Transcriptional activation by the Antennapedia and fushi tarazu proteins in cultured Drosophila cells

Regulation of Silk Genes by Hox and Homeodomain Proteins in the Terminal Differentiated Silk Gland of the Silkworm Bombyx mori

The silk gland of the silkworm Bombyx mori is a long tubular organ that is divided into several subparts along its anteroposterior (AP) axis. As a trait of terminal differentiation of the silk gland, several silk protein genes are expressed with unique regional specificities. Most of the Hox and some of the homeobox genes are also expressed in the differentiated silk gland with regional specificities. The expression patterns of Hox genes in the silk gland roughly correspond to those in embryogenesis showing "colinearity". The central Hox class protein Antennapedia (Antp) directly regulates the expression of several middle silk gland-specific silk genes, whereas the Lin-1/Isl-1/Mec3 (LIM)-homeodomain transcriptional factor Arrowhead (Awh) regulates the expression of posterior silk gland-specific genes for silk fiber proteins. We summarize our results and discuss the usefulness of the silk gland of Bombyx mori for analyzing the function of Hox genes. Further analyses of the regulatory mechanisms underlying the region-specific expression of silk genes will provide novel insights into the molecular bases for target-gene selection and regulation by Hox and homeodomain proteins.

Expression and purification of HER2 extracellular domain proteins in Schneider2 insect cells

Overexpression of human epidermal growth factor receptor 2 (HER2/ErbB2/Neu) results in ligand independent activation of kinase signaling and is found in about 30% of human breast cancers, and is correlated with a more aggressive tumor phenotype. The HER2 extracellular domain (ECD) consists of four domains - I, II, III and IV. Although the role of each domain in the dimerization and activation of the receptor has been extensively studied, the role of domain IV (DIV) is not clearly understood yet. In our previous studies, we reported peptidomimetic molecules inhibit HER2:HER3 heterodimerization. In order to study the binding interactions of peptidomimetics with HER2 DIV in detail, properly folded recombinant HER2 protein in pure form is important. We have expressed and purified HER2 ECD and DIV proteins in the Drosophila melanogaster Schneider2 (S2) cell line. Using the commercial Drosophila expression system (DES), we transfected S2 cells with plasmids designed to direct the expression of secreted recombinant HER2 ECD and DIV proteins. The secreted proteins were purified from the conditioned medium by filtration, ultrafiltration, dialysis and nickel affinity chromatography techniques. The purified HER2 proteins were then analyzed using Western blot, mass spectrometry and circular dichroism (CD) spectroscopy.

Control of the spineless antennal enhancer: direct repression of antennal target genes by Antennapedia

It is currently thought that antennal target genes are activated in Drosophila by the combined action of Distal-less, homothorax, and extradenticle, and that the Hox gene Antennapedia prevents activation of antennal genes in the leg by repressing homothorax. To test these ideas, we analyze a 62bp enhancer from the antennal gene spineless that is specific for the third antennal segment. This enhancer is activated by a tripartite complex of Distal-less, Homothorax, and Extradenticle. Surprisingly, Antennapedia represses the enhancer directly, at least in part by competing with Distal-less for binding. We show that Antennapedia is required in the leg only within a proximal ring that coexpresses Distal-less, Homothorax and Extradenticle. We conclude that the function of Antennapedia in the leg is not to repress homothorax, as has been suggested, but to directly repress spineless and other antennal genes that would otherwise be activated within this ring.

Stripy Ftz target genes are coordinately regulated by Ftz-F1

During development, cascades of regulatory genes act in a hierarchical fashion to subdivide the embryo into increasingly specified body regions. This has been best characterized in Drosophila, where genes encoding regulatory transcription factors form a network to direct the development of the basic segmented body plan. The pair-rule genes are pivotal in this process as they are responsible for the first subdivision of the embryo into repeated metameric units. The Drosophila pair-rule gene fushi tarazu (ftz) is a derived Hox gene expressed in and required for the development of alternate parasegments. Previous studies suggested that Ftz achieves its distinct regulatory specificity as a segmentation protein by interacting with a ubiquitously expressed cofactor, the nuclear receptor Ftz-F1. However, the downstream target genes regulated by Ftz and other pair-rule genes to direct segment formation are not known. In this study, we selected candidate Ftz targets by virtue of their early expression in Ftz-like stripes. This identified two new Ftz target genes, drumstick (drm) and no ocelli (noc), and confirmed that Ftz regulates a serotonin receptor (5-HT2). These are the earliest Ftz targets identified to date and all are coordinately regulated by Ftz-F1. Engrailed (En), the best-characterized Ftz/Ftz-F1 downstream target, is not an intermediate in regulation. The drm genomic region harbors two separate seven-stripe enhancers, identified by virtue of predicted Ftz-F1 binding sites, and these sites are necessary for stripe expression in vivo. We propose that pair-rule genes, exemplified by Ftz/Ftz-F1, promote segmentation by acting at different hierarchical levels, regulating first, other segmentation genes; second, other regulatory genes that in turn control specific cellular processes such as tissue differentiation; and, third, 'segmentation realizator genes' that are directly involved in morphogenesis.

Transformation of Drosophila cell lines: an alternative approach to exogenous protein expression

Techniques and experimental applications are described for exogenous protein expression in Drosophila cell lines. Ways in which the Drosophila cell lines and the baculovirus expression vector system differ in their applications are emphasized.

HOXB6 protein is bound to CREB-binding protein and represses globin expression in a DNA binding-dependent, PBX interaction-independent process

Although HOXB6 and other HOX genes have previously been associated with hematopoiesis and leukemias, the precise mechanism of action of their protein products remains unclear. Here we use a biological model in which HOXB6 represses alpha- and gamma-globin mRNA levels to perform a structure/function analysis for this homeodomain protein. HOXB6 protein represses globin transcript levels in stably transfected K562 cells in a DNA-binding dependent fashion. However, the capacity to form cooperative DNA-binding complexes with the PBX co-factor protein is not required for HOXB6 biological activity. Neither the conserved extreme N-terminal region, a polyglutamic acid region at the protein C terminus, nor the Ser(214) CKII phosphorylation site was required for DNA binding or activity in this model. We have previously reported that HOX proteins can inhibit CREB-binding protein (CBP)-histone acetyltransferase-mediated potentiation of reporter gene transcription. We now show that endogenous CBP is co-precipitated with exogenous HOXB6 from nuclear and cytoplasmic compartments of transfected K562 cells. Furthermore, endogenous CBP co-precipitates with endogenous HOXB6 in day 14.5 murine fetal liver cells during active globin gene expression in this tissue. The CBP interaction motif was localized to the homeodomain but does not require the highly conserved helix 3. Our data suggest that the homeodomain contains most or all of the important structures required for HOXB6 activity in blood cells.

The nuclear receptor Ftz-F1 and homeodomain protein Ftz interact through evolutionarily conserved protein domains

The Drosophila homeodomain protein Fushi Tarazu (Ftz) and its partner, the orphan receptor Ftz-F1, are members of two distinct families of DNA binding transcriptional regulators. Ftz and Ftz-F1 form a novel partnership in vivo as a Hox/orphan receptor heterodimer. Here we show that the murine Ftz-F1 ortholog SF-1 functionally substitutes for Ftz-F1 in vivo, rescuing the defects of ftz-f1 mutants. This finding identified evolutionarily conserved domains of Ftz-F1 as critical for activity of this receptor in vivo. These domains function, at least in part, by mediating direct protein interactions with Ftz. The Ftz-F1 DNA binding domain interacts strongly with Ftz and dramatically facilitates the binding of Ftz to target DNA. This interaction is augmented by a second interaction between the AF-2 domain of Ftz-F1 and the N-terminus of Ftz via an LRALL sequence in Ftz that is reminiscent of LXXLL motifs in nuclear receptor coactivators. We propose that Ftz-F1 serves as a cofactor for Ftz by facilitating the selection of target sites in the genome that contain Ftz/Ftz-F1 composite binding sites. Ftz, on the other hand, influences Ftz-F1 activity by interacting with its AF-2 domain in a manner that mimics a nuclear receptor coactivator.

Mechanisms regulating target gene selection by the homeodomain-containing protein Fushi tarazu

Homeodomain proteins are DNA-binding transcription factors that control major developmental patterning events. Although DNA binding is mediated by the homeodomain, interactions with other transcription factors play an unusually important role in the selection and regulation of target genes. A major question in the field is whether these cofactor interactions select target genes by modulating DNA binding site specificity (selective binding model), transcriptional activity (activity regulation model) or both. A related issue is whether the number of target genes bound and regulated is a small or large percentage of genes in the genome. In this study, we have addressed these issues using a chimeric protein that contains the strong activation domain of the viral VP16 protein fused to the Drosophila homeodomain-containing protein Fushi tarazu (Ftz). We find that genes previously thought not to be direct targets of Ftz remain unaffected by FtzVP16. Addition of the VP16 activation domain to Ftz does, however, allow it to regulate previously identified target genes at times and in regions that Ftz alone cannot. It also changes Ftz into an activator of two genes that it normally represses. Taken together, the results suggest that Ftz binds and regulates a relatively limited number of target genes, and that cofactors affect target gene specificity primarily by controlling binding site selection. Activity regulation then fine-tunes the temporal and spatial domains of promoter responses, the magnitude of these responses, and whether they are positive or negative.

How does the fushi tarazu gene activate engrailed in the Drosophila embryo?

In the even-numbered parasegments of the Drosophila embryo, expression of the fushi tarazu (ftz) gene is necessary for transcription of engrailed (en). Yet those cells expressing ftz+ in a stripe, only the anteriormost come to express en. One explanation is that the level of ftz+ might be graded across the stripe and in order to express en, it would be sufficient for cells to exceed a threshold concentration of Ftz protein. We use photographs and microspectrophotometry to measure differences in Ftz antigen concentration; we do not find a gradient within the Ftz stripe. Rather, the stripe appears to contain cells with similar amounts of antigen plus a few weakly staining cells that are usually at the posterior edge. Further, varying the amount of Ftz protein has no effect on en expression. Finally, embryos lacking the even-skipped gene have normal levels of Ftz but do not express en. Our observations appear to rule out the threshold hypothesis.

A gamete-specific, sex-limited homeodomain protein in Chlamydomonas

During fertilization in Chlamydomonas, gametes of opposite mating types interact with each other through sex-specific adhesion molecules on their flagellar surfaces. Flagellar adhesion brings the cell bodies of the gametes into close contact and initiates a signal transduction pathway in preparation for cell-cell fusion. We have identified a cDNA, gsp1, whose transcript levels are upregulated during flagellar adhesion. The GSP1 polypeptide is a novel, gamete-specific homeodomain protein, the first to be identified in an alga. Its homeodomain shows significant identity with several higher plant homeodomain proteins. Although encoded by a single copy gene present in cells of both mating types, immunoblot analysis showed that GSP1 was expressed in mating type (mt)+ gametes, but was not detectable in mt- gametes or in vegetative cells of either mating type. Moreover, GSP1 appeared late during gametogenesis, suggesting that it may function during adhesion with mt- gametes or after zygote formation. GSP1 is expressed in imp11, mt- mutant gametes, which have a lesion in the mid gene involved in sex determination and exhibit many phenotypic characteristics of mt+ gametes. Thus, gsp1 is negatively regulated by mid and is the first molecule to be identified in Chlamydomonas that shows sex-limited expression.

Molecular mechanism of polyhomeotic activation by Engrailed

The Drosophila Engrailed homeoprotein has been shown to activate directly a Polycomb-group gene, polyhomeotic, during embryogenesis. The molecular mechanism involved in this activation has been studied. Two different types of Engrailed-binding fragments have been detected within the polyhomeotic locus. The P1 and D1 fragments contain several 'TTAATTGCAT' motifs, whereas the D2 fragment contains a long 'TAAT' stretch to which multiple copies of Engrailed bind cooperatively. Another homeodomain-containing protein, Extradenticle, establishes protein-protein interactions with Engrailed on the D2 fragment. We have shown by CAT assays that both types of Engrailed-binding sites (P1 or D1 and D2), as well as Extradenticle, are necessary to obtain activation by Engrailed. In vivo, we have also shown that normal polyhomeotic expression depends on extradenticle expression. Moreover, in the absence of Extradenticle, overexpression of Engrailed protein represses polyhomeotic expression.

Two distinct types of repression domain in engrailed: one interacts with the groucho corepressor and is preferentially active on integrated target genes

Active transcriptional repression has been characterized as a function of many regulatory factors. It facilitates combinatorial regulation of gene expression by allowing repressors to be dominant over activators under certain conditions. Here, we show that the Engrailed protein uses two distinct mechanisms to repress transcription. One activity is predominant under normal transient transfection assay conditions in cultured cells. A second activity is predominant in an in vivo active repression assay. The domain mediating the in vivo activity (eh1) is highly conserved throughout several classes of homeoproteins and interacts specifically with the Groucho corepressor. While eh1 shows only weak activity in transient transfections, much stronger activity is seen in culture when an integrated target gene is used. In this assay, the relative activities of different repression domains closely parallel those seen in vivo, with eh1 showing the predominant activity. Reducing the amounts of repressor and target gene in a transient transfection assay also increases the sensitivity of the assay to the Groucho interaction domain, albeit to a lesser extent. This suggests that it utilizes rate-limiting components that are relatively low in abundance. Since Groucho itself is abundant in these cells, the results suggest that a limiting component is recruited effectively by the repressor-corepressor complex only on integrated target genes.

A phosphorylation site in the ftz homeodomain is required for activity

The Drosophila homeodomain-containing protein Fushi tarazu (Ftz) is expressed sequentially in the embryo, first in alternate segments, then in specific neuroblasts and neurons in the central nervous system, and finally in parts of the gut. During these different developmental stages, the protein is heavily phosphorylated on different subsets of Ser and Thr residues. This stage-specific phosphorylation suggests possible roles for signal transduction pathways in directing tissue-specific Ftz activities. Here we show that one of the Ftz phosphorylation sites, T263 in the N-terminus of the Ftz homeodomain, is phosphorylated in vitro by Drosophila embryo extracts and protein kinase A. In the embryo, mutagenesis of this site to the non-phosphorylatable residue Ala resulted in loss of ftz-dependent segments. Conversely, substitution of T263 with Asp, which is also non-phosphorylatable, but which successfully mimics phosphorylated residues in a number of proteins, rescued the mutant phenotype. This suggests that T263 is in the phosphorylated state when functioning normally in vivo. We also demonstrate that the T263 substitutions of Ala and Asp do not affect Ftz DNA-binding activity in vitro, nor do they affect stability or transcriptional activity in transfected S2 cells. This suggests that T263 phosphorylation is most likely required for a homeodomain-mediated interaction with an embryonically expressed protein.

A silencer is required for maintenance of transcriptional repression throughout Drosophila development

Transcriptional silencing by the Polycomb Group of genes maintains the position-specific repression of homeotic genes throughout Drosophila development. The Polycomb Group of genes characterized to date encode chromatin-associated proteins that have been suggested to form heterochromatin-like structures. By studying the expression of reporter genes, we have identified a 725 bp fragment, called MCP725, in the homeotic gene Abdominal-B, that accurately maintains position-specific silencing during proliferation of imaginal cells. Silencing by MCP725 requires the Polycomb and the Polycomblike genes, indicating that it contains a Polycomb response element To investigate the mechanisms of transcriptional silencing by MCP725, we have studied its temporal requirements by removing MCP725 from the transgene at various times during development. We have discovered that excision of MCP725 during larval stages leads to loss of silencing. Our findings indicate that the silencer is required for the maintenance of the repressed state throughout cell proliferation. They also suggest that propagation of the silenced state does not occur merely by templating of a heterochromatin structure by virtue of protein-protein interactions. Rather, they suggest that silencers play an active role in the maintenance of the position-specific repression throughout development.

Transcriptional regulation of the Drosophila homeotic gene teashirt by the homeodomain protein Fushi tarazu

The Drosophila melanogaster gene teashirt (tsh) is essential for segment identity of the embryonic thorax and abdomen. A deletion 3' to the tsh transcription unit causes the loss of tsh early expression in the even-numbered parasegments, and the corresponding larval cuticular patterns are disrupted. tsh function in the odd-numbered parasegments in these mutants is normal by both criteria. The in vivo activities of genomic fragments from the deleted region were tested in transgenic embryos. A 2.0 kb enhancer from the 3' region acts mainly in the even-numbered parasegments and is dependent on fushi tarazu (ftz) activity, which encodes a homeodomain protein required for the development of even-numbered parasegments. Ftz protein binds in vitro to four distinct sequences in a 220 bp sub-fragment; these and neighboring sequences are conserved in the equivalent enhancer isolated from Drosophila virilis. Tsh protein produced under the control of the 220 bp enhancer partially rescues a null tsh mutation, with its strongest effect in the even-numbered parasegments. Mutation of the Ftz binding sites partially abrogates the capacity for rescue. These results suggest a composite mechanism for regulation of tsh, with different activators such as ftz contributing to the overall pattern of expression of this key regulator.

Mobile element 297 in the Abd-B gene of Drosophila melanogaster, not Delta 88, is responsible for the tuh-3 mutation

The tumorous-head-3 (tuh-3) mutation has been associated with the insertion of mobile element Delta 88 at +200 on the bithorax complex (BX-C) DNA map, 5' of all Abdominal-B (Abd-B) transcripts. Different phenotypes of tuh-3 are regulated by the tumorous-head-1 (tuh-1) maternal effect locus. In the presence of the recessive tuh-1h maternal effect, tuh-3 offspring produce homeotic abdominal and genital tissue in the head. In the presence of the dominant tuh-1g maternal effect, tuh-3 offspring have normal heads but now show genital defects. One other mutant, I127B, produces flies with identical defects to that of tuh-3 in the presence of both maternal effects. Molecular analysis of I127B revealed the insertion of mobile element 297 in the Abd-B gene, approximately 25 kb downstream of the Delta 88 insertion in tuh-3. No other abnormalities were detected. Reexamination of our tuh-3 strain revealed a 297 insertion in an identical region to that of I127B, in addition to the Delta 88 insertion. Recombinants of tuh-3, carrying 297 only, produced homeotic head defects and genital defects in the presence of the tuh-1h and tuh-1g maternal effects, respectively. Recombinants of tuh-3, carrying Delta 88 only, failed to produce any defects in the presence of either maternal effect. Based upon these results, we propose that it is the 297 insertion in the Abd-B gene, not Delta 88, that is responsible for the tuh-3 mutation.

A conserved region of engrailed, shared among all en-, gsc-, Nk1-, Nk2- and msh-class homeoproteins, mediates active transcriptional repression in vivo

The engrailed homeoprotein is a dominantly acting or ‘active’ transcriptional repressor both in cultured cells and in vivo. When retargeted via a homeodomain swap to the endogenous fushi tarazu gene (ftz), it actively represses it, resulting in a ftz mutant phenocopy. We have mapped functional regions of engrailed using this in vivo repression assay. In addition to a region containing an active repression domain identified in cell culture assays (K. Han and J. L. Manley (1993) EMBO J. 12, 2723–2733), we find that two evolutionarily conserved regions contribute to activity. The one of these that does not flank the HD is particularly crucial to repression activity in vivo. We find that this domain is present not only in all engrailed-class homeoproteins but also in all known members of several other classes, including goosecoid, Nk1, Nk2 and msh. Thus engrailed's active repression function in vivo is dependent on a highly conserved interaction that was established early in the evolution of the homeobox gene superfamily. We further show using rescue transgenes that the widely conserved in vivo repression domain is required for the normal function of engrailed in the embryo.

Functional domains in the Deformed protein

A chimeric protein consisting of Deformed with a substituted Abdominal-B homeodomain (Dfd/Abd-B) is used to identify protein domains outside the homeodomain that are required for regulatory activity in vivo. A series of deletion proteins were generated based on regions showing amino acid composition similar to known regulatory domains. Each mutant protein can influence regulation of homeotic genes in a manner distinct from the intact protein. Activity was also tested using promoter elements from empty spiracles and Distal-less, two genes known to be directly regulated by Abdominal-B. Removal of the acidic region and the C-tail region convert the chimera from a strong activator to a repressor of the Distal-less element, but had comparatively little effect on the activation of the empty spiracles element. Constructs without a third domain, the N domain, fail to show any regulatory activity. The N domain is the only domain of the Dfd/Abd-B protein which exhibits significant activation activity when fused to a heterologous DNA binding domain. Our results suggest transcriptional activity of the N domain can be modulated by the acidic and C-tail domains.

DNA binding specificity of two homeodomain proteins in vitro and in Drosophila embryos

In previous experiments, the homeodomain proteins even-skipped and fushi-tarazu were found to UV cross-link to a surprisingly wide array of DNA sites in living Drosophila embryos. We now show that UV cross-linking gives a highly accurate measure of DNA binding by these proteins. In addition, the binding of even-skipped and fushi-tarazu proteins has been measured in vitro to the same DNA fragments that were examined in vivo. This analysis shows that these proteins have broad DNA recognition properties in vitro that are likely to be important determinants of their distribution on DNA in vivo, but it also shows that in vitro DNA binding specificity alone is not sufficient to explain the distribution of these proteins in embryos.

A possible role in chemical carcinogenesis for epigenetic, heritable changes in gene expression

Although genetic changes are clearly important in the initiation of carcinogenesis, there is reason to think that epigenetic changes may also play a role in the process. A key feature of carcinogenesis is the long latency between exposure to carcinogenic insults and the appearance of malignancy. Thus, if epigenetic changes are to be involved, they must somehow be inherited at each cell division without the continued presence of the carcinogen. I propose that self-perpetuating changes in patterns of gene expression are a plausible mechanism for an epigenetic component of carcinogenesis. Networks of transcription factors that regulate each other's and their own expression are known to control important developmental processes, particularly the determination of entire cell lineages. An inherent property of many such autoregulatory networks is the existence of two very distinct, stable steady-states, defined in terms of the concentration of each transcription factor in the network. In this report, I present a model in which an acute carcinogen exposure is postulated to shift such a network from one steady-state to the other, effectively turning on or off the expression of at least one of the genes. Because of the autoregulatory nature of the network, this new steady-state is stably inherited at each cell division. Such changes in gene expression may ultimately contribute to the malignant phenotype if the regulatory network affects genes important in cell-cycle checkpoints, maintenance of genome stability, signal transduction, or other processes that are altered in tumor cells.

Homeotic response elements are tightly linked to tissue-specific elements in a transcriptional enhancer of the teashirt gene

Along the anterior-posterior axis of animal embryos, the choice of cell fates, and the organization of morphogenesis, is regulated by transcription factors encoded by clustered homeotic or ‘Hox’ genes. Hox genes function in both epidermis and internal tissues by regulating the transcription of target genes in a position- and tissue-specific manner. Hox proteins can have distinct targets in different tissues; the mechanisms underlying tissue and homeotic protein specificity are unknown. Light may be shed by studying the organization of target gene enhancers. In flies, one of the target genes is teashirt (tsh), which encodes a zinc finger protein. tsh itself is a homeotic gene that controls trunk versus head development. We identified a tsh gene enhancer that is differentially activated by Hox proteins in epidermis and mesoderm. Sites where Antennapedia (Antp) and Ultrabithorax (Ubx) proteins bind in vitro were mapped within evolutionarily conserved sequences. Although Antp and Ubx bind to identical sites in vitro, Antp activates the tsh enhancer only in epidermis while Ubx activates the tsh enhancer in both epidermis and in somatic mesoderm. We show that the DNA elements driving tissue-specific transcriptional activation by Antp and Ubx are separable. Next to the homeotic protein-binding sites are extensive conserved sequences likely to control tissue activation by different homeodomain proteins. We propose that local interactions between homeotic proteins and other factors effect activation of targets in proper cell types.

Inserting the Ftz homeodomain into engrailed creates a dominant transcriptional repressor that specifically turns off Ftz target genes in vivo

The Engrailed homeodomain protein is an ‘active’ or dominant transcriptional repressor in cultured cells. In contrast, the Fushi Tarazu homeodomain protein is an activator, both in cultured cells and in Drosophila embryos, where it activates several known target genes, including its own gene. This auto-activation has been shown to depend on targeting to a fushi tarazu enhancer by the Fushi Tarazu homeodomain. We combined Fushi Tarazu targeting and Engrailed active repression in a chimeric regulator, EFE. When EFE is ubiquitously expressed, it overrides endogenous Fushi Tarazu and causes a fushi tarazu mutant phenotype. Normal Fushi Tarazu target genes are affected as they are in fushi tarazu mutants. One such target gene is repressed by EFE even where Fushi Tarazu is not expressed, suggesting that the repression is active. This is confirmed by showing that the in vivo activity of EFE depends on a domain that is required for active repression in culture. A derivative that lacks this domain, while it cannot repress the endogenous fushi tarazu gene, can still reduce the activity of the fushi tarazu autoregulatory enhancer, suggesting that it competes with endogenous Fushi Tarazu for binding sites in vivo. However, this passive repression is much less effective than active repression.

Extradenticle protein is a selective cofactor for the Drosophila homeotics: role of the homeodomain and YPWM amino acid motif in the interaction

The Drosophila homeotic selector (HOM) genes encode a family of DNA binding transcription factors that specify developmental fates of different body segments by differentially regulating the activity of downstream target genes. A central question is how the HOM proteins achieve their developmental specificity despite the very similar DNA binding specificities of isolated HOM proteins in vitro. Specificity could be achieved by differential interactions with protein cofactors. The extradenticle gene might encode such a cofactor since it interacts genetically in parallel with Ultrabithorax, abdominal-A, and perhaps other HOM genes. By using a yeast two-hybrid system, we demonstrate selective interaction of the extradenticle homeodomain protein with certain Ultrabithorax and abdominal-A proteins but not with an Antennapedia protein or a more distant homeodomain protein. Strong interaction with Ultrabithorax proteins requires only the Ultrabithorax homeodomain and a 15-residue N-terminal extension that includes Tyr-Pro-Trp-Met (YPWM), a tetrapeptide motif found near the homeodomain in most HOM proteins and their mammalian Hox counterparts. The size and sequence of the region between the YPWM element and the homeodomain differ among Ultrabithorax isoforms, and this variable region appears to affect the interaction detected in the assay.

The C-terminus of the homeodomain is required for functional specificity of the Drosophila rough gene

In contrast to most Drosophila homeobox genes, which are required during embryogenesis, the rough gene is involved in photoreceptor cell specification in the compound eye. Taking advantage of the viability of null rough alleles and the small size of the rough gene, we have combined in vivo and in vitro mutagenesis to define important functional domains in the rough protein. All missense mutations found to disrupt rough function mapped to highly conserved amino acids in the homeodomain (HD), suggesting that the nature of few, if any, single amino acids outside the HD is critical for rough activity. The analysis of chimeric proteins, in which the whole HD or parts of it were swapped between the rough and Antennapedia (Antp) proteins, revealed that the C-terminus of the rough HD is important for rough activity in vivo. This C-terminal region was also found to be required for the recognition of rough binding sites in vitro. Our data suggest that amino acids located in the C-terminus of the homeodomain may play important roles in selective binding site recognition.

Diploidy of Drosophila imaginal cells is maintained by a transcriptional repressor encoded by escargot

The Drosophila escargot (esg) gene encodes a C2-H2-type zinc finger protein that is expressed in the imaginal discs and histoblasts. In some esg mutants, the abdominal histoblasts become polyploid. It has therefore been suggested that the role of esg is to maintain diploidy of the imaginal cells. We show that esg encodes a DNA-binding protein with high affinity for G/ACAGGTG, the consensus-binding sequence for the basic helix-loop-helix (bHLH) family of transcription factors (E2 box). This DNA-binding activity is essential for esg function in vivo as the strong embryonic lethal allele esgVS8 is caused by an amino acid change within the zinc finger region, leading to reduced affinity for DNA. In cultured cells, a heterodimer of the bHLH proteins Scute and Daughterless activates transcription from promoters containing E2 boxes. The esg protein strongly inhibits this activation, suggesting that esg may regulate developmental processes dependent on bHLH proteins. In larvae, esg protein expressed by the heat shock promoter can rescue the polyploid phenotype of abdominal histoblasts, demonstrating that the phenotype is attributable to a loss of esg function. esg must be expressed continuously during the larval period for efficient rescue. Ectopic expression of esg in the salivary glands inhibits endoreplication of DNA. These results suggest that esg is involved in transcriptional inhibition of genes required for endoreplication.

bHLH proteins encoded by the Enhancer of split complex of Drosophila negatively interfere with transcriptional activation mediated by proneural genes

The Enhancer of split complex [E(SPL)-C] of Drosophila participates in the control of cell fate choice by uncommitted neuroectodermal cells in the embryo. It encodes seven proteins that belong to the basic helix-loop-helix (bHLH) family, six of which are expressed in very similar patterns in the neuroectoderm. Here we describe experiments aimed at unravelling the molecular basis of their function. We found that two products of the complex, HLH-M5 and ENHANCER OF SPLIT, are capable of binding as homo-and heterodimers to a sequence in the promoters of the Enhancer of split and achaete genes, called the N-box, which differs slightly from the consensus binding site (the E-box) for other bHLH proteins. In transient expression assays in cell culture, both proteins were found to attenuate the transcriptional activation mediated by the proneural bHLH proteins LETHAL OF SCUTE and DAUGHTERLESS at the Enhancer of split promoter.

A differential response element for the homeotics at the Antennapedia P1 promoter of Drosophila

Homeotic genes encode DNA-binding transcription factors that specify the identity of a segment or segments in particular body regions of Drosophila. The developmental specificity of these proteins results from their differential regulation of various target genes. This specificity could be achieved by use of different regulatory elements by the homeoproteins or by use of the same elements in different ways. The Ultrabithorax (UBX), abdominal-A (ABD-A), and Antennapedia (ANTP) homeoproteins differentially regulate the Antennapedia P1 promoter in a cell culture cotransfection assay: UBX and ABD-A repress, whereas ANTP activates P1. Either of two regions of P1 can confer this pattern of differential regulation. One of the regions lies downstream and contains homeoprotein-binding sites flanking a 37-bp region called BetBS. ANTP protein activates transcription through the binding sites, whereas UBX and ABD-A both activate transcription through BetBS and use the flanking binding sites to prevent this effect. Thus, homeoproteins can use the same regulatory element but in very different ways. Chimeric UBX-ANTP proteins and UBX deletion derivatives demonstrate that functional specificity in P1 regulation is dictated mainly by sequences outside the homeodomain, with important determinants in the N-terminal region of the proteins.

Two homeo domain proteins bind with similar specificity to a wide range of DNA sites in Drosophila embryos

We have used in vivo UV cross-linking to directly measure DNA binding by the homeo domain proteins even-skipped (eve) and fushi tarazu (ftz) in Drosophila embryos. Strikingly, these two proteins bind at uniformly high levels throughout the length of their genetically identified target genes and at lower, but significant, levels to genes that they are not expected to regulate. The data also suggest that these two proteins have very similar DNA-binding specificities in vivo. In contrast, a non-homeo domain transcription factor, zeste, is only detected on short DNA elements within a target promoter and not on other genes. These results are consistent with the in vitro properties of these various proteins, their respective concentrations in the nucleus, and with earlier predictions of how transcription factors bind DNA in vivo. We propose that these data favor the model that eve, ftz, and closely related homeo domain proteins act by directly regulating mostly the same target genes.

Identification of homeobox genes expressed in human haemopoietic progenitor cells

Homeodomain (HD)-containing proteins have been shown to regulate cellular commitment and differentiation in fungal, invertebrate and vertebrate systems. Bone marrow cells synthesizing the CD34 antigen are a complex mix of early, stem and progenitor cells at various stages of commitment to the many haemopoietic lineages. Here, we report the cloning and sequencing of 31 homeobox (HB) sequences, identified using degenerate oligodeoxyribonucleotide primers, in a polymerase chain reaction with cDNA derived from a purified CD34+ population of human haemopoietic cells. Of these sequences, 16 correspond to previously identified genes, and 13 are located within the HOX A, B and C clusters. Ten of the clones most likely represent human homologues of genes identified previously in other species. Five of the clones reported here represent novel HD sequences. The identification of five new genes using a subclass-specific 5' primer, designed from the engrailed and Xanf1 sequences, suggests that there still remain several uncharacterized HB genes in the human genome. Haemopoietic cells purified on the basis of CD34 antigen synthesis are a rich source of regulatory genes consistent with their ability to differentiate into diverse haemopoietic cell types.

Characterisation of T48, a target of homeotic gene regulation in Drosophila embryogenesis

In Drosophila, the homeotic genes encode transcription factors which specify segmental identities during embryogenesis by directly regulating subordinate target genes. A chromatin immunopurification approach has been used to identify targets of the homeotic gene Ultrabithorax (Ubx). We report here the characterisation of a candidate target gene which is positively regulated by the homeotic genes Ubx and abdominal-A in both the epidermis and the midgut visceral mesoderm and encodes a novel transmembrane protein. By mapping the cis-regulatory regions of this gene, we have identified elements responsible for mediating homeotic regulation, and show that they contain in vitro Ubx binding sites. Therefore, we suggest that this is an example of a direct target of regulation by homeotic genes in two germlayers.

Functional analysis of mouse Hoxa-7 in Saccharomyces cerevisiae: sequences outside the homeodomain base contact zone influence binding and activation

The murine developmental control gene product, Hoxa-7, was shown to function as a DNA-binding transactivator in Saccharomyces cerevisiae. The importance of the ATTA core, the preference for antp class flanking nucleotides, the importance of Asn-51 of the homeodomain (HD), and the synergism of multiple binding sites all reflect properties that have previously been described for HOM or Hox proteins in tissue culture systems. A comparison of contact positions among genes of paralog groups and classes of mammalian HDs points to a lack of diversity in positions that make base contact, suggesting that besides the combination of HD amino acid-base pair contacts, another means of recognizing differences between targets must exist if Hox genes select different targets. The HD of antennapedia is identical to the Hoxa-7 HD. The interaction of Hoxa-7 with the exact sequence used in the nuclear magnetic resonance three-dimensional structural analysis on the antennapedia HD was studied. Hoxa-7 binding and transactivation was influenced by sequences outside of the known base contact zone of this site. We conclude that Hoxa-7 protein has a second means to interact with DNA or/and that the sequences flanking the base contact zone influence HD interactions by distorting DNA within the contact zone (base or backbone). This result is discussed in terms of DNA flexure and two modes of transcription used in S. cerevisiae.

Functional analysis of mouse Hoxa-7 in Saccharomyces cerevisiae: sequences outside the homeodomain base contact zone influence binding and activation

The murine developmental control gene product, Hoxa-7, was shown to function as a DNA-binding transactivator in Saccharomyces cerevisiae. The importance of the ATTA core, the preference for antp class flanking nucleotides, the importance of Asn-51 of the homeodomain (HD), and the synergism of multiple binding sites all reflect properties that have previously been described for HOM or Hox proteins in tissue culture systems. A comparison of contact positions among genes of paralog groups and classes of mammalian HDs points to a lack of diversity in positions that make base contact, suggesting that besides the combination of HD amino acid-base pair contacts, another means of recognizing differences between targets must exist if Hox genes select different targets. The HD of antennapedia is identical to the Hoxa-7 HD. The interaction of Hoxa-7 with the exact sequence used in the nuclear magnetic resonance three-dimensional structural analysis on the antennapedia HD was studied. Hoxa-7 binding and transactivation was influenced by sequences outside of the known base contact zone of this site. We conclude that Hoxa-7 protein has a second means to interact with DNA or/and that the sequences flanking the base contact zone influence HD interactions by distorting DNA within the contact zone (base or backbone). This result is discussed in terms of DNA flexure and two modes of transcription used in S. cerevisiae.

An indirect negative autoregulatory mechanism involved in hepatocyte nuclear factor-1 gene expression

Recent studies have revealed that hepatocyte nuclear factor 4 (HNF-4) is an essential positive regulator of another liver enriched transcription factor HNF-1, defining a transcriptional hierarchy between the two factors operating in hepatocytes. To assess the possible autoregulation of the HNF-1 gene we have examined the effect of HNF-1 on its own transcription. In transient transfection assays, HNF-1 strongly down-regulated transcription driven by its own promoter in HepG2 cells. In addition HNF-1 also repressed the activity of HNF-4 dependent ApoCIII and ApoAI promoters. The same effect was observed using vHNF-1, a distinct but highly related protein to HNF-1. Both HNF-1 and vHNF-1 downregulated HNF-4 activated transcription from intact and chimeric promoter constructs carrying various HNF-4 binding sites implying that they act by impeding HNF-4 binding or activity. DNA binding and cell free transcription experiments however failed to demonstrate any direct or indirect interaction of HNF-1 and vHNF-1 with the above regulatory regions. Both factors repressed HNF-4 induced transcription of the ApoCIII and HNF-1 genes in HeLa cells, arguing against the requirement of a hepatocyte specific function. These findings define an indirect negative autoregulatory mechanism involved in HNF-1 gene expression, which in turn may affect HNF-4 dependent transcription of other liver specific genes.

Expression of class I homeobox genes in fetal and adult murine skin

We examined the expression patterns of several class I homeobox genes in mouse fetal and adult skin. All the genes of the Hox-B locus, except Hoxb-1, are expressed in skin from murine fetuses of 17 and 18 d gestation, at which time the epidermis is undergoing stratification and differentiation. The amount of individual Hox gene message varies considerably, but expression of all genes is detectable by RNase protection except Hoxb-1, which could not be detected even by the reverse transcription-polymerase chain reaction (RT-PCR) assay. Homeobox gene expression in skin is not confined to the Hox-B locus; the paralogous genes Hoxa-4, -b-4, -c-4, and -d-4 are all expressed. The amount of Hoxb-4, -b-2, and -c-4 message in skin is relatively constant from the earliest gestational day examined (day 16) through birth at day 19. Expression of several homeobox genes is also seen in adult skin.

extradenticle, a regulator of homeotic gene activity, is a homolog of the homeobox-containing human proto-oncogene pbx1

Mutations in the Drosophila gene extradenticle (exd) cause homeotic transformations by altering the morphological consequences of homeotic selector gene activity. We have cloned and sequenced exd: it encodes a homeodomain protein with extensive identity (71%) to the human proto-oncoprotein Pbx1. exd is expressed during embryogenesis when the selector homeodomain proteins of the Antennapedia and bithorax complexes establish segmental identity. Maternally expressed exd is uniform and can suppress the segmental transformations of embryos lacking zygotic exd. While zygotic exd expression is also at first uniform, later expression is modulated by the homeotic selector genes. These studies support the view that exd acts with the selector homeodomain proteins as a DNA-binding transcription factor, thereby altering their regulation of downstream target genes.

Local alterations of Krox-20 and Hox gene expression in the hindbrain suggest lack of rhombomeres 4 and 5 in homozygote null Hoxa-1 (Hox-1.6) mutant embryos

It is unknown whether cross-regulatory interactions between homeotic genes, which have been shown to play an important role in the maintenance of their expression domains during Drosophila development, are also important during mammalian development. We have analyzed here the expression of Hox genes in Hoxa-1 (Hox-1.6) null mutant embryos to investigate the possible existence of regulatory interactions between Hoxa-1 and other Hox genes. We show that the absence of a functional Hoxa-1 gene product does not globally interfere with the expression of other Hox genes in terms of both spatial boundaries and transcript abundance. However, a limited area of the hindbrain shows a strong reduction in Hoxb-1 (Hox-2.9) and Krox-20 transcripts, which most likely reflects a marked reduction in size of the former fourth and fifth rhombomeres. These alterations coincide with the region that is subsequently affected in Hoxa-1 null mutant mice and suggest that the primary defects in this mutation are spatially restricted deletions of some rhombomeric structures.

Cooperative binding at a distance by even-skipped protein correlates with repression and suggests a mechanism of silencing

In this study, we examined how the Drosophila developmental control gene even-skipped (eve) represses transcription. Tissue culture cells were used to show that eve contains domains which inhibit transcriptional activators present at the Ultrabithorax (Ubx) proximal promoter when bound up to 1.5 kb away from these activators. Different portions of eve were fused to a heterologous DNA binding domain to show that three adjacent regions of eve contribute to silencing. There appear to be two mechanisms by which eve protein represses transcription. In this study, we used in vitro transcription and DNA binding experiments to provide evidence for one of these mechanisms. Repression in vitro correlates with binding of eve protein to two low-affinity sites in the Ubx proximal promoter. Occupancy of these low-affinity sites is dependent upon cooperative binding of other eve molecules to a separate high-affinity site. Some of these sites are separated by over 150 bp of DNA, and the data suggest that this intervening DNA is bent to form a looped structure similar to those caused by prokaryotic repressors. One of the low-affinity sites overlaps an activator element bound by the zeste transcription factor. Binding of eve protein is shown to exclude binding by zeste protein. These data suggest a mechanism for silencing whereby a repressor protein would be targeted to DNA by a high-affinity element, which itself does not overlap activator elements. Cooperative binding of further repressor molecules to distant low-affinity sites, and competition with activators bound at these sites lead to repression at a distance.

Cooperative binding at a distance by even-skipped protein correlates with repression and suggests a mechanism of silencing

In this study, we examined how the Drosophila developmental control gene even-skipped (eve) represses transcription. Tissue culture cells were used to show that eve contains domains which inhibit transcriptional activators present at the Ultrabithorax (Ubx) proximal promoter when bound up to 1.5 kb away from these activators. Different portions of eve were fused to a heterologous DNA binding domain to show that three adjacent regions of eve contribute to silencing. There appear to be two mechanisms by which eve protein represses transcription. In this study, we used in vitro transcription and DNA binding experiments to provide evidence for one of these mechanisms. Repression in vitro correlates with binding of eve protein to two low-affinity sites in the Ubx proximal promoter. Occupancy of these low-affinity sites is dependent upon cooperative binding of other eve molecules to a separate high-affinity site. Some of these sites are separated by over 150 bp of DNA, and the data suggest that this intervening DNA is bent to form a looped structure similar to those caused by prokaryotic repressors. One of the low-affinity sites overlaps an activator element bound by the zeste transcription factor. Binding of eve protein is shown to exclude binding by zeste protein. These data suggest a mechanism for silencing whereby a repressor protein would be targeted to DNA by a high-affinity element, which itself does not overlap activator elements. Cooperative binding of further repressor molecules to distant low-affinity sites, and competition with activators bound at these sites lead to repression at a distance.

Identification and characterization of a gene activated by the deformed homeoprotein

In Drosophila, the homeotic genes encode transcription factors which control segment identity during embryogenesis by specifying the appropriate set of ‘target’ genes necessary to produce the individual segmental characteristics. Though we know much about the homeotic genes and the proteins they encode, we know little of their targets. Here we identify and characterize one such target gene, a gene activated by the product of the homeotic gene Deformed. DNA binding assays and expression of reporter gene constructs indicate that activation of this gene requires a direct interaction between the Deformed protein and an upstream enhancer element at this target gene. However, although Deformed is required to activate this gene in cells of the maxillary segment, ectopically expressed Deformed does not activate the gene in other regions of the embryo. We conclude from this and other observations that additional factors may be required to activate the target gene, and, therefore, Deformed may participate in either a combinatorial or hierarchical activation signal in the maxillary cells. This newly identified gene encodes a novel protein of unknown function, though proteins with similar amino acid composition have been found. The pattern of transcript accumulation during embryogenesis indicates that this gene may be regulated by other homeoproteins in addition to Deformed.

Rough genes with Deformed homeobox substitutions exhibit rough regulatory specificity during Drosophila eye development

In certain cases, homeobox genes with different in vivo roles encode proteins with similar in vitro DNA binding specificities. To test the role of the homeobox in the regulatory specificity of such genes, rough homeobox sequences were changed in part or entirely to those of the Deformed gene, and the modified rough genes tested for their ability to rescue the rough mutant phenotype. Surprisingly, the chimaeric genes retained levels of rough regulatory specificity but acquired no novel functions. These results suggest that factors other than the DNA binding specificity of the homeodomain play crucial roles in determining the target, and thus the regulatory specificity, of such proteins.

Synergistic activation of transcription is mediated by the N-terminal domain of Drosophila fushi tarazu homeoprotein and can occur without DNA binding by the protein

Synergistic activation of transcription by Drosophila segmentation genes in tissue culture cells provides a model with which to study combinatorial regulation. We examined the synergistic activation of an engrailed-derived promoter by the pair-rule proteins paired (PRD) and fushi tarazu (FTZ). Synergistic activation by PRD requires regions of the homeodomain or adjacent sequences, and that by FTZ requires the first 171 residues. Surprisingly, deletion of the FTZ homeodomain does not reduce the capacity of the protein for synergistic activation, although this mutation abolishes any detectable DNA-binding activity. This finding suggests that FTZ can function through protein-protein interactions with PRD or other components of the homeoprotein transcription complex, adding a new layer of mechanisms that could underlie the functional specificities and combinatorial regulation of homeoproteins.

A cis-element mediating Ultrabithorax autoregulation in the central nervous system

We dissected an upstream control region (a BXD fragment) from the homeotic gene Ultrabithorax (Ubx) of Drosophila which confers a Ubx-like expression pattern in the embryonic ectoderm. We found several distinct enhancer elements spread through the whole BXD fragment each of which is active in transformed embryos, mediating a different pattern of beta-galactosidase expression in the ventral nerve cord. The strongest of these patterns mimics Ubx expression within the Ubx domain. This pattern is strictly dependent on Ubx function. Thus, the BXD control region contains a Ubx response element, suggesting that positive autoregulation of Ubx may occur in the central nervous system of the developing embryo.

Transcriptional repression by the Drosophila even-skipped protein: definition of a minimal repression domain

We have used a transient expression assay employing Drosophila tissue culture cells to study the transcriptional repression activity of the homeo domain protein Even-skipped (Eve). Eve was found to repress all promoters that contained Eve-binding sites, including both TATA-containing and TATA-lacking minimal promoters, as well as promoters activated by several different classes of activator proteins. These findings suggest that the general transcription machinery can be a target of Eve. By analyzing properties of a variety of Eve mutants and chimeric fusion proteins, we have identified several features important for efficient repression. In addition to the DNA-binding domain, a potent repressor requires a repression domain, which can be as small as 27 residues. The minimal 57-residue Eve repression domain, as well as several others studied here, were all found to be proline rich and to contain a high percentage of hydrophobic residues. An intriguing feature of the strong repressors was that their DNA-binding activities, measured by gel retention assays with nuclear extracts, were significantly less than those of derivatives inactive in repression.

Synergistic activation of transcription is mediated by the N-terminal domain of Drosophila fushi tarazu homeoprotein and can occur without DNA binding by the protein

Synergistic activation of transcription by Drosophila segmentation genes in tissue culture cells provides a model with which to study combinatorial regulation. We examined the synergistic activation of an engrailed-derived promoter by the pair-rule proteins paired (PRD) and fushi tarazu (FTZ). Synergistic activation by PRD requires regions of the homeodomain or adjacent sequences, and that by FTZ requires the first 171 residues. Surprisingly, deletion of the FTZ homeodomain does not reduce the capacity of the protein for synergistic activation, although this mutation abolishes any detectable DNA-binding activity. This finding suggests that FTZ can function through protein-protein interactions with PRD or other components of the homeoprotein transcription complex, adding a new layer of mechanisms that could underlie the functional specificities and combinatorial regulation of homeoproteins.

Connectin, a target of homeotic gene control in Drosophila

The homeotic genes of Drosophila encode transcription factors that specify morphological differences between segments. To identify the genes that they control, we developed a chromatin immunopurification approach designed to isolate in vivo binding sites for the products of the homeotic gene Ultrabithorax. Here, we report the analysis of one immunopurified binding site. This 110 bp fragment maps within a regulatory region of a gene under homeotic control, connectin. A 4 kb DNA fragment, including the immunopurified binding site, is sufficient to reproduce the appropriate homeotic control within a subset of the full tissue distribution of connectin. Analysis of the role of the 110 bp binding site indicates that it mediates transcriptional controls by Ultrabithorax and other homeotic genes. This is the first report of a functional in vivo binding site isolated using the chromatin immunopurification method. We also show that the protein product of the connectin gene is predicted to be a cell-surface molecule containing leucine-rich repeats. The protein, connectin, can mediate cell-cell adhesion thus suggesting a direct link between homeotic gene function and processes of cell-cell recognition.

Concentration-dependent activities of the even-skipped protein in Drosophila embryos

The Drosophila pair-rule gene even-skipped (eve) encodes a homeo-domain-containing protein (Eve) that is required for the development of both odd- and even-numbered parasegments. We have used a heat shock-inducible eve transgene to study the regulatory functions of Eve in vivo. Transcripts encoded by eight other segmentation genes were monitored for changes in distribution and abundance following short pulses of ectopic Eve expression. Two tiers of response times appeared to distinguish between genes that were direct [fushi tarazu (ftz), odd-skipped (odd), runt (run), paired, and wingless] and indirect [eve, hairy, and engrailed (en)] targets of Eve. Genes that appeared to be directly regulated by Eve were differentially repressed in a concentration-dependent fashion. Interestingly, the run and ftz genes could also be activated by Eve during a brief 20- to 30-min stage in development. The delayed actions upon the eve and en genes appeared to be mediated by run and odd. As in eve- embryos, these effects on segmentation gene expression patterns caused defects in both odd- and even-numbered parasegments. Four sequential phenotypes could be induced, each of which was attributable to the altered expression of a unique subset of target genes.

Homeotic genes of the bithorax complex repress limb development in the abdomen of the Drosophila embryo through the target gene Distal-less

Homeotic genes encode transcription factors that are thought to specify segmental identity by regulating expression of subordinate genes. Limb development is repressed in the abdominal segments of the Drosophila embryo by the hometic genes of the Bithorax complex (BX-C). Localized expression of the homeobox gene Distal-less (DII) is required for leg development in thoracic segments. We have identified a minimal cis-regulatory enhancer element that directs DII expression in the larval leg primordia. We present evidence that the BX-C proteins repress DII expression in abdominal segments by binding to a small number of specific sites in this element. Mutating these sites eliminates BX-C protein binding and renders the element insensitive to BX-C-mediated repression in vivo. Repression of limb development in the abdomen appears to be controlled at the DII enhancer. Thus DII may serve as a downstream target gene through which the homeotic genes control abdominal segment identity in the Drosophila embryo.

Cell-surface changes induced by ectopic expression of the murine homeo☐ gene Hox-3.3

Murine homeobox-containing genes (Hox genes) are postulated as playing key roles in the establishment of the anterior-posterior embryonic body axis, possibly providing cells with positional cues. Little is known, however, concerning how cells might respond to homeobox gene expression to interpret these cues. Since changes in the cell-surface are central to many processes in early development we reasoned that cells expressing different complements of Hox genes might have different surface properties. In order to investigate this we have used the sensitive, non-disruptive technique of multiple two-phase aqueous partition, which is able to detect small differences on the surface of intact cells. Using this technique we have found that ectopic expression of the murine Hox-3.3 gene in cultured cells induces reproducible changes in the cell surface. Changes only occurred above a threshold level of gene expression, but above this level a correlation between surface change and gene expression was seen. The implications for the establishment of a 'Hox' code of homeobox genes acting to specifically change cell-surface properties are discussed.