Identification of a second promoter in the human c-ets-2 proto-oncogene

We localized and characterized a new regulatory element with promoter activity in the human c-ets-2 intron 1. This promoter governs the expression of 5' divergent c-ets-2 transcripts through multiple start sites dispersed within 300 bp. Among the multiple start sites detected, three are major transcriptional initiation points. We detected transcripts initiated from this new promoter in various cell lines such as COLO 320, NBE, or HepG2 cells. This promoter exhibits transcriptional activity when linked to the CAT gene, and deletion constructs reveal that it contains activating and repressing elements. The sequence of the promoter reveals putative binding sites for ETS, MYB, GATA, and Oct factors. In addition, we show that this promoter is functionally conserved in the chicken.

Expression of the human fetal bac h19 gene in invasive cancers

We have isolated a new -ene, named BAC, which is the human equivalent of the murine H19 gene and is highly expressed in most fetal tissues and in a variety of fresh tumors. BAC was analyzed in 130 untreated invasive carcinomas of different types. The frequency of BAC-expressing cancers as well as the level of expression greatly varied among the different types of cancer and within the same type of cancer. For example, the 2.3 kb BAC transcript band was detected in 94% of breast adenocarcinomas and in only 35% of epidermoid lung carcinomas with differences of 100-fold in the level of expression between tumor specimens. The majority of tumor tissues displayed BAC expression while their normal counterpart did not with the exception of normal breast tissues which contained low but significant level of BAC transcript. It is possible that BAC expression was influenced by the presence of gene deletions in tumors. Indeed, this gene is located in chromosome 11p15, a region in which deletions have frequently been observed in human cancers. Therefore, the variable levels of expression could have a biological significance and be used as a marker of tumor progression.

Homologous T and B cells immortalized in vitro by the Epstein-Barr virus exhibit differential genetical and functional features

After in vitro EBV infection of peripheral blood lymphocytes (PBL), we previously obtained IL-2-independent T-cell lines expressing EBNA1 and LMP1 viral latent genes. One tumorigenic clone, NC5, was further characterized for chromosomal abnormalities, rearrangement and expression of oncogenes, and constitutive or induced activation of cellular transduction pathways. NC5 as well as TC cells derived from an NC5-induced tumor exhibited the same few chromosomal abnormalities absent in normal PBL and B-cell lines (LCLs) from the same donor. No rearrangement or altered expression of C-MYC, BCL-2 and NF-KB2 oncogenes could be detected. In contrast, we found high levels of BCL-X and thioredoxin (TRX), as markers of EBV infection or T-cell activation/transformation status. No constitutive activation of NF-kappa B or STAT transcriptional complexes was observed in these cells. For NF-kappa B, this was in apparent contradiction with its reported inducibility mediated by LMP1, taking into account that NF-kappa B was still inducible by TNF alpha or PMA and ionomycin. Our results highlight independence of EBV protein-mediated transformation towards classical cellular pathways in T-lymphocytes.

Molecular cloning and characterization of the mouse E2F6 gene

E2F6 is the most recently identified member of the E2F family. In this study, the murine E2F6 gene was cloned and found to consist of eight exons. Analysis of its 5' flanking region revealed two transcription start sites. The proximal promoter region contained no TATA or CAAT box. We also identified a novel E2F6 mRNA containing the alternative exon 2. The E2F6 mRNAs are highly expressed during mouse embryogenesis and are present in all adult tissues examined. Moreover, E2F6 shows a unique expression pattern in synchronized mouse embryonic fibroblasts. E2F6 expression rapidly increases during the G0-G1 transition, reaching its higher level in mid-G1, and remains relatively constant thereafter. These findings suggest that E2F6 may contribute to the regulation of events throughout the cell cycle. Isolation of the murine E2F6 gene is a step toward generation of genetically modified mouse models that will help to understand the functions of E2F6.

The Ets family member Erg gene is expressed in mesodermal tissues and neural crests at fundamental steps during mouse embryogenesis

The Erg gene belongs to the Ets family encoding a class of transcription factors. To gain new insight on the in vivo functional specificity of the Erg gene within the wide Ets family, we used in situ hybridization to determine its expression pattern during murine embryogenesis. We found that the Erg gene expression predominates in mesodermal tissues, including the endothelial, precartilaginous and urogenital areas. A specific Erg gene expression was also identified in migrating neural crest cells. A comparison with Fli-1, the most closely Erg-related gene, revealed that both gene expressions partially overlap, suggesting that they may contribute to related functions in these tissues. Like other Ets family genes, Erg seems involved in several fundamental developmental steps in murine embryogenesis, including epithelio-mesenchymal transition, cell migration, settlement and differentiation.

The carboxy-terminal end of the candidate tumor suppressor gene HIC-1 is phylogenetically conserved

HIC-1 (hypermethylated in cancer), a new candidate tumor suppressor gene located in 17p13.3, encodes a protein with five Krüppel-like C2H2 zinc finger motifs and a N-terminal protein-protein interaction domain called BTB/POZ. These two domains appeared as the only conserved regions found between human HIC-1 and its avian homologue, gammaFBP-B, isolated as a transcriptional repressor of the gammaF-Crystallin gene. We have recloned the HIC-1 gene and found four nucleotide differences within the 3' part of its published coding sequence. The corrected HIC-1 C-terminal end exhibits now significant homology (70%) to the chicken gammaFBP-B C-terminal end and appears thus as a third phylogenetically conserved domain that may serve an important, yet unknown function in HIC-1.

Erg proteins, transcription factors of the Ets family, form homo, heterodimers and ternary complexes via two distinct domains

The ets genes family encodes a group of proteins which function as transcription factors under physiological conditions. We report here that the Erg proteins, members of the Ets family, form homo and heterodimeric complexes in vitro. We demonstrate that the Ergp55 protein isoform forms dimers with itself and with the two other isoforms, Ergp49 and Ergp38. Using a set of Erg protein deletion mutants, we define two distinct domains independently involved in dimerization. The first one is located in the amino-terminal part of the protein containing the pointed domain (PNT), conserved in a subset of Ets proteins. The second one resides within the ETS domain, the DNA-binding domain. We also show that the Erg protein central region behaves as an inhibitory domain of dimerization and its removal enhances the Ergp55 transactivation properties. Furthermore, Ergp55 forms heterodimers with some other Ets proteins. Among the latter, we show that Fli-1, Ets-2, Er81 and Pu-1 physically interact with Erg. Finally, we show that the formation of the previously described ternary complex Ergp55/Fos/jun is mediated by ETS domain and Jun protein, while the ternary complex Ergp49/Fos/Jun is mediated by Fos protein.

Isolation and characterization of a chicken homologue of the Spi-1/PU.1 transcription factor

Spi-1/PU.1 is a member of the Ets family of transcription factors important in regulation of hematopoiesis. We have isolated a chicken cDNA homologuous to the mammalian Spi-1/PU.1 gene with an open reading frame of 250 amino acids (aa). The chicken Spi-1/PU.1 protein is 14 aa and 16 aa shorter than its human and mouse counterparts but is extremely well conserved with 78.8% and 75.2% identity respectively. The carboxy terminal DNA binding region, or ETS binding domain, is 100% identical to that of human and mouse. Some differences with the mammalian homologues are seen in the N-terminal part of the protein and in the PEST connecting domain. However, the differences are mainly conservative and all the features underlying functional aspects seem preserved. The major discrepancy lies in a 12 aa deletion in an already poorly conserved part of the PEST sequence. Spi-1/PU.1 transcripts were detected at high levels in spleen and Fabricius bursa of chick embryos by Northern blot and in situ hybridization. Our results show that the chicken Spi-1/PU.1 protein behaves like a bonafide Spi-1/PU.1 transcription factor in its DNA binding and transactivating properties.

The avian transcription factor c-Rel is expressed in lymphocyte precursor cells and antigen-presenting cells during thymus development

Transcription factors of the Rel/NF-kappaB family are widely involved in the immune system. In this study, we investigate the in vivo expression of the avian protein c-Rel in the T-cell lineage during thymus development. The majority of thymocytes do not express the c-Rel protein. However, lymphocyte precursor cells that colonize the thymus express the c-Rel protein shortly after their homing in the organ and before they begin to differentiate. c-Rel is also detected in different subsets of antigen-presenting cells such as epithelial cells, dendritic cells, and macrophages. In vitro studies have shown that Rel/NF-kappaB proteins are sequestered in an inactive form in the cytoplasm by interaction with the IkappaBalpha inhibitory protein. By immunocytochemistry, we show that in vivo c-Rel is localized in the cytoplasm of antigen-presenting cells but in both the cytoplasm and nucleus of lymphocyte precursor cells. The cytoplasmic localization of c-Rel in antigen-presenting cells correlates with a high expression of IkappaBalpha, whereas the nuclear localization of c-Rel in lymphocyte precursor cells correlates with a much lower expression of IkappaBalpha. These results suggest that c-Rel might be constitutively activated in lymphocyte precursor cells.

Ligand binding was acquired during evolution of nuclear receptors

The nuclear receptor (NR) superfamily comprises, in addition to ligand-activated transcription factors, members for which no ligand has been identified to date. We demonstrate that orphan receptors are randomly distributed in the evolutionary tree and that there is no relationship between the position of a given liganded receptor in the tree and the chemical nature of its ligand. NRs are specific to metazoans, as revealed by a screen of NR-related sequences in early- and non-metazoan organisms. The analysis of the NR gene duplication pattern during the evolution of metazoans shows that the present NR diversity arose from two waves of gene duplications. Strikingly, our results suggest that the ancestral NR was an orphan receptor that acquired ligand-binding ability during subsequent evolution.

Isolation and characterization of transformed human T-cell lines infected by Epstein-Barr virus

Epstein-Barr virus (EBV) is a human lymphotropic virus whose main targets have traditionally been described as B lymphocytes and epithelial cells. Here we report the isolation and characterization of largely monoclonal transformed human T-cell lines infected by EBV. The transformed T cells expressed CD2, CD3, and either CD4 or CD8 surface molecules and more generally displayed the phenotype of naive T cells with a complete and clonal rearrangement of the T-cell receptor. None of the cell lines expressed B cells, natural killer, or myeloid antigens or had immunoglobulins genes rearrangement. They grew in the absence of growth factor; however, they all secreted interleukin-2 after mitogenic activation. Polymerase chain reaction (PCR) analysis showed the presence of EBV DNA in all these cell lines. Moreover, Southern blot analysis of one of these cell lines shows the presence of circular episomic EBV DNA, and by Northern blot or reverse transcriptase-PCR analysis, only the expression of Epstein-Barr nuclear antigen-1 (EBNA-1) and latent membrane protein-1 (LMP-1) genes was detected. Finally, the complete transformed phenotype of this T-cell line was shown by its injection into nude or recombination activating gene 2 (RAG2)-deficient mice that led to the formation of solid tumors.

Thyroid hormone receptor genes of neotenic amphibians

Since thyroid hormones play a pivotal role in amphibian metamorphosis we used PCR to amplify DNA fragments corresponding to a portion of the ligand-binding domain of the thyroid hormone receptor (TR) genes in several neotenic amphibians: the obligatory neotenic members of the family Proteidea the mudpuppy Necturus maculosus and Proteus anguinus as well as two members of the facultative neotenic Ambystoma genus: the axolotl Ambystoma mexicanum and the tiger salamander Ambystoma tigrinum. In addition, we looked for TR genes in the genome of an apode Typhlonectes compressicaudus. TR genes were found in all these species including the obligatory neotenic ones. The PCR fragments obtained encompass both the C and E domains and correspond to alpha and beta genes. Their sequences appear to be normal, suggesting that there is no acceleration of evolutionary rates in the TR genes of neotenic amphibians. This result is not surprising for Ambystomatidae, which are known to respond to T3 (3,3',5-triiodothyronine) but is not in agreement with biochemical and biological data showing that Proteidea cannot respond to thyroid hormones. Interestingly, by RT-PCR analysis we observed a high expression levels of TRalpha in gills, intestine, and muscles of Necturus as well as in the liver of Ambystoma mexicanum, whereas TRbeta expression was only detected in Ambystoma mexicanum but not in Necturus. Such a differential expression pattern of TRalpha and TRbeta may explain the neoteny in Proteidea. The cloning of thyroid-hormone-receptor gene fragments from these species will allow the molecular study of their failure to undergo metamorphosis.

Hydrocortisone modulates the expression of c-ets-1 and 72 kDa type IV collagenase in chicken dermis during early feather morphogenesis

At the onset of chicken feather morphogenesis, dermal cells migrate along bundles of collagen fibers to colonize areas where bud outgrowth takes place. Chicken embryos treated with hydrocortisone during the critical phase of dermal rearrangement show featherless skin areas in which the dermis exhibits an increase of interstitial collagen. We had previously demonstrated that c-ets-1 is a nuclear transcription factor expressed in the dermis at the beginning of feather morphogenesis. Here we study, by in situ mRNA hybridization, the expression of c-ets-1 in the dermis of chicken embryos treated with hydrocortisone. We found that, among the two distinct products (p54 and p68) encoded by the chicken c-ets-1, the expression of the p68 product increased while expression of p54 decreased after hydrocortisone treatment. Since Ets-1 regulates matrix-metalloproteinases genes, we analyzed the expression of the 72 kDa type IV collagenase in both normal and hydrocortisone-treated embryos. We demonstrated that 72 kDa type IV collagenase mRNA expression decreased in the dermis after hydrocortisone treatment and that its expression correlated with that of p54c-ets-1. Taken together, these results indicate that hydrocortisone modulates c-ets-1 expression. In addition, they raise the interesting possibility that c-ets-1 might be involved in an altered pattern of feather development mediated by the accumulation of collagen due to a decrease in collagenase activities.

TATA-less promoters of some Ets-family genes are efficiently repressed by wild-type p53

p53 has been reported to repress a number of TATA-containing promoters in transient transfection assays. TATA-less promoters are generally believed to be refractive to p53 repression. We report here that the TATA-less promoters of Ets-family genes (Ets-1 and Ets-2) are efficiently repressed by wild-type but not mutant p53 in transient co-transfection assays. Moreover, p53 was immunologically detected in protein complexes formed on oligonucleotides from both the TATA-containing and TATA-less promoters. Our data suggest that p53 is involved in the regulation of the expression of both promoter types, most probably by protein-protein interaction. A model for p53 function in promoter repression is proposed.

The 5' part of the human H19 RNA contains cis-acting elements hampering its translatability

H19 is an imprinted gene developmentally regulated in man and mouse and implicated in various neoplasms. No corresponding protein product has yet been detected, although several open reading frames (ORFs) could be identified along its RNA. The largest ORF found in the human gene could encode a putative 26 kDa protein. We have isolated two H19 cDNAs (AP and ES) that contain this ORF4 and correspond to incomplete copies of the unique 2.3 kb H19 RNA. In transient expression assays, AP was able to synthesize a 26 kDa protein whereas ES was not. With respect to ORF4, ES exhibits a 536 bp long GC-rich 5' untranslated region, whereas AP contains the last 22 nucleotides of this 5'UTR. Using deletions and point mutations, we have found that the length and probably the secondary structure of the 5'UTR strongly hampers the translatability of the RNA. In addition, a potential role of upstream ORFs (uORFs) was detected as stressed by the enhances translation of a construct mutated in uORF3 overlapping ORF4. Interactions between H19 and proteins are indicated by a specific binding between 5'UTR derived RNA segments and two nuclear proteins of about 27 kDa. Our results favor a potential role of these particular structures and binding properties in general trans-regulation of RNA post-transcriptional processes rather than in normal control of H19 mRNA translation.

A cAMP response element and an Ets motif are involved in the transcriptional regulation of flt-1 tyrosine kinase (vascular endothelial growth factor receptor 1) gene

The flt-1 gene encodes a transmembrane tyrosine kinase, Flt-1, a receptor for vascular endothelial growth factor. The expression of flt-1 gene is restricted to endothelial cells in vivo. To understand the molecular mechanism underlying endothelial-specific expression of this gene, we studied the functional significance of transcriptional motifs in the 200-base pair region of the human flt-1 gene promoter, which has been identified to confer cell type specificity. By point mutation analysis using chloramphenicol acetyltransferase plasmids in 293E1 cells, which express significant levels of flt-1 mRNA, we found that an Ets motif, E4, at -54 to -51 and a cAMP response element (CRE) at -83 to -76 are involved in the transcriptional regulation of this gene. Disruption of either this CRE or E4 within the promoter sequence of 90 base pairs resulted in a decrease in chloramphenicol acetyltransferase activity of 90%, indicating that co-existence of both of CRE and Ets motif E4 is necessary for transcription of the flt-1 gene. Co-transfection of an expression vector containing c-ets-1, c-ets-2, or c-erg cDNA with this 90-base pair sequence yielded a 5-8-fold elevation of chloramphenicol acetyltransferase activity, further supporting the idea that Ets family protein(s) participates in the regulation of the flt-1 gene. Gel shift assays using nuclear extracts of 293E1 and endothelial cells demonstrated the existence of protein factor(s) that specifically binds to CRE and Ets motif E4, respectively. Taken together, our results strongly suggest cooperation of a CRE and an Ets motif for the function of the flt-1 gene promoter.

PEA3 transactivates vimentin promoter in mammary epithelial and tumor cells

We have used differential display RT-PCR method to detect the genes specifically activated or repressed between mammary tumor and normal mammary epithelial cells. One of the genes identified is vimentin. The vimentin gene is abundantly expressed in both human and mouse mammary tumor cells and its expression decreased dramatically in normal mammary epithelial cells. The expression of vimentin gene correlates with the expression of transcription factor PEA3. Since the promoters of human and mouse vimentin genes contain one PEA3 binding site we investigated the ability of PEA3 to transactivate the vimentin promoter in mouse mammary epithelial cell CLS1, mouse mammary tumor MMT and human mammary tumor cell lines MCF7 and MDA231. Our results suggest that PEA3 specifically transactivates vimentin promoter through PEA3 site. Among members of the ETS transcription factor family only Erg showed ability to transactivate vimentin promoter besides PEA3. Our results also suggest that NFkB site on the vimentin promoter may act as a positive regulatory element for the transcription of vimentin. In metastatic mammary tumors derived from mice carrying the polyoma middle T or neu transgene, PEA3 is overexpressed and vimentin has been shown to play a key role in the motility of cells. Our results suggest that one of the roles of PEA3 in mammary tumor is to participate the activation of vimentin gene whose gene product in turn contributes to the metastatic potential of mammary tumors.

Sensitivity to the parvovirus minute virus of mice as a probe for azatyrosine-mediated phenotypic reversion of spontaneously transformed cells

It has been shown that cells transformed by known oncogenes could be reverted to an untransformed phenotype by the antibiotic Azatyrosine (AzTyr). In order to evaluate the reverting effect of AzTyr on five spontaneously transformed FR3T3C cell clones, we performed three assays: soft agar clonability, tumorigenicity in nude mice and susceptibility to killing by the parvovirus minute virus of mice (MVMp). In contrast to untransformed cells, transformed or tumorigenic cells are permissive for the lytic replication of MVMp and are killed. Our results demonstrate that although the cell populations that emerged after AzTyr treatment of FR3T3C clones had different phenotypes (two were untransformed and two had an altered transformed phenotype), they all behaved like untransformed cells, as judged from their resistance to MVMp infection. Our results demonstrate that susceptibility to MVMp is a valuable way to monitor the reversion.

Down-regulation of fibronectin gene expression by the p53 tumor suppressor protein

The tumor suppressor p53 protein down-regulates in vitro the expression of several cellular and viral promoters. However, it is not clear whether this down-regulation reflects equivalent modulation of the activity of these promoters in vivo. Here, we propose a suitable system to assess the effect of p53 on gene expression in vivo: the pair of p53 antisense-transfected and parental HeLa cells. The low amount of free wild-type p53 in HeLa cells seems still sufficient for the repression of several promoters that might be derepressed in p53 antisense-transfected HeLa cells. We have used this system for the demonstration both in vivo and in vitro of the repression of the fibronectin (FN) gene promoter by wild-type p53. The protein and mRNA amounts for FN were increased in the p53 antisense-transfected HeLa clones. This was accompanied by the restoration of the FN network in these cells. FN promoter constructs fused to the chloramphenicol acetyltransferase reporter gene were specifically repressed by wild-type p53 in different cell lines. Integrin alpha 5 beta 1 clustering was changed in the sites of focal contacts, most probably representing its relocalization as a consequence of the increased amounts of fibronectin.

Two functionally distinct domains responsible for transactivation by the Ets family member ERM

The recently cloned human Ets transcription factor ERM is closely related to the ER81 and PEA3 genes. Here, we report the functional analysis of the DNA-binding and transactivation properties of ERM. Specific DNA-binding by ERM requires the ETS domain, conserved in all members of the Ets family and is inhibited by an 84 residue long central region and the carboxy-terminal tail. Two fragments of ERM are transferrable activation domains: alpha, which sits in the 72 first residues and encompasses the acidic domain conserved between ERM, ER81 and PEA3, and the carboxy-terminal tail which also bears a DNA-binding inhibition function. Deletion of alpha strongly reduces transactivation by ERM. Moreover, alpha and the carboxy-terminal tail exhibit functional synergism, suggesting that they activate transcription through different mechanisms. In support of this idea, we demonstrate that VP16 squelches transactivation by alpha but not by the carboxy-terminal tail. This result also indicates that alpha and VP16 may share common limiting cofactors. alpha and the carboxy-terminal tail do not seem to be conserved within the whole Ets family, indicating that the specificity of ERM may rely on interactions with distinct cofactors.

Antisense p53 provokes changes in HeLa cell growth and morphology

Human cervical carcinoma HeLa cells, containing the wild-type p53 protein, were stably transfected with a p53 antisense encoding plasmid. Differences in the p53 content in extracts from the p53 antisense transfected HeLa clones compared to the parental HeLa cells were demonstrated by protein binding to the p53 consensus oligonucleotide and in transactivation assays. Striking morphological alterations were observed in several HeLa clones stably transfected with the p53 antisense encoding plasmid. Giant multinucleated cells appeared several passages after transfection in four p53 antisense transfected HeLa clones derived from three independent transfection experiments, and were not observed in parental HeLa cells, or in p53 sense or vector-transfected HeLa clones. A strong growth inhibitory effect was observed for the HeLa clones stably transfected with p53 antisense encoding plasmid. Reconstitution experiments with HeLa cells treated with a short p53 antisense oligonucleotide gave similar results: growth inhibition and giant cells. These giant multinucleated cells were negative for [3H]thymidine incorporation and displayed dispersed nuclear staining for proliferating cell nuclear antigen, indicating the absence of DNA replication.

Isopropanol precipitation removes PCR inhibitors from ancient bone extracts

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Mesodermal expression of the chicken erg gene associated with precartilaginous condensation and cartilage differentiation

The ets gene superfamily encodes a class of transcription factors that bind to a purine rich sequence through a 85 amino-acid ETS domain. Among them, the human erg gene has been found to be involved in Ewing's sarcoma, primitive neurectodermal tumour of childhood and acute myeloid leukaemia. Nevertheless, little is known about human erg expression. Northern blot analyses have shown a human erg expression restricted to few cell lines and thymus, but the status concerning expression during development remains unknown probably because no homologue of this gene has yet been isolated and studied in other vertebrates. We thus choose to clone the chicken erg gene (ck-erg) and to study its expression during chicken development. We obtained a bona fide clone of ck-erg and defined the transcriptional modulating properties of its product. The ck-Erg protein acts as a transcriptional activator through a conventional consensus ETS binding site. Northern blot studies on various chicken tissues, in situ analyses and comparison with the well-characterised c-ets-1 expression show that ck-erg is expressed in mesoderm- and, to a lesser extent, in ectoderm-derived tissues. During chicken development, two salient features could be observed. From stage E1 to E3.5, ck-erg expression was widely distributed in mesodermal derivatives and neural crest, resembling c-ets-1 expression. However, by E6, the expression of ck-erg exhibited, unlike c-ets-1, a drastically new and strong signal in precartilaginous condensation zones and cartilaginous skeletal primordia. These stages are the first steps of bone formation during skeletal elaboration. Our results show for the first time a possible specific involvement of ck-erg in cartilage morphogenesis.

Expression of interstitial collagenase is restricted to skeletal tissue during mouse embryogenesis

Collagenases are thought to be involved in physiological and pathological processes that require extracellular matrix remodeling. Using the in situ hybridization technique, we describe the expression of interstitial collagenase gene during mouse embryogenesis between E6.5 and E17. We demonstrate that interstitial collagenase expression is exclusively detected in one event, namely the onset of bone formation. Transcripts accumulate in hypertrophied chondrocytes, found in the mature cartilaginous matrix of long-bone growth plates or ribs, and in osteoblasts and/or in endothelial cells that have migrated into the shafts of developing long bones. The expression of the tissue inhibitor of metalloproteinases (TIMP-2) gene precedes the expression of interstitial collagenase in developing bones. These data suggest that interstitial collagenase plays a specific role in bone development and that the tight regulation of its activity during development is achieved not only by post-translational mechanisms with TIMPs, as previously suggested, but also at the transcriptional level.

Rab2 nucleotide coding sequence in gallus gallus and it phylogenetic position

The nucleic acid sequence of the chicken rab2 mRNA was determined by sequencing a full length cDNA. The phylogeny of rab2 sequences was established.

The H19 gene is expressed within both epithelial and stromal components of human invasive adenocarcinomas

In a previous work, we have isolated the human H19 gene and shown accumulation of transcripts in various human tumors including breast carcinomas (Douc-Rasy et al (1993) Int J Oncol 2, 753-758). Questions arose, after Northern blot results, about the precise H19 mRNA location, specially in normal breast tissues and benign or malign primary breast tumors. Then we performed molecular in situ hybridization to get insight into tissue expression of the H19 gene. Examined resections included one normal tissue, one fibroadenoma and 13 cancers. Results obtained with the H19 probe can be summarized as follows: 1) in normal breast tissues signals were focally observed in epithelial cells, but more predominantly in the palleal tissue which is sensitive to hormones; 2) in the fibroadenoma, fibroblastic cells were extensively labeled at the stroma-epithelium boundary, but epithelial cells were negative; and 3) in primary cancers, eight specimens exhibited signals on stromal cells, one specimen on epithelial cells and four on both epithelial and stromal cells. Data provide the following evidence: 1) usually labeled cells are clustered, either within normal or pathological tissues; 2) the labeling pattern highly differs from one tumor to another; and 3) H19 probe displays very different signals from one cell to another in given compartment of a given tissue section. In conclusion, it seems that a high H19 expression matches the tumor invasion. Our results suggest that the expression of this gene is concerned by the relationships between epithelial and stromal cells, and can reflect peculiar physiological states of the cells. Furthermore, we discuss results showing an abundant expression of H19 gene in some adenocarcinomas of bad prognosis, in the context of the otherwise established tumor-suppressor role of this gene, or the strictly controlled gene dosage, which could be overridden in these particular cases.

Tracking the origins of the cave bear (Ursus spelaeus) by mitochondrial DNA sequencing

The different European populations of Ursus arctos, the brown bear, were recently studied for mitochondrial DNA polymorphism. Two clearly distinct lineages (eastern and western) were found, which may have diverged approximately 850,000 years ago. In this context, it was interesting to study the cave bear, Ursus spelaeus, a species which became extinct 20,000 years ago. In this study, we have amplified and sequenced a fragment of 139-bp in the mitochondrial DNA control region of a 40,000-year-old specimen of U. spelaeus. Phylogenetic reconstructions using this sequence and the European brown bear sequences already published suggest that U. spelaeus diverged from an early offshoot of U. arctos--i.e., approximately at the same time as the divergence of the two main lineages of U. arctos. This divergence probably took place at the earliest glaciation, likely due to geographic separation during the earlier Quaternary cold periods. This result is in agreement with the paleontological data available and suggests a good correspondence between molecular and morphological data.

Rev-erb beta, a new member of the nuclear receptor superfamily, is expressed in the nervous system during chicken development

We have identified and characterized a new orphan member of the nuclear hormone receptor superfamily in the chicken. This new gene, called Rev-erb beta, exhibits strong homologies with the Rev-erb alpha/ear-1 orphan receptor gene, which partially overlaps the thyroid hormone receptor alpha gene in opposite orientation. We demonstrate that both Rev-erb alpha and Rev-erb beta genes are conserved in their C and E domains. Rev-erb beta binds to DNA as a monomer and recognizes the same binding motif as the alpha gene product. The Rev-erb beta gene product does not interact with retinoid X receptors, as revealed by gel shift experiments. In situ hybridization experiments show that Rev-erb beta is expressed in the central and peripheric nervous system, spleen, and mandibular and maxillar processes, as well as in blood islands. During embryonic development, we noticed a striking specific distribution of Rev-erb beta transcripts in the notochord at 24 h and later on, in the floor plate of the neural tube. We propose that Rev-erb beta may play an important role in the complex network of inductive signals, which control neuron differentiation.

A model for gene evolution of the ets-1/ets-2 transcription factors based on structural and functional homologies

The chicken c-ets-1 locus encodes two transcription factors, p54c-ets-1 and p68c-ets-1 that differ in their N-termini, encoded respectively by the I54 and alpha beta exons. p68c-ets-1 equivalents are only found in birds and reptiles while p54c-ets-1 is widely conserved in vertebrates, from amphibians to mammals. Thus, the classical view concerning the evolution of the c-ets-1 gene has been to consider that I54 is of ancient origin whereas alpha and beta, which provide an additional activating domain in p68c-ets-1, would have been acquired much more recently. Sequencing the alpha and beta exons in various species pinpointed a highly conserved region of 13 amino acids which is rich in acidic and hydrophobic residues, a feature of some other transactivating domains. Strikingly, this subdomain is also present in the otherwise unrelated N-terminal activating region of p58c-ets-2 and was thus named BEC for Ets-1-beta/Ets-2-Conserved sequence. Moreover, the two N-termini share the BEC sequence at a homologous position in their highly similar genomic organization indicating a common origin. This structural homology underlies a functional similarity since fusion of the heterologous GAL4 DNA-binding domain with either of the two isolated domains demonstrates that BEC is essential in both cases for the transactivating activity. The function of the alpha beta domain in the context of p68c-ets-1 also strictly depends on the presence of the BEC sequence. Finally, the whole N-terminus of p58c-ets-2 can functionally substitute for its counterpart in p68c-ets-1 further demonstrating that p68c-ets-1 and p58c-ets-2 are structurally and functionally more closely related than previously thought. Besides, we also found BEC in the N-terminus of the Drosophila pointed gene which may be considered as closely related to the uncommitted 'ets1/2' common ancestor. These data demonstrate that the alpha and beta exons are not a recent and specific acquisition but stem, like the p58c-ets-2 N-terminus, from the invertebrate unduplicated 'ets 1/2' gene. This work unravels a new model for the ets-1/ets-2 gene's evolution, based for the first time on both structural and functional evidences. Accordingly, p68c-ets-1 and p58c-ets-2 are the direct descendants of the ancestral 'ets1/2' gene whereas I54 may have been acquired as a second promoter in the c-ets-1 gene after the duplication. Indeed, I54 is not found in the Drosophila pointed gene. The high degree of similarity, and hence of functional redundancy, between p68c-ets-1 and p58c-ets-2 may have led to the rapid divergence (and even loss in mammals) of alpha and beta during evolution whereas I54, which provided a novel function unique to c-ets-1, was maintained within the presently widespread p54c-ets-1 version.

[Does the c-ets 1 oncogene participate in the regulation of tumor angiogenesis?]

Does the c-ets 1 protooncogene take part in the regulation of tumor angiogenesis? The formation of new blood vessels is an essential process in embryonic development and wound healing, for tumor growth and metastasis. In situ hybridization studies have revealed that the protooncogene c-ets 1 is expressed in endothelial cells at the beginning of blood vessel formation, in normal and pathological conditions. C-ets 1 encodes a transcription factor, a protein which binds specifically to DNA and which regulates the transcription of genes containing these specific binding sequences in their promotors. Thus in vitro experiments suggest that c-ets 1 may activate the transcription of genes encoding collagenase 1, stromelysine 1 and urokinase plasminogen activator, proteases involved in extracellular matrix degradation. A working hypothesis is that c-ets 1 takes part in regulating angiogenesis by controlling the transcription of these genes whose activity is necessary for the migration of endothelial cells from preexisting capillaries. This hypothesis is discussed with respect to current experimental evidences and to the complexity of the regulatory network controlling gene transcription and extracellular matrix degradation.

In vivo cooperation of two nuclear oncogenic proteins, P135gag-myb-ets and p61/63myc, leads to transformation and immortalization of chicken myelomonocytic cells

To investigate a possible in vivo cooperation between the p61/63myc and P135gag-myb-ets proteins, we used a previously constructed retrovirus, named MHE226, which contains the fused v-myb and v-ets oncogenes of the E26 retrovirus and the v-myc oncogene of MH2. For that purpose, chicken neuroretina cells producing MHE226 and pseudotyped with the Rous associated virus-1 (RAV-1) helper virus were injected in 1-day-old chickens. In control experiments, we also injected chicken neuroretina cells producing E26 (RAV-1), RAV-1 alone, or constructs lacking one of the oncogenes of MHE226. The average life span of MHE226-infected chickens is half that of E26-infected chickens. MHE226-infected chickens harbor tumors scattered in many organs, but compared with E26, MHE226 induced a weak leukemia. Study of integration sites suggests that the majority of the tumors results from clonal or oligoclonal events. Cell cultures were derived from the tumors of MHE226-infected chickens and grown in standard medium without addition of exogenous chicken myelomonocytic growth factor. These cells still divide at high rate after more than 100 passages and can thus be considered immortalized. By using several criteria, these cells were characterized as precursors of the myelomonocytic lineages.

Induced expression of the conditionally cytotoxic herpes simplex virus thymidine kinase gene by means of a parvoviral regulatory circuit

As a step toward the achievement of targeted expression of toxic genes, we have established a model system using the selective trans-activation of the late promoter P38 of Minute Virus of Mice (MVMp) by the parvoviral nonstructural protein NS-1. The conditionally toxic herpes simplex virus type 1 thymidine kinase (tk) gene (HSV1-tk) was cloned under the control of the P38 promoter and transfected into NIH-3T3 TK- cells. Treatment of the stably transfected cells with acyclovir (ACV) followed by infection with MVMp reduced cell survival by 3.5- to 5-fold compared to the toxic effects of ACV or MVMp alone. These results indicate that it should be possible to combine the genuine cytopathic action of parvoviruses with a specific activation of toxic genes driven by parvoviral promoters, to achieve the targeted destruction of parvovirus-expressing (in particular tumor) cells.

Pax-QNR/Pax-6, a paired- and homeobox-containing protein, recognizes Ets binding sites and can alter the transactivating properties of Ets transcription factors

We have previously isolated a cDNA clone encoding a protein with a paired- and homeodomain from MC29-transformed quail neuroretina cells that we have termed Pax-QNR. Pax-QNR is homologous to the murine Pax-6, which is mutated in the autosomal dominant mutation small eye (Sey) of the mouse and aniridia in man. The 46 kDa Pax-QNR protein binds specifically to the e5 DNA recognition sequence present upstream of the Drosophila even-skipped gene. The Pax-QNR paired and homeobox domains expressed separately in bacteria are both able to recognize this sequence. The core sequence recognized by the paired domain of Pax genes is TTCC (GGAA), and this sequence is also present in the core recognition site bound specifically by Ets family-encoded proteins. Ets proteins are a family of transcription factors sharing a highly conserved 85 amino acid DNA binding domain. In this article we demonstrate that Pax-QNR/Pax-6 expressed in reticulocyte lysate is able to specifically recognize several Ets binding sites. In addition, we found that the transactivation mediated by the p68c-ets-1 pr p55erg through the Polyomavirus enhancer sequence is specifically inhibited by the p46kDaPax-QNR in transient transfection assay.

Predicted common structural features of DNA-binding domains from Ets, Myb and HMG transcription factors

The Ets family of transcription factors shares a 85 amino acid domain, named the ETS domain, which appears responsible for their DNA binding activity. This domain did not show any clear similarity with already known DNA binding motifs. Hydrophobic Cluster Analysis (HCA), a sensitive method able to detect protein structural relationships even at low sequence identity, was chosen in order to compare the ETS domain with other conventional DNA binding motifs. HCA analysis combined with known three-dimensional NMR data, suggests that the ETS domain may be structurally related to the Myb DNA binding domain and possibly to the HMG one. Indeed, the ETS domain is likely to contain two helix-loop-helix motifs.

Involvement of the proto-oncogene c-ets 1 and the urokinase plasminogen activator during mouse implantation and placentation

Many of the Ets proteins have been shown to be transcription activators. In vitro, Ets 1 proteins are involved in the transcriptional induction of genes such as stromelysin 1, collagenase 1 or urokinase type plasminogen activator, which are proteases responsible for extracellular matrix degradation. In vivo, c-ets 1 is expressed in a wide variety of embryonic tissues in migrating cells, especially in endothelial cells during blood vessel formation. C-ets 1 is also expressed in stromal cells of invasive carcinomas. In the present work, we have investigated the expression of both c-ets 1 and u-PA, a putative target gene of the Ets 1 proteins, within a biological model which includes both embryonic and tumoral aspects. Implantation and placentation of the mouse embryo display migration of the trophoblastic cells, which invade the stroma of the uterine endometrium and trigger the establishment of a new vascular frame. Using in situ hybridization, we show that the overlapping of expression of c-ets 1 and u-PA is restricted to some maternal cell populations from the invasive front and to the endothelial cells of the endometrial vasculature. C-ets 1 is never expressed in trophoblasts. In contrast, u-PA expression in trophoblasts is strong and coincides with the embryo invasive phase. In the embryo proper, c-ets 1 displays a spatio-temporal expression pattern similar to that described in the chick embryo. Until E 10.5, u-PA is expressed neither in embryonic nor in extra-embryonic structures. The respective roles of c-ets 1 and u-PA and their relationship during mammalian placentation are discussed.

Characterization of quail Pax-6 (Pax-QNR) proteins expressed in the neuroretina

After differential screening of a cDNA library constructed from quail neuroretina cells (QNR) infected with the v-myc-containing avian retrovirus MC29, we have isolated a cDNA clone, Pax-QNR, homologous to the murine Pax-6, which is mutated in the autosomal dominant mutation small eye of mice and in the disorder aniridia in humans. Here we report the characterization of the Pax-QNR proteins expressed in the avian neuroretina. From bacterially expressed Pax-QNR peptides, we obtained rabbit antisera directed against different domains of the protein: paired domain (serum 11), domain between the paired domain and homeodomain (serum 12), homeodomain (serum 13), and carboxyl-terminal part (serum 14). Sera 12, 13, and 14 were able to specifically recognize five proteins (48, 46, 43, 33, and 32 kDa) in the neuroretina. In contrast to proteins of 48, 46, and 43 kDa, proteins of 33 and 32 kDa were not recognized by the paired antiserum (serum 11). Paired-less and paired-containing proteins exhibited the same half-life (6 h) and were phosphorylated mostly on serine residues. Immunoprecipitations performed with subcellular fractions of neuroretinas showed that the paired-containing proteins were located in the nucleus, whereas the 33- and 32-kDa proteins were found essentially in the cytoplasmic compartment. However, immunofluorescence experiments performed after transient transfections showed that p46 and p33/32 were also located in vivo into the nucleus. Thus, the Pax-QNR/Pax-6 gene can produce proteins with two DNA-binding domains as well as proteins containing only the DNA-binding homeodomain.

New human erg isoforms generated by alternative splicing are transcriptional activators

The erg gene is a member of the ets gene family. ETS proteins have been shown to bind specifically the (GGA-A/T) motif and to transactivate via this consensus sequence. The human erg products exhibit approximately 70% homology with ETS proteins in their DNA-binding domain. We have isolated three erg cDNAs from a human fetal liver library. Two of them are different from the previously described erg-1 and erg-2 cDNAs (Rao et al., Science, 1987, 237, 635-639), in the middle of their coding sequence and in their 5' part where a novel initiation codon is introduced. These isoforms are generated by alternative RNA splicing from a single gene that leads to the inclusion or exclusion of different exon sequences. The three cDNAs expressed by an in vitro transcription-translation system direct the synthesis of proteins of approximately 38, 49 and 55 kDa. These in vitro erg products were tested for their DNA-binding activity by gel mobility-shift assays with different probes containing the ETS-specific binding site. The results indicated that all these erg isoforms are able to bind the ETS binding site in a specific manner. Our data using transient transfection assays indicate that erg protein isoforms function as transcriptional activators.

A new mechanism of oncogenic activation: E26 retroviral v-ets oncogene has inverted the C-terminal end of the transcription factor c-ets-1

The v-ets-encoded domain in the P135gag-myb-ets transforming protein of the E26 retrovirus differs mainly from its cellular progenitor, p68c-ets-1 by two point mutations and by the replacement of the 13 last C-terminal amino acids present in c-ets-1 by 16 unrelated residues of previously unknown origin in v-ets. Here, we demonstrate that these v-ets C-terminal specific residues are in fact encoded by the opposite strand of the c-ets-1 C-terminus.

Ancestry and diversity of the HMG box superfamily

The HMG box is a novel type of DNA-binding domain found in a diverse group of proteins. The HMG box superfamily comprises a.o. the High Mobility Group proteins HMG1 and HMG2, the nucleolar transcription factor UBF, the lymphoid transcription factors TCF-1 and LEF-1, the fungal mating-type genes mat-Mc and MATA1, and the mammalian sex-determining gene SRY. The superfamily dates back to at least 1,000 million years ago, as its members appear in animals, plants and yeast. Alignment of all known HMG boxes defined an unusually loose consensus sequence. We constructed phylogenetic trees connecting the members of the HMG box superfamily in order to understand their evolution. This analysis led us to distinguish two subfamilies: one comprising proteins with a single sequence-specific HMG box, the other encompassing relatively non sequence-specific DNA-binding proteins with multiple HMG boxes. By studying the extent of diversification of the superfamily, we found that the speed of evolution was very different within the various groups of HMG-box containing factors. Comparison of the evolution of the two boxes of ABF2 and of mtTF1 implied different diversification models for these two proteins. Finally, we provide a tree for the highly complex group of SRY-like ('Sox' genes), clustering at least 40 different loci that rapidly diverged in various animal lineages.

Characterization of a functional promoter for the human thyroid hormone receptor alpha (c-erbA-1) gene

The thyroid hormone receptor alpha (THRA or c-erbA-1) gene belongs to a family of genes that encode nuclear receptors for various hydrophobic ligands such as steroids, retinoic acid and thyroid hormones. We have previously described the genomic organization of the human THRA gene, which comprises 10 exons distributed along 27 kbp of genomic DNA. We describe here a promoter that initiates THRA transcription. This promoter contains no obvious TATA-like element but is very GC rich and harbors numerous Sp1 sites. It also contains several sites similar to previously described cis-acting sequences including hormone-responsive elements (HREs). When transfected into cultured HeLa cells, it drives the expression of a CAT reporter gene. The activity of this human THRA promoter is enhanced by the synthetic glucocorticoid dexamethasone but seems unaffected by thyroid hormones.

Evolution of the ets gene family

Over the past few years a variety of genes have been described whose protein products share similarity with that of the c-ets-1 proto-oncogene, the cellular counterpart of the v-ets oncogene of the avian E26 retrovirus. This so-called "ets family" of transcription factors includes at least a dozen members present in several organisms. We have questioned the common evolutionary origin of these various gene products. By constructing phylogenetical trees with different methods, we show that the ets family is very ancient since the duplication of the various groups of ets related proteins occurred before the Arthropods/Vertebrates split (ca. 500 million years).

[Does oncogene c-ets 1 participate in the regulation of tumor angiogenesis?]

The formation of new blood vessels is an essential process in embryonic development and wound healing, for tumor growth and metastasis. In situ hybridization studies have revealed that the protooncogene c-est1 is expressed in endothelial cells at the beginning of blood vessel formation, in normal and pathological conditions. c-ets1 encodes a transcription factor, a protein which binds specifically to DNA and which regulates the transcription of genes containing these specific binding sequences in their promotors. Thus, in vitro experiments suggest that c-ets1 may activate the transcription of genes encoding collagenase 1, stromelysine 1 and urokinase plasminogen activator, proteases involved in extracellular matrix degradation. A working hypothesis is that c-ets1 takes part in regulating angiogenesis by controlling the transcription of these genes whose activity is necessary for the migration of endothelial cells from pre-existing capillaries. This hypothesis is discussed with respect to current experimental evidence and to the complexity of the regulatory network controlling gene transcription and extracellular matrix degradation.

New chromosome 21 DNA markers isolated by pulsed field gel electrophoresis from an ETS2-containing Down syndrome chromosomal region

To generate new chromosome 21 markers in a region that is critical for the pathogenesis of Down syndrome (D21S55-MX1), we used pulsed field gel electrophoresis (PFGE) to isolate a 600-kb NruI DNA fragment from the WA17 hybrid cell line, which has retained chromosome 21 as the only human material. This fragment, which contains the oncogene ETS2, was used to construct a partial genomic library. Among the 14 unique sequences that were isolated, 3 were polymorphic markers and contained sequences that are conserved in mammals. Five of these markers mapped on the ETS2-containing NruI fragment and allowed us to define an 800-kb high-resolution PFGE map.

The various domains of v-myb and v-ets oncogenes of E26 retrovirus contribute differently, but cooperatively, in transformation of hematopoietic lineages

The genome of the avian leukemia virus E26 is a unique example of association between two transcription factors which appear as a fused composite nuclear oncoprotein, P135gag-myb-ets. Previous studies with E26 have shown that v-myb and v-ets must cooperate to fully transform both erythrocytic and myelomonocytic precursor cells in vivo and in vitro. To analyse further the contribution of the individual domains involved in the transformation of various hematopoietic lineages, we have constructed several mutant viruses expressing a fusion protein with deletions in either v-myb or v-ets. We show here that integrity of the v-ets oncogene is necessary for transformation of the erythrocytic cells but that neither the DNA-binding domain nor the trans-activating domain of v-myb is required for this transformation. The DNA-binding domain of v-ets is necessary to transform myelomonocytic cells. Furthermore, we show that E26 onco-protein also transforms granulocytic cells. The v-ets DNA-binding domain is not necessary to transform them, whereas deleting the v-myb DNA-binding domain strongly reduces transformation of these cells. These data show that the v-myb and v-ets DNA-binding domains provide quite different contributions to the transformation of various hematopoietic lineages by E26.

Protein truncation is not required for c-myb proto-oncogene activity in neuroretina cells

The v-myb oncogene of avian myeloblastosis virus (AMV) differs from its normal cellular counterpart by a truncation at both its amino and carboxyl termini and by a substitution of 11 amino acid residues. We had previously shown that v-myb-containing AMV, in the presence of basic fibroblast growth factor, transformed chicken neuroretina (CNR) cells. To understand the mechanism of c-myb activation, we have tested whether avian retroviruses that express the full-length c-Myb are also active on CNR cells. We have found that c-Myb, like v-Myb, strongly increases the basic fibroblast growth factor response of CNR cells and that these c-myb-expressing cells are able to grow in soft agar in the presence of the growth factor. We have also found that, in contrast to normal or v-myb-expressing AMV-transformed CNR cells, c-Myb-transformed cells express mim-1, a granulocyte-specific gene. However, normal v-Myb- and c-Myb-expressing CNR cells all express the pax-QNR gene, a newly described paired and homeobox-containing gene specifically expressed in the neuroretina. We conclude that, in contrast to what has been described for hematopoietic cells, overexpression of c-Myb is sufficient to activate gene expression and to induce an abnormal behavior of CNR cells.

Characterization of a paired box- and homeobox-containing quail gene (Pax-QNR) expressed in the neuroretina

The retina is an integral part of the central nervous system, and consists of two layers, the outer pigmented layer and the inner sensory layer or neuroretina (NR). The NR layer contains several strata of cells (glial and neuronal) derived from proliferating neuroectodermal precursors that differentiate after terminal mitosis. In vitro, NR cells can differentiate not only into neuronal and glial types, but also into pigment and lens cells. Quail (Coturnix coturnix japonica) NR cells (QNR) infected with MC29 transforming retrovirus become pigmented after several passages in vitro. In order to characterize the genes expressed in these pigmented MC29 QNR, a cDNA library was prepared from these cells. After differential screening we have isolated a cDNA clone which identifies an RNA expressed in NR but not in the pigmented layer of the retina. This cDNA encodes a protein related to that of Drosophila, mouse and zebrafish paired box- and homeobox-containing segmentation genes and is called Pax-QNR. The expression of Pax-QNR in the NR is confined to the ganglionic cell layer and to the lower part of the inner nuclear layer containing the amacrine or correlation neurones.

Phylogeny of the p68c-ets-1 amino-terminal transactivating domain reveals some highly conserved structural features

The chicken c-ets-1 locus gives rise to two distinct transcription factors differing only in their structurally and functionally unrelated N-termini. One of these transcription factors, p54c-ets-1, contains a specific, short (27 amino acids), hydrophilic N-terminus encoded by a single exon, I54, that is widely conserved among vertebrates. The other one, p68c-ets-1, the cellular counterpart of the viral ets oncogene product, differs in the replacement of the I54 by two exons, termed alpha and beta, encoding a larger (71 amino acids), hydrophobic N-terminus which, in contrast to I54, exhibits properties of a transactivating domain. To date the alpha and beta exons have only been found in chicken. Here, we demonstrate the existence of the alpha and beta exons in other avian species (quail and duck) and the existence of the alpha exon in reptiles (turtle). However, none of them could be detected in mammals. Our results strongly suggest that, in contrast to the phylogenetically well-conserved I54 exon, the alpha exon is restricted to reptilian species (birds and 'true' reptiles), whereas the beta exon is detectable so far only in birds. Comparison of their amino acid sequences reveals that the alpha exon and to a much greater extent the beta exon have diverged faster than the I54 exon. In addition, we show that the N- and C-terminal thirds of the alpha exon and the highly hydrophobic nature of the alpha beta-encoded sequence are heavily conserved features and thus likely to be required for function as a transactivating domain in p68c-ets-1 and possibly in the viral P135gag-myb-ets transforming protein.

The ets gene family

Ets proteins have a conserved DNA-binding domain and regulate transcriptional initiation from a variety of cellular and viral gene promoter and enhancer elements. Some members of the Ets family, Ets-1 and Ets-2, cooperate in transcription with the AP-1 transcription factor, the product of the proto-oncogene families, fos and jun, while others, Elk-1 and SAP-1, form ternary complexes with the serum response factor (SRF). Certain ets gene family members possess transforming activity while others are activated by proviral integration in erythroleukaemias.