appearance of T160 nuclear foci is specific of mid to late S phase. Furthermore, while T160 expression does not change during the cell cycle, it is dramatically down-regulated when cells begin to differentiate, as highlighted in C2C12 myoblasts and myotubes. The disappearance of T160 nuclear staining in multinucleated myotubes is shown. Taken together, these data suggest that its function may be less specific than V-(D)-J recombination and more related to some cellular basic process, such as DNA replication or repair.

The murine homolog of the HIN 200 family, Ifi 204, is constitutively expressed in myeloid cells and selectively induced in the monocyte/macrophage lineage

To examine the expression of Ifi 200 genes in vivo and add new information about their function, polyclonal monospecific rabbit antibodies, designated N-term or C-term, were raised against both the N-terminus and C-terminus of the 204 protein (p204) respectively. Western blotting analysis demonstrated that p204 and D3, another member of the Ifi 200 gene family, are constitutively expressed, though at different degrees, in bone marrow, thymus and lymph nodes, and barely detectable in the spleen. Poly rI:rC treatment did not modulate their expression. Peritoneal resident macrophages (Mphi) from untreated mice were negative, but displayed high levels of both p204 and D3 on poly rI:rC treatment. A significant increase of these proteins is also observed when Mphi are cultured overnight in vitro with IFNs or LPS. Lung, kidney and brain were negative for p204 and D3 expression. These results, together with immunohistochemical analysis, demonstrate that the 204 gene has an expression pattern restricted to cells of the myelomonocytic lineage similar to that observed for the human homolog, the myeloid nuclear differentiation antigen (MNDA) suggesting its potential involvement in the differentiation and maturation of this cell lineage.

Murine cytomegalovirus induces expression and enzyme activity of cellular dihydrofolate reductase in quiescent cells

Murine cytomegalovirus (MCMV) productively infects quiescent fibroblasts in which the levels of nucleoside triphosphate precursors and cell functions involved in DNA metabolism are minimal. It appears that MCMV has evolved molecular pathways in order to ensure the presence of nucleoside triphosphate precursors for the viral DNA polymerase. Here, we report that MCMV infection of quiescent NIH 3T3 cells markedly stimulates transcription, expression and activity of the cellular dihydrofolate reductase (DHFR), a key enzyme in the synthesis of DNA precursors. DHFR stimulation by MCMV is sensitive to UV irradiation and seems to depend on expression of the viral immediate-early protein pp89. Finally, it has been demonstrated that suppression of virus-induced DHFR activity by the specific inhibitor methotrexate prevents MCMV DNA replication. These observations indicate that induction of host cell DHFR activity by MCMV is required for viral DNA synthesis in quiescent fibroblasts.

The Ifi 200 genes: an emerging family of IFN-inducible genes

The biological activities of interferons (IFNs) are mediated by IFN-induced proteins. One family is encoded by several structurally related genes located on murine chromosome 1 (Ifi 200 cluster) and three homologous genes (MNDA, IFI 16 and AIM2) located on human chromosome 1 as well, within a linkage group highly conserved between mouse and human. All the proteins of this family contain at least one copy of a conserved 200 amino acid domain, in addition to other regions that are different or missing among the various family members. Conservation of the 200 amino acid segment, therefore, may be responsible for a common function, while individually expressed domains may afford other tissue- or cell-specific functions. The data available demonstrate that at least two members of the Ifi 200 protein family, p202 and p204, inhibit cell proliferation in vitro. Moreover, high constitutive levels of p204 expression impair normal embryo development in transgenic animals. Here, we will review the principal features of murine and human proteins belonging to this family and their function in the cell growth-regulatory activities mediated by IFNs.

Inhibition of cell proliferation by the interferon-inducible 204 gene, a member of the Ifi 200 cluster

The role of the IFN-inducible p204 as growth regulator was investigated by transfecting an expression vector constitutively expressing p204 into several cell lines. Like pRB and p107, p204 is a potent growth inhibitor in sensitive cells, as demonstrated by the cell focus assay. Since stable transfectants of sensitive lines constitutively overexpressing p204 could not be established in vitro, we inserted the 204 cDNA into a vector bearing an heavy-metal-inducible promoter. Here we show that proliferation of B6MEF fibroblasts lacking endogenous p204 is strongly inhibited by transient p204 expression in the nucleus. p204 delays G1 progression into the S-phase and cells accumulate with a DNA content equivalent to cells arrested in late G1. Moreover, the role of p204 in the control of cell growth in vivo was investigated by generating transgenic mice in which the Ifi 204 gene was constitutively expressed in all tissues. To this end, expression vectors bearing the 204 cDNA under the control of the SV40 viral promoter were constructed. The overexpression of the p204 transgene achieved by injecting fertilized mouse eggs with these vectors was compatible with embryo development up to the four-cell stage in an in vitro follow-up of 4.5 days. However, no viable animals with an intact copy of the transgene were obtained, suggesting that high and constitutive levels of p204 expression can impair normal embryo development. These findings indicate that p204 plays a negative role in growth regulation and provide new information about the molecular mechanisms exploited by IFNs to inhibit cell proliferation.

Decreased expression of the high-mobility group protein T160 by antisense RNA impairs the growth of mouse fibroblasts

The T160 protein belongs to the HMG-1 box protein family and preferentially binds to non-B-DNA conformations with no sequence specificity. Its exact role has yet to be defined, though it seems to participate in processes involving DNA, such as replication, transcription and recombination. We have used an antisense RNA strategy to investigate its role in cell growth and proliferation. T160 expression is strongly suppressed by stable introduction of an antisense construct into NIH3T3 cells, and this decrease is accompanied by substantial changes in the growth properties of the stable transfectants. Impaired growth of T160- cells was mainly related to two mechanisms: i) decreased rates of cell proliferation at normal serum concentration; and ii) occurrence of cell death by apoptosis at low serum concentration, as demonstrated by both flow cytometry and microscopy. The finding that decreased T160 availability affects cell proliferation, provides further evidence of its involvement in a basic cell function, such as DNA replication.

The high-mobility group protein T160 binds to both linear and cruciform DNA and mediates DNA bending as determined by ring closure

The high-mobility group protein T160 was isolated by screening a phage library from a murine pre-B-cell line L1210. South-Western experiments have previously shown that this protein binds to V-(D)-J recombination signal sequences, suggesting that it may be a sequence-specific DNA-binding protein. However, neither gel-shift nor footprinting analyses have been successfully employed with the T160 protein, despite an extensive effort. In this study, the T160 protein or truncated forms made soluble through denaturing and renaturing cycles in urea were successfully used in gel-shift experiments showing that T160 binds to cruci-form or linear duplex DNA with no apparent sequence specificity. Furthermore, fragments longer than 100 bp efficiently formed covalently closed circular monomers in the presence of T160 and T4 DNA ligase, indicating that the protein is capable of introducing bends into the duplex. Last, tissue distribution by Western blotting analysis showed that the T160 protein is expressed in various murine tissues in addition to those of lymphoid origin. Considering its broad evolutionary conservation (from plants to mammals) also, these results suggest that the functional role of the T160 protein is not limited to V-(D)-J recombination, but might be involved in basic processes such as DNA replication and repairing, where irregular DNA structures are generated and very likely recognized by HMG domain proteins.

Molecular cloning and expression of an interferon-inducible protein encoded by gene 203 from the gene 200 cluster

We report here the complete coding sequence of a 203 cDNA, a member of the interferon-inducible Ifi 200 gene family. By combining reverse-transcriptase PCR and rapid amplification of cDNA ends (RACE) techniques we have obtained a 3.8-kb cDNA corresponding to a 203 mRNA. When used as a probe in northern analysis, its 3' segment hybridized to a 3.8-kb interferon-inducible mRNA, whereas the 5'-end additionally hybridized to a less abundant interferon-inducible 1.8-kb mRNA. Nucleotide sequence analysis revealed that the two mRNAs share the 5'-untranslated region and the same open reading frame, which encodes a hydrophilic protein composed of 408 amino acids. The difference between them is due to a 3'-untranslated region extended by alternative polyadenylation site selection. Furthermore, 203 mRNA was found to be inducible by interferon-alpha in various murine cell lines. Using polyclonal antibodies raised against a segment specific for the 203 protein, we established that p203 protein levels increase on treatment with interferon-alpha in murine fibroblasts and that p203 is located in the nucleus.

Suppression of high mobility group protein T160 expression impairs mouse cytomegalovirus replication

The high mobility group (HMG-1) box proteins bind both non-B-DNA conformations and specific nucleotide sequences. They have been implicated in a wide variety of cellular functions involving DNA, such as transcription, replication and recombination. To determine whether HMG-1 box protein T160 plays a role in virus replication, we employed an antisense strategy to inhibit its expression in NIH 3T3 cells. The two T160 clones that expressed levels of T160 50% lower than those expressed by clones transfected with the empty vector (Neo+ clones) were investigated with respect to their permissiveness to the growth of viruses representing three families: Rhabdoviridae, vesicular stomatitis virus (VSV); Picornaviridae, encephalomyocarditis virus (EMCV), and Alpha- and Betaherpesviridae, herpes simplex virus 1 (HSV-1) and mouse cytomegalovirus (MCMV), respectively. They displayed a high degree of resistance to MCMV replication, but were fully permissive to the other viruses. Competitive PCR and probing IE-1 products by Western blot analysis showed that this resistance was not due to depressed levels of virus adsorption during the early phases of infection. We therefore conclude that T160 is involved in replication of the betaherpesvirus MCMV.

The murine cytomegalovirus immediate-early 1 protein stimulates NF-kappa B activity by transactivating the NF-kappa B p105/p50 promoter

The transcription of murine cytomegalovirus (MCMV) immediate-early (IE) genes is regulated by a large and complex enhancer containing several consensus binding sites for the ubiquitous transcription factor NF-kappa B. To verify whether MCMV, like the human CMV, can activate NF-kappa B-dependent transcription, we transfected murine embryo fibroblasts cells with a construct containing three copies of the NF-kappa B element in front of the homologous minimal MCMV IE1-3 promoter. Upon MCMV infection the reporter gene activity was transactivated to about three-fold above the basal level. The specificity of this transactivation was demonstrated by the lack of any significant effect on the activity of DNA constructs containing either a mutated NF-kappa B trimer or an ATF/CRE trimer. Gel shift assays with a NF-kappa B probe revealed that MCMV infection activated DNA binding proteins showing NF-kappa B characteristics. The DNA-binding activity remained elevated during the course of infection and was associated to an increase in the steady-state mRNA levels for the NF-kappa B subunit p105/p50. Since the promoter of the p105/p50 gene was transactivated by MCMV infection during the period in which the IE proteins are expressed, the role of the two major IE transcriptional regulatory proteins was examined. In cotransfection experiments, the IE1 protein transactivated the p105/p50 promoter, whereas the IE3 was ineffective in increasing the transcription of the reporter gene. Taken as a whole, these results demonstrate that MCMV, like its human counterpart, regulates the cellular NF-kappa B activity needed for the initial induction of the IE genes and the progression of the viral replicative cycle.

Ring chromosome 9: an atypical case

A new case of ring chromosome 9 in a 36-month-old child is presented. In addition to the pathognomonic features of this rare disorder (only 21 cases reported), our patient presents some peculiarities, such as corpus callosum hypoplasia and epileptic seizures (infantile periodic spasms). We also observed a reduced level of leukocyte interferon alpha whose synthesis is controlled by a gene on chromosome 9 and which could be responsible for the recurrent respiratory tract infections, typical and sometimes fatal in these patients.

Host genotype controls the ability of the ISGF3 complex to activate transcription of IFN-inducible genes

C57BL/6 mice are unable to express the Ifi 202 type genes upon injection in vivo of multiple dsRNA, poly rl:rC, or IFN-treatment in vitro. For this purpose the 5' terminal flanking region (called the b segment of 804 bp) was linked to a heterologous reporter gene chloramphenicol acetyl transferase (CAT) and transfected into NIH3T3 cells or BLK cells derived from the C57BL/6 strain. IFN-alpha induced strong CAT activity in NIH3T3 but not in BLK cells. This lack of transcription activation was not due to a defect in STAT factor activity, since IFN-alpha treatment in the presence of IFN-gamma priming induced translocation of the ISGF3 into the nucleus, and binding to the ISRE (IFN-Stimulated Response Element) of the 202 gene even in C57BL/6 derived cells. Surprisingly when three tandem copies of the 202 ISRE (42 bp) were linked to a heterologous promoter (c-fos promoter) driving the reporter CAT gene, activation was also observed in C57BL/6 cells upon IFN-treatment. Finally, another IFN-inducible gene, namely the Mx, was activated in C57BL/6 mice. Thus, the primary defect of the C57BL/6 strain leading to an impaired Ifi 202 type gene response to IFN appears to be an inability of the ISGF3 complex to activate the endogenous promoter. Altogether these results suggest that unidentified nuclear factors related to the host genotype control the ability of the STAT factors to activate transcription upon IFN-treatment.

Interferon-alpha inhibits the murine cytomegalovirus immediate-early gene expression by down-regulating NF-kappa B activity

Transcription of murine cytomegalovirus (MCMV) immediate-early (IE) genes is regulated by the interaction of cellular transcription factors with a strong viral enhancer controlling promoters flanking both sides of the regulatory sequence. We have previously demonstrated that interferon-alpha (IFN-alpha) inhibits MCMV replication by impairing the transcription of IE genes. To define the cis-acting elements and trans-acting factors involved in this inhibition, permissive murine fibroblasts were transferred with DNA constructs containing the chloramphenicol acetyl transferase reporter gene and portions of the IE enhanced. The region spanning -1185 to -259 relative to the IE1-3 promoter was sufficient to allow IFN-alpha-induced inhibition. Since this segment contains several NF-kappa B sites, cells were transfected with a construct containing three copies of NF-kappa B element in front of the homologous minimal IE1-3 promoter. Upon IFN-alpha treatment the reporter gene activity was strongly reduced, indicating that NF-kappa B binding site is sufficient to confer inhibition. The specificity of this inhibition was demonstrated by the lack of a significant effect on the activity of DNA constructs containing either a mutated NF-kappa B trimer or an ATF/CRE trimer. Gel shift assays with NF-kappa B probes revealed that MCMV infection activated NF-kappa B proteins, whereas IFN-alpha treatment significantly reduced their ability to bind NF-kappa B sites. In cotransfection experiments using various NF-kappa B subunit expression vectors and a reporter driven by three copies of an NF-kappa B element, activation of NF-kappa B-dependent transcription was observed with expression of p65 or combinations of p50-p65. Taken as a whole, these results suggest that IFN-alpha inhibits MCMV IE gene enhancer activity by mechanisms that decrease the availability of virus-induced NF-kappa B transcriptionally active in the nuclei of infected cells.

cAMP response element of murine cytomegalovirus immediate early gene enhancer is transactivated by ras oncogene products

Products of ras oncogenes strongly stimulate the activity of the reporter gene, chloramphenicol acetyltransferase (CAT), driven by a 1.2 kb fragment of the murine cytomegalovirus (MCMV) immediate early (IE) gene enhancer (pCMVCAT). To define the role of proteins binding to the unique cAMP response element (CRE) present in the IE enhancer, NIH 3T3 cells were cotransfected with prasZip6 plasmid, a mammalian expression vector containing a v-Ha-ras cDNA, together with p(delta)ACMVCAT (pCMVCAT without the CRE sequence). Lower stimulation of CAT activity was indeed observed upon deletion of the CRE sequence. Decreased levels of p(delta)ACMVCAT were also observed in cell lines carrying stably transfected ras oncogenes. Further support for the role of the CRE sequence in MCMV enhancer activation comes from the finding that v-Ha-ras expression increases the activity of a reporter gene, beta-galactosidase, driven by three tandem copies of CRE sequence about six-fold. Moreover, this transactivation was prevented by cotransfection of the dominant inhibitor mutant Ha-ras (Leu-61; Ser-186) and was not suppressed by cotransfection of Ha-ras (Asn-17), suggesting that the effect is due to activated ras protein, rather than normal p21ras. Finally the transactivation observed is accompanied by an increase in nuclear proteins binding to a labelled oligonucleotide homologous to the CRE sequence, as shown in a gel retardation assay. These results suggest that the CRE element contributes to the transactivation of the MCMV IE gene enhancer by ras oncogenes.

Constitutive expression of the interferon-inducible protein p202 in NIH 3T3 cells affects cell cycle progression

p202 is a protein expressed in murine cells after Interferon treatment. Although the function of p202 is still basically unknown, its ability to bind the hypophosphorylated form of the retinoblastoma protein pRb suggests a possible role in the control of cell proliferation. To investigate the role of p202 we have generated several cell clones of NIH 3T3 fibroblasts that constitutively express p202. Here we show that proliferation of quiescent cells on stimulation by serum addition is strongly inhibited by constitutive p202 expression. Moreover, when growth arrested cells are stimulated to proliferate, expression of p202 inhibits G0/G1 progression into the S phase and the cells accumulate with a DNA content that is equivalent to cells arrested in the G0/G1 phase of the cell cycle. Taken together, these studies suggest that p202 may play a negative role in growth regulation.

Mechanisms of viral inhibition by interferons

Interferons (IFNs) are a family of related proteins grouped in four species (alpha, beta, gamma and omega) according to their cellular origin, inducing agents and antigenic and functional properties. Their binding to specific receptors leads to the activation of signal transduction pathways that stimulate a defined set of genes, whose products are eventually responsible for the IFN antiviral effects. Their action against viruses is a complex phenomenon. It has been reported that IFNs restrict virus growth at the levels of penetration, uncoating, synthesis of mRNA, protein synthesis and assembly. This review will attempt to evaluate evidence of the involvement of the IFN-inducible proteins in the expression of the antiviral state against RNA or DNA viruses.

Characterization of nuclear factors involved in 202 gene induction by IFN-alpha, viruses or dsRNA in murine leukemia cells

When treated with IFN-alpha, L1210 leukemia cells express high levels of the mouse 202 gene mRNA after a few hours. Three tandem copies of a 43 bp fragment (GAbox) homologous to the IFN-stimulatable response element (ISRE), located in the 5'-flanking region of the 202 gene, were linked to the reporter CAT gene and transiently transfected into L1210 cells. The data suggest that the GA box is sufficient to confer transcriptional inducibility upon IFN stimulation. Binding assays, using the labeled GA box as a probe, demonstrated the presence of a retarded complex, designated GAbfl, in the nuclear extracts of L1210 cells treated with IFN-alpha. This complex is absent in the extracts of L1210 cells treated with ssRNA viruses or synthetic dsRNA. Moreover, photoaffinity cross-linking experiments revealed that GAbfl contains a protein of about 50 kDa. Altogether these results demonstrate that antiviral state induction by IFN-alpha in L1210 cells is preceded by GAbfl binding to the ISRE of the IFN-inducible genes.

Trans-activation of the mouse cytomegalovirus immediate early gene enhancer by ras oncogenes

The ras gene family encodes 21K proteins that reside on the inner face of the plasma membrane and bind GTP and GDP with an equally high affinity. Cotransfection of NIH 3T3 cells with a mammalian expression vector containing a viral Harvey-ras (v-Ha-ras) cDNA, together with a plasmid (pCMVCAT) carrying the immediate early (IE) enhancer of the murine cytomegalovirus (MCMV) linked to the chloramphenicol acetyltransferase (CAT) reporter gene strongly stimulated CAT activity. Basal levels of pCMVCAT expression as well as trans-activation by v-ras plasmid were both inhibited by cotransfection of an expression vector containing the dominant inhibitory mutant gene Ha-ras Asn-17. This indicates that the p21ras protein is responsible for these activities. High pCMVCAT activation was also observed in cell lines carrying stably transfected ras oncogenes, activated by point mutation or amplification. To define the cis-acting DNA elements in the MCMV IE enhancer responsible for this trans-activation by p21ras protein, we constructed several plasmids containing the CAT gene under control of MCMV IE enhancers that were deleted in different regions. The CAT assays demonstrated that several sequences were responsive to p21ras protein. These sequences are scattered throughout the IE enhancer, upstream of the transcription start site, and contain responsive elements that are homologous to the binding sites for cellular transcription factors such as NF kappa B, AP1, ATF and SP1. Activation of the p21ras protein may thus be one of the signals that regulate IE genes transcription during MCMV infection.

Characterization of nuclear factors involved in 202 gene induction by interferon-alpha in murine leukemia cells

The 5' terminal flanking region of the interferon-inducible gene, 202, contains an interferon-stimulable response element (ISRE), called a GA box, that confers inducibility by interferon(IFN)-alpha, but not by IFN-gamma, on a reporter gene, such as the chloramphenicol acetyltransferase (CAT). Nuclear extracts from L1210 murine leukemia cells, stimulated for various periods of time with IFN-alpha, were mixed with 32P-labeled GA box and analyzed for the presence of retarded complexes in electrophoretic-mobility-shift assays. In addition to a few constitutive retarded complexes, an inducible GA box-binding activity (GAbf-1) appeared after 5 min, peaked at about 2 h, and was still abundant 12 h after IFN-alpha treatment. In the cytoplasmic fraction GAbf-1 was not detectable before 30 min, continued to increase up to 2 h, but had disappeared within 12 h. GAbf-1 activity was not observed in nuclear extracts treated with IFN-gamma, and was not inhibited by prior treatment with the protein-synthesis inhibitor cycloheximide. When the binding properties of GAbf-1 were compared with those of ISGF-3, the primary transcriptional activator for IFN-alpha-induced genes, a different pattern of retarded complexes was observed. Moreover, as observed by immunoblotting analysis, nuclear extracts from IFN-alpha-treated L1210 cells did not contain the p91/84 subunit of the ISGF3, the best characterized nuclear complex activated by IFN-alpha. Altogether these results indicate that GAbf-1 may be a novel transcription factor exploited by IFN-alpha to activate the 202 inducible gene in murine pre-B leukemia cells.

[Serum stimulates the transcriptional activity of the enhancer of the immediate-early genes of the murine cytomegalovirus through p21 ras]

The analysis of the MCMV IE enhancer revealed the presence of many putative binding sites for the transcription factors AP-1 and NFkB. Previous studies suggested that such factors represent a final target for the metabolic cascade triggered by serum and growth factors. On these basis we wanted to verify if serum stimulates the transcriptional activity of the MCMV IE enhancer through p21ras and AP-1 and NFkB according to the actual model of transduction of the mitogenic signal. Our data demonstrate that serum stimulates the MCMV IE enhancer through a pathway in which the p21ras is involded, as demonstrated by using the dominant inhibitory mutant ras(Asn 17). Moreover deletion mutant analysis of the enhancer showed that the serum responsive region lies between nucleotides -1280 and -285 and contains a high concentration of putative AP-1 and NFkB binding sites.

[Protein p53 inhibits the activity of the enhancer of the immediate-early genes of murine cytomegalovirus]

The protein encoded by the tumor suppressor gene p53 can complex and functionally interact with cytomegalovirus proteins produced during the immediate-early phase of infection. The functions of these complex are unclear but there is some evidence to suggest that binding of p53 to these viral proteins may inactivate p53 functions. Recent reports have shown that p53 is involved in regulation of transcription. In this study we have considered the possibility that p53 may regulate transcription of cytomegalovirus immediate early genes which play a crucial role for virus replication. Here we report that experiments in which NIH 3T3 cells were cotransfected with a p53 expression plasmid together with a reporter gene linked to the mouse cytomegalovirus immediate-early enhancer/promoter revealed that wild type p53 could efficiently reduce the transcriptional activity of this viral regulatory sequence. By contrast expression of a mutated p53 correlated with a much smaller reduction of transcription. Deletion mutants analysis of the enhancer revealed that repression of transcription by p53 requires a minimal promoter containing an SP1 consensus sequence and a TATA box.

Interferons inhibit onset of murine cytomegalovirus immediate-early gene transcription

Treatment of NIH 3T3 fibroblasts with interferon-alpha or interferon-gamma (IFN-alpha or IFN-gamma) significantly reduced murine cytomegalovirus (MCMV) replication. Determination of viral DNA in the nuclei of the infected cells before onset of DNA replication demonstrated that virus uptake, transport to the nucleus, and DNA stability were not decreased. Analysis of the virus specified mRNAs soon after infection revealed that in the cells exposed to IFNs expression of the immediate early (IE) genes was strongly reduced. Nuclear run-off transcription analysis showed that this inhibition is due to significant reduction of IE gene transcription rates following IFN treatment. Since transcription of the MCMV IE region is regulated by a strong enhancer element, a construct containing the chloramphenicol acetyltransferase (CAT) reporter gene, driven by an 1.2 kb segment spanning the enhancer and IE1/3 promoter region of the IE transcription unit, was transfected into NIH 3T3 cells. Treatment with IFN-alpha or IFN-gamma after transfection strongly reduced CAT activity compared to untreated controls. In an attempt to define a negative IFN-responsive element in the IE enhancer, a series of deletion mutants driving the CAT reporter gene were transfected into NIH 3T3 cells that were then treated with IFN-alpha. With the sole exception of the construct containing the minimal MCMV IE1/3 promoter (-102 to the cap site), all other deletion mutants were strongly down-regulated by IFN-alpha-treatment. Taken as a whole, these results suggest that IFNs inhibit MCMV replication by impairing the transcription of the IE transcription units, and that this negative regulation is carried out by sequences scattered throughout the IE enhancer region.

Effect of interferon-alpha on immediate early gene expression of murine cytomegalovirus

Interferon-alpha (IFN-alpha) significantly reduced the replication of murine cytomegalovirus (MCMV) in mouse embryo fibroblasts derived from the susceptible mouse strain C3H/HeJ. When infectious virus production was measured, a strong decrease in virus titer was observed in IFN-treated cells at a multiplicity of infection (moi) of 1 and 0.5 pfu/cell. Analysis of virus-specified mRNAs by Northern blot assay revealed that IFN-alpha had a significant effect on the expression of viral mRNAs at 48h. In particular, the mRNAs of the major immediate early (IE) transcription units, IE1, IE2, and IE3, were impaired by IFN-alpha. In addition, decrease of IE1 mRNA synthesis was accompanied by a reduction of the major IE product (pp89), as revealed by Western blot assay. These results suggest that IFN-alpha may inhibit MCMV replication by directly impairing IE gene transcription.

Cloned transcription factor MTF-1 activates the mouse metallothionein I promoter

Metallothioneins (MTs) are small cysteine-rich proteins whose structure is conserved from fungi to man. MTs strongly bind heavy metals, notably zinc, copper and cadmium. Upon exposure of cells to heavy metal and other adverse treatments, MT gene transcription is strongly enhanced. Metal induction is mediated by several copies of a 15 bp consensus sequence (metal-responsive element, MRE) present in the promoter region of MT genes. We and others have demonstrated the presence of an MRE-binding factor in HeLa cell nuclear extracts. We found that this factor, termed MTF-1 (MRE-binding transcription factor) is inactivated/reactivated in vitro by zinc withdrawal/addition. Here we report that the amounts of MTF-1-DNA complexes are elevated several-fold in zinc-treated cells, as measured by bandshift assay. We have also cloned the cDNA of mouse MTF-1, a 72.5 kDa protein. MTF-1 contains six zinc fingers and separate transcriptional activation domains with high contents of acidic and proline residues. Ectopic expression of MTF-1 in primate or rodent cells strongly enhances transcription of a reporter gene that is driven by four consensus MREd sites, or by the complete mouse MT-I promoter, even at normal zinc levels.