A Drosophila homologue of human Sp1 is a head-specific segmentation gene

Integration of the head and trunk segmentation systems controls cephalic furrow formation in Drosophila

Genetic and molecular analyses of patterning of the Drosophila embryo have shown that the process of segmentation of the head is fundamentally different from the process of segmentation of the trunk. The cephalic furrow (CF), one of the first morphological manifestations of the patterning process, forms at the juxtaposition of these two patterning systems. We report here that the initial step in CF formation is a change in shape and apical positioning of a single row of cells. The anteroposterior position of these initiator cells may be defined by the overlapping expression of the head gap gene buttonhead (btd) and the primary pair-rule gene even-skipped (eve). Re-examination of the btd and eve phenotypes in live embryos indicated that both genes are required for CF formation. Further, Eve expression in initiator cells was found to be dependent upon btd activity. The control of eve expression by btd in these cells is the first indication of a new level of integrated regulation that interfaces the head and trunk segmentation systems. In conjunction with previous data on the btd and eve embryonic phenotypes, our results suggest that interaction between these two genes both controls initiation of a specific morphogenetic movement that separates two morphogenetic fields and contributes to patterning the hinge region that demarcates the procephalon from the segmented germ band.

Krüppel target gene knockout participates in the proper innervation of a specific set of Drosophila larval muscles

Krüppel (Kr) encodes a zinc finger-type transcription factor expressed in spatially and temporally restricted patterns during Drosophila embryogenesis. Molecular targets and the mechanism of Kr action have been studied extensively during the early segmentation process, but only little is known about Kr requirement during later development. We report the identification of a target gene of Kr, termed knockout (ko), isolated by virtue of Krüppel in vitro binding sites. Loss and gain of function experiments show that Kr activity maintains ko expression in a subset of muscles. ko encodes a novel protein expressed in several embryonic tissues including Kr-expressing muscles. Movements of embryos deficient for ko activity are uncoordinated. Their muscle pattern is normal, but the patterns of neuromuscular innervation are specifically disarranged. The results suggest that the Kr target gene ko is required for proper innervation of specific muscles by RP motoneurons.

Promoter selective transcriptional synergy mediated by sterol regulatory element binding protein and Sp1: a critical role for the Btd domain of Sp1

Cellular cholesterol and fatty acid levels are coordinately regulated by a family of transcriptional regulatory proteins designated sterol regulatory element binding proteins (SREBPs). SREBP-dependent transcriptional activation from all promoters examined thus far is dependent on the presence of an additional binding site for a ubiquitous coactivator. In the low-density lipoprotein (LDL) receptor, acetyl coenzyme A carboxylase (ACC), and fatty acid synthase (FAS) promoters, which are all regulated by SREBP, the coactivator is the transcription factor Sp1. In this report, we demonstrate that Sp3, another member of the Sp1 family, is capable of substituting for Sp1 in coactivating transcription from all three of these promoters. Results of an earlier study showed that efficient activation of transcription from the LDL receptor promoter required domain C of Sp1; however, this domain is not crucial for activation of the simian virus 40 promoter, where synergistic activation occurs through multiple Sp1 binding sites and does not require SREBP. Also in the present report, we further localize the critical determinant of the C domain required for activation of the LDL receptor to a small region that is highly conserved between Sp1 and Sp3. This crucial domain encompasses the buttonhead box, which is a 10-amino-acid stretch that is present in several Sp1 family members, including the Drosophila buttonhead gene product. Interestingly, neither the buttonhead box nor the entire C domain is required for the activation of the FAS and ACC promoters even though both SREBP and Sp1 are critical players. ACC and FAS each contain two critical SREBP sites, whereas there is only one in the LDL receptor promoter. This finding suggested that buttonhead-dependent activation by SREBP and Sp1 may be limited to promoters that naturally contain a single SREBP recognition site. Consistent with this model, a synthetic construct containing three tandem copies of the native LDL receptor SREBP site linked to a single Sp1 site was also significantly activated in a buttonhead-independent fashion. Taken together, these studies indicate that transcriptional activation through the concerted action of SREBP and Sp1 can occur by at least two different mechanisms, and promoters that are activated by each one can potentially be identified by the number of critical SREBP binding sites that they contain.

The transcriptional activator Sp1, a novel autoantigen

OBJECTIVE

To identify one nuclear autoantigenic protein within a complex of DNA binding proteins that bind to GC-rich sequences in Epstein-Barr virus and cellular DNA, and to describe the clinical characteristics of patients whose sera contained autoantibodies to this novel autoantigen.

METHODS

Antibodies to autoantigen Sp1 were initially measured by an electrophoretic mobility shift assay to detect DNA binding proteins. Nuclear extracts and purified Sp1 protein were used in these assays. Recognition of the autoantigen by autoimmune sera was confirmed by immunoprecipitation and immunoblotting.

RESULTS

The autoantigen was identified as Sp1. Anti-Sp1 was detected in sera from 8 (3%) of 230 patients. These sera contained antinuclear antibodies, but lacked antibodies to double-stranded DNA or to several extractable nuclear antigens. The patients whose sera contained antibodies to Sp1 were white women with fatigue, arthritis, Raynaud's phenomenon, malar rash, and photosensitivity.

CONCLUSION

Sp1 is the first described example of an RNA polymerase II transcription activator as an autoantigen. The presence of Sp1 autoantibodies is associated with undifferentiated connective tissue disease.

buttonhead does not contribute to a combinatorial code proposed for Drosophila head development

The Drosophila gap-like segmentation genes orthodenticle, empty spiracles and buttonhead (btd) are expressed and required in overlapping domains in the head region of the blastoderm stage embryo. Their expression domains correspond to two or three segment anlagen that fail to develop in each mutant. It has been proposed that these overlapping expression domains mediate head metamerization and could generate a combinatorial code to specify segment identity. To test this model, we developed a system for targeted gene expression in the early embryo, based on region specific promoters and the flp-out system. Misexpression of btd in the anterior half of the blastoderm embryo directed by the hunchback proximal promoter rescues the btd mutant head phenotype to wild-type. This indicates that, while btd activity is required for the formation of specific head segments, its ectopic expression does not disturb head development. We conclude that the spatial limits of btd expression are not instructive for metamerization of the head region and that btd activity does not contribute to a combinatorial code for specification of segment identity.

Regulation of gene expression in the preimplantation mouse embryo: temporal and spatial patterns of expression of the transcription factor Sp1

Activation of the embryonic genome during preimplantation mouse development entails a dramatic reprogramming of the pattern of gene expression. The complement of transcription factors that are present in the early embryo and that must intrinsically be involved in this reprogramming is essentially uncharacterized. We and others have demonstrated that transcription factor Sp1 is present in the mouse oocyte and early cleavage stage preimplantation embryo. Due to Sp1's prominent role in regulating the expression of a vast array of genes that are involved in cell proliferation and differentiation, as well as in general housekeeping functions, we characterized the temporal and spatial patterns of Sp1 expression during preimplantation development. The relative abundance of Sp1 transcripts, as well as transcripts for the TATA box-binding protein TBP, decreases during oocyte maturation and reaches a minimum level in the two-cell stage, after which time the abundance of these transcripts increases progressively to the blastocyst stage. Immunoblotting experiments detect Sp1 species of Mr = 95,000 and 105,000 at all stages of preimplantation development. The amount of Sp1 increases about 8-fold during preimplantation development, and an alpha-amanitin-insensitive increase is observed between G1 and G2 of the one-cell embryo; this increase may reflect the mobilization of a maternal Sp1 transcript. Immunocytochemical experiments also reveal a similar increase in the amount of Sp1 during preimplantation; the nuclear concentration of Sp1 is greater in the trophectoderm cells than in the inner cell mass cells. Finally, gel-shift experiments document an increase during preimplantation development of a DNA-binding activity that is likely due to Sp1. These increases in the abundance of the Sp1 protein and an Sp1-like DNA-binding activity parallel increases in the rate of transcription that occur during preimplantation development.

Control of early neurogenesis of the Drosophila brain by the head gap genes tll, otd, ems, and btd

The progenitors of the Drosophila central nervous system (CNS), called neuroblasts, segregate from the neurectoderm of the early embryo in a stereotyped pattern. The neuroblasts that give rise to the brain segregate from the procephalic neurectoderm and form three neuromeres, called protocerebrum, deuterocerebrum, and tritocerebrum. The expression of the proneural genes of the achaete-scute complex (AS-C) is required for neurectodermal cells to acquire the competency to form neuroblasts. We show here that the expression of the proneural gene lethal of scute (l'sc) is required for the development of the majority of the procephalic neuroblasts. l'sc expression in the procephalic neurectoderm is controlled by the head gap genes tailless (tll), orthodenticle (otd), buttonhead (btd), and empty spiracles (ems), which are expressed in partially overlapping domains of the head neurectoderm. Loss of function of a given head gap gene results in the absence of l'sc expression in its domain, followed by the absence of neuroblasts that would normally segregate from this domain. Loss of tll function results in the absence of all protocerebral neuroblasts, otd functions in a domain that includes a large part of the protocerebrum and a smaller part of the adjacent deuterocerebrum. Both ems and btd are required in partially overlapping subsets of neuroblasts of the deuterocerebrum and tritocerebrum.

Activation of transcription of the melanoma inducing Xmrk oncogene by a GC box element

Melanoma formation in Xiphophorus is caused by overexpression of the Xmrk gene. The promoter region of the Xmrk oncogene differs strikingly from the corresponding proto-oncogenic sequences and was acquired in the course of a nonhomologous recombination with another gene locus, D. In order to identify regulatory elements leading to the strong transcriptional activation of Xmrk in melanoma tissue and to contribute to an understanding of the role the regulatory locus R might play in suppressing the tumor phenotype in wild-type Xiphophorus, we performed functional analysis of the Xmrk oncogene promoter. Transient transfections in melanoma and nonmelanoma cells revealed the existence of a potent positive regulatory element positioned close to the transcriptional start site. Contained within this promoter segment is a GC-rich sequence identical to the binding site described for human Sp1. In vitro binding studies and biochemical characterizations demonstrated the existence of GC-binding proteins in fish that share immunological properties with members of the human Sp family of transcription factors and appear to be involved in the high transcriptional activation of the Xmrk oncogene. Since the identified cis element is functional in both melanoma and nonmelanoma cells, additional silencer elements suppressing Xmrk expression in nonpigment cells must exist, thereby suggesting a negative regulatory function for the genetically defined R locus.

Structure of the insect head in ontogeny and phylogeny: a view from Drosophila

Evolutionary, developmental and insect biologists are currently using a three-pronged approach to study the evolution and development of the insect head. First, genetic manipulation of the fruit fly Drosophila melanogaster has led to the identification of many genes, including the segmentation and homeotic genes, that are important for embryonic pattern formation and development. Second, a comparison of orthologous gene expression patterns in other insects reveals that these regulatory genes are deployed in similar, yet distinct, patterns in different insects. Third, comparisons of embryonic morphology with gene expression patterns suggest that in general these genes promote a common insect body plan, but that variations in gene expression can often be correlated to variations in morphology. Here, we present a detailed review of the development of the cephalic ectoderm of Drosophila and extrapolate to development of a generalized insect head. Our analysis of the variations among insect species, in both morphology and gene expression patterns, conducted within an evolutionary framework supported by traditional phylogenies and paleontology provides the basis for hypotheses about the genetic factors governing morphologic and developmental evolution.

buttonhead and D-Sp1: a novel Drosophila gene pair

The Drosophila gene buttonhead (btd) is a gap-like head segmentation gene which encodes a triple zinc finger protein structurally and functionally related to the human transcription factor Spl. Here we report the pattern of btd expression during embryogenesis. btd is not only expressed and required in the blastoderm anlagen of the antennal, intercalary and mandibular segments as reported previously, but both expression and requirement extend into the anlage of the maxillary segment. From gastrulation onwards, btd is expressed in distinct spatial and temporal patterns, suggesting that btd might be required for a number of developmental processes beyond head segmentation. In fact, analysis of btd mutant embryos revealed that btd participates in the formation of the peripheral nervous system. However, no other morphologically apparent phenotype was observed. We identified a btd-related gene, termed D-Sp1, which is expressed in temporal and spatial patterns similar to btd during postblastodermal development. No localized expression domains of D-Sp1, which is located in the same X-chromosomal band as btd, were seen during the blastoderm stage. The results suggest that D-Sp1 and btd represent a novel gene pair with partially redundant functions after the blastoderm stage.

P-OTX: a PIT-1-interacting homeodomain factor expressed during anterior pituitary gland development

A novel OTX-related homeodomain transcription factor has been identified on the basis of its ability to interact with the transactivation domain of the pituitary-specific POU domain protein, Pit-1. This factor, referred to as P-OTX (pituitary OTX-related factor), is expressed in primordial Rathke's pouch, oral epithelium, first bronchial arch, duodenum, and hindlimb. In the developing anterior pituitary, it is expressed in all regions from which cells with distinct phenotypes will emerge in the mature gland. P-OTX is able to independently activate and to synergize with Pit-1 on pituitary-specific target gene promoters. Therefore, P-OTX may subserve functions in generating both precursor and specific cell phenotypes in the anterior pituitary gland and in several other organs.

Collier, a novel regulator of Drosophila head development, is expressed in a single mitotic domain

BACKGROUND

Segmentation of the Drosophila embryo is based on a cascade of hierarchical gene interactions that is initiated by maternal morphogens; these interactions define spatially restricted domains of zygotic gene expression within the blastoderm. Although the hierarchy of the segmentation genes that subdivide the trunk is well established, the patterning of the head is less well understood. Seven head segments can be assigned on the basis of metameric patterns of segment-polarity gene expression and internal sensory organs. The domains of expression of head gap-like genes broadly overlap, with their posterior margins out of phase by one segment. Taken together with the lack of pair-rule gene expression in the head, these observations led to the suggestion that head gap genes act in a combinatorial manner, determining head segmental borders and segmental identity at the same time.

RESULTS

We have identified a new Drosophila gene, collier (col), whose expression at the blastoderm stage is restricted to a single stripe of cells corresponding to part of the intercalary and mandibular segment primordia, possibly parasegment O. Reduction of col activity in early gastrula embryos by antisense RNA expression results in a specific lack of head structures derived from these segments. The expression of col coincides with a mitotic domain, which supports the proposal that cells in this domain undergo a concerted mitotic and differentiation program that is orchestrated at the transcriptional level. Col is an ortholog of mammalian early B-cell factor/Olfactory-1. These proteins define a new family of transcription factors that contain a helix-loop-helix dimerization motif and a new type of DNA-binding domain that is highly conserved during evolution.

CONCLUSIONS

Here we describe Col, the first Drosophila member of a new family of transcription factors. Col may act as a "second-level regulator' of head patterning. The structural conservation of Col during evolution raises the questions of its conservation of function in head specification and its interactions with other factors conserved between insects and vertebrates.

The Drosophila morphogenetic protein Bicoid binds DNA cooperatively

The Drosophila morphogenetic protein Bicoid, encoded by the maternal gene bicoid, is required for the development of the anterior structures in the embryo. Bicoid, a transcriptional activator containing a homeodomain, is distributed in an anterior-to-posterior gradient in the embryo. In response to this gradient, the zygotic gene hunchback is expressed uniformly in the anterior half of the embryo in a nearly all-or-none manner. In this report we demonstrate that a recombinant Bicoid protein binds cooperatively to its sites within a hunchback enhancer element. A less than 4-fold increase in Bicoid concentration is sufficient to achieve an unbound/bound transition in DNA binding. Using various biochemical and genetic methods we further demonstrate that Bicoid molecules can interact with each other. Our results are consistent with previous studies performed in the embryo, and they suggest that one mechanism to achieve a sharp on/off switch of gene expression in response to a morphogenetic gradient is cooperative DNA binding facilitated by protein-protein interaction.

Identification of Sp1-binding sites in the CD11c (p150,95 alpha) and CD11a (LFA-1 alpha) integrin subunit promoters and their involvement in the tissue-specific expression of CD11c

The leukocyte integrins LFA-1 (CD11a/CD18) and p150,95 (CD11c/CD18) mediate cell-cell and cell-extracellular matrix interactions during inflammatory responses and signal transduction into the cytoplasm. While the CD11a integrin subunit is expressed on all leukocytes, CD11c is almost exclusively expressed on cells of the myeloid lineage and on activated B lymphocytes. Its expression is regulated during cell activation and differentiation by transcriptional mechanisms. We have previously demonstrated that the proximal region of the CD11c promoter directs tissue-restricted and developmentally-regulated expression of reporter genes. Structural studies by electrophoretic mobility shift assays have demonstrated the presence of two Sp1-binding sites at -70 (Sp1-70) and -120 (Sp1-120) which mediate the Sp1 transactivation of the CD11c promoter in Sp1-defective SL2 cells, and which are involved in cell lineage-specific DNA-protein interactions, as demonstrated by footprinting in vivo. More importantly, mutation of either Sp1 site inhibited the activity of the CD11c promoter both in myeloid U937 cells and the CD11c-expressing B lymphoblastoid JY cell line, while the opposite effect was observed in the CD11c-negative epithelial HeLa cell line, demonstrating the involvement of both Sp1-binding sites in the basal and the tissue-restricted expression of the CD11c integrin subunit gene. Interestingly, the analysis of the CD11a proximal promoter also revealed the existence of an Sp1-binding site at -70, indicating a common role for these cis-acting elements in the transcription of the leukocyte integrin alpha subunit genes. The binding of Sp1 to the regulatory regions of the leukocyte integrin genes raises the possibility that the retinoblastoma susceptibility gene product is implicated in integrin expression through its functional interaction with Sp1, thus establishing a link between integrin-dependent leukocyte adhesiveness and the state of cellular differentiation/proliferation.

Emergence of the ZNF91 Krüppel-associated box-containing zinc finger gene family in the last common ancestor of anthropoidea

The ZNF91 gene family, a subset of the Krüppel-associated box (KRAB)-containing group of zinc finger genes, comprises more than 40 loci; most reside on human chromosome 19p12-p13.1. We have examined the emergence and evolutionary conservation of the ZNF91 family. ZNF91 family members were detected in all species of great apes, gibbons, Old World monkeys, and New World monkeys examined but were not found in prosimians or rodents. In each species containing the ZNF91 family, the genes were clustered at one major site, on the chromosome(s) syntenic to human chromosome 19. To identify a putative "founder" gene, > 20 murine KRAB-containing zinc finger protein (ZFP) cDNAs were randomly cloned, but none showed sequence similarity to the ZNF91 genes. These observations suggest that the ZNF91 gene cluster is a derived character specific to Anthropoidea, resulting from a duplication and amplification event some 55 million years ago in the common ancestor of simians. Although the ZNF91 gene cluster is present in all simian species, the sequences of the human ZNF91 gene that confer DNA-binding specificity were conserved only in great apes, suggesting that there is not a high selective pressure to maintain the DNA targets of these proteins during evolution.

Trans- and cis-acting requirements for blastodermal expression of the head gap gene buttonhead

The Drosophila gene buttonhead (btd) encodes a zinc-finger protein related to the human transcription factor Sp1. btd is expressed in the syncytial blastoderm embryo in a stripe covering the anlagen of the antennal, intercalary and mandibular head segments. btd has been characterized as a head gap gene, since these segments are deleted in btd mutant embryos. We report here that the cis-acting elements required for btd head stripe expression are contained in a 1 kb DNA fragment, located about 3 kb upstream of the promoter. The four maternal coordinate systems are necessary for correct btd head stripe expression, likely by acting through the 1 kb cis-acting control region. Expression of the btd head stripe depends on the anterior morphogen encoded by the gene bicoid (bcd). bcd-dependent activation also involves the activity of the morphogens of the posterior and dorsoventral systems, hunchback and dorsal, respectively, which act together to control the spatial limits of the expression domain. Finally, the terminal system takes part in the regulation of btd head stripe expression by enhancing activation at low levels of activity and repression at high levels of activity.

Developmental defects in brain segmentation caused by mutations of the homeobox genes orthodenticle and empty spiracles in Drosophila

We have studied the roles of the homeobox genes orthodenticle (otd) and empty spiracles (ems) in embryonic brain development of Drosophila. The embryonic brain is composed of three segmental neuromeres. The otd gene is expressed predominantly in the anterior neuromere; expression of ems is restricted to the two posterior neuromeres. Mutation of otd eliminates the first (protocerebral) brain neuromere. Mutation of ems eliminates the second (deutocerebral) and third (tritocerebral) neuromeres. otd is also necessary for development of the dorsal protocerebrum of the adult brain. We conclude that these homeobox genes are required for the development of specific brain segments in Drosophila, and that the regionalized expression of their homologs in vertebrate brains suggests an evolutionarily conserved program for brain development.

An alternatively spliced form of the transcription factor Sp1 containing only a single glutamine-rich transactivation domain

Protein-protein interactions involving specific transactivation domains play a central role in gene transcription and its regulation. The promoter-specific transcription factor Sp1 contains two glutamine-rich transcriptional activation domains (A and B) that mediate direct interactions with the transcription factor TFIID complex associated with RNA polymerase II and synergistic effects involving multiple Sp1 molecules. In the present study, we report the complementary DNA sequence for an alternatively spliced form of mouse Sp1 (mSp1-S) that lacks one of the two glutamine-rich activation regions present in the full-length protein. Corresponding transcripts were identified in mouse tissues and cell lines, and an Sp1-related protein identical in size to that predicted for mSp1-S was detected in mouse nuclear extracts. Cotransfection analysis revealed that mSp1-S lacks appreciable activity at promoters containing a single Sp1 response element but is active when multiple Sp1 sites are present, suggesting synergistic interactions between multiple mSp1-S molecules. The absence of a single glutamine-rich domain does not fully explain the properties of the smaller protein and indicates that additional structural features account for its unique transcriptional activity. The functional implications of this alternatively spliced form of Sp1 are discussed.

Analysis of neural elements in head-mutant Drosophila embryos suggests segmental origin of the optic lobes

We describe the development of 20 sensory organs in the embryonic Drosophila head, which give rise to 7 sensory nerves of the peripheral nervous system (PNS), and 4 ganglia of the stomatogastric nervous system (SNS). Using these neural elements and the optic lobes as well as expression domains of the segment polarity gene engrailed in the wild-type head of Drosophila embryos as markers we examined the phenotype of different mutants which lack various and distinct portions of the embryonic head. In the mutants, distinct neural elements and engrailed expression domains, serving as segmental markers, are deleted. These mutants also affect the optic lobes to various degrees. Our results suggest that the optic lobes are of segmental origin and that they derive from the ocular segment anteriorly adjacent to the antennal segment of the developing head.

Transcriptional activation of the neuronal peripherin-encoding gene depends on a G + C-rich element that binds Sp1 in vitro and in vivo

Peripherin (Prph) is a type-III intermediate filament (IF) protein principally synthesized in peripheral nervous system neurons. We have previously shown that three regulatory elements, PER1, PER2 and PER3, in the first 98 bp of the Prph gene promoter, were sufficient to direct cell-type specific expression of a reporter gene [Desmarais et al., EMBO J. 11 (1992) 2971-2980]. Of these elements, PER1 was found to be important for cell-type specificity, but required the presence of other elements for transcriptional activity. Here, we show that PER3 is a stronger activator than PER2 and that it can stimulate cell-type-specific transcription when combined with PER1. We have characterized the G + C-rich PER3 element for its ability to bind trans-acting factors. Gel retardation and methylation interference (MI) assays show that PER3 binds transcription factor Sp1. In addition, an anti-Sp1 antibody recognizes the PER3 DNA-binding protein. A 3-bp mutation abrogating the capacity of PER3 to bind Sp1 in vitro completely abolished expression of the reporter gene construct containing only PER3 and PER1, while in a construct containing the first 256 bp of the Prph promoter, it led to an 80% decrease with respect to the control wild-type construct. Finally, by co-transfection of a Sp1-expressing plasmid, we show that Sp1 can stimulate transcription from a reporter gene containing the PER3 sequence. Together, these results indicate that interactions between Sp1 and the proteins binding PER1 are involved in the control of the Prph gene.

Spatial regulation of segment polarity gene expression in the anterior terminal region of the Drosophila blastoderm embryo

The effects of mutations in five anterior gap genes (hkb, tll, otd, ems and btd) on the spatial expression of the segment polarity genes, wg and hh, were analyzed at the late blastoderm stage and during subsequent development. Both wg and hh are normally expressed at blastoderm stage in two broad domains anterior to the segmental stripes of the trunk region. At the blastoderm stage, each gap gene acts specifically to regulate the expression of either wg or hh in the anterior cephalic region: hkb, otd and btd regulate the anterior blastoderm expression of wg, while tll and ems regulate hh blastoderm expression. Additionally, btd is required for the first segmental stripe (mandibular segment) of both hh and wg at blastoderm stages. The subsequent segmentation of the cephalic segments (preantennal, antennal and intercalary) appears to be dependent on the overlap of the wg and hh cephalic domains as defined by these gap genes at the blastoderm stage. None of these five known gap genes are required for the activation of the labral segment domains of hh and wg, which are presumably either activated directly by maternal pathways or by an unidentified gap gene.

Expression of two zebrafish orthodenticle-related genes in the embryonic brain

To analyze the molecular mechanism of pattern formation in the anteriormost regions of the zebrafish embryo, we isolated two zebrafish sequences, zOtx1 and zOtx2, related to the Drosophila orthodenticle (otd) and two murine Otx genes. zOtx1 and zOtx2 encode predicted gene products which are 82% and 94% identical to the corresponding mouse proteins. Transcripts of both zebrafish genes appear abruptly at high levels in a triangular patch at the animal pole of the mid-gastrula, a region which contains cells fated to become midbrain and forebrain. Between 9 and 14 h of development, zOtx transcripts disappear from forebrain regions in a manner characteristic for each gene, and from 14 to 24 h, particular regions of the forebrain and midbrain express one or both genes. The posterior limit of expression of both genes in 10-30-h embryos forms a sharp boundary at the posterior border of the midbrain. As in the mouse, the early expression patterns of the zOtx genes are consistent with a role in defining midbrain and forebrain territories. However, there are a number of interesting differences between the forebrain and midbrain regions which express the genes in the two species.

Different spatio-temporal expressions of three otx homeoprotein transcripts during zebrafish embryogenesis

Three zebrafish otx homeoproteins containing a homeodomain homologous to that of the Drosophila orthodenticle head gap gene product have been identified by cloning and sequencing of cDNAs. The zebrafish otx2 homeoprotein shares high amino-acid sequence identity with the mouse Otx2 homeoprotein, whereas the zebrafish otx1 and otx3 homeoproteins exhibit moderate homology with the mouse Otx1 and Otx2 homeoproteins. Three otx homeoprotein mRNAs show different spatio-temporal expression patterns during zebrafish embryogenesis as revealed by Northern blot and whole mount in situ hybridization analyses. Large amounts of the otx1 homeoprotein mRNA are found in fertilized uncleaving eggs. The otx3 homeoprotein mRNA appears in the embryonic shield, the site of the organizer. In the developing brain, three zebrafish otx mRNAs are distributed in the diencephalon and the midbrain, but their fine expression patterns are different. These results suggest that three zebrafish otx homeoproteins, alone or in combination, may play roles in very early embryogenesis, gastrulation, and the development and subdivision of the diencephalon and the midbrain.

Three maternal coordinate systems cooperate in the patterning of the Drosophila head

In contrast to the segmentation of the embryonic trunk region which has been extensively studied, relatively little is known about the development and segmentation of the Drosophila head. Proper development of the cephalic region requires the informational input of three of the four maternal coordinate systems. Head-specific gene expression is set up in response to a complex interaction between the maternally provided gene products and zygotically expressed genes. Several zygotic genes involved in head development have recently been characterized. A genetic analysis suggests that the segmentation of the head may use a mechanism different from the one acting in the trunk. The two genes of the sloppy paired locus (slp1 and slp2) are also expressed in the embryonic head. slp1 plays a predominant role in head formation while slp2 is largely dispensible. A detailed analysis of the slp head phenotype suggests that slp is important for the development of the mandibular segment as well as two adjacent pregnathal segments (antennal and ocular). Our analysis of regulatory interactions of slp with maternal and zygotic genes suggests that it behaves like a gap gene. Thus, phenotype and regulation of slp support the view that slp acts as a head-specific gap gene in addition to its function as a pair-rule and segment polarity gene in the trunk. We show that all three maternal systems active in the cephalic region are required for proper slp expression and that the different systems cooperate in the patterning of the head. The terminal and anterior patterning system appear to be closely linked. This cooperation is likely to involve a direct interaction between the bcd morphogen and the terminal system. Low levels of terminal system activity seem to potentiate bcd as an activator of slp, whereas high levels down-regulate bcd rendering it inactive. Our analysis suggests that dorsal, the morphogen of the dorsoventral system, and the head-specific gap gene empty spiracles act as repressor and corepressor in the regulation of slp. We discuss how positional information established independently along two axes can act in concert to control gene regulation in two dimensions.

Transcriptional regulation of string (cdc25): a link between developmental programming and the cell cycle

During postblastoderm embryogenesis in Drosophila, cell cycles progress in an invariant spatiotemporal pattern. Most of these cycles are differentially timed by bursts of transcription of string (cdc25), a gene encoding a phosphatase that triggers mitosis by activating the Cdc2 kinase. An analysis of string expression in 36 pattern-formation mutants shows that known patterning genes act locally to influence string transcription. Embryonic expression of string gene fragments shows that the complete pattern of string transcription requires extensive cis-acting regulatory sequences (> 15.3 kb), but that smaller segments of this regulatory region can drive proper temporal expression in defined spatial domains. We infer that string upstream sequences integrate many local signals to direct string's transcriptional program. Finally, we show that the spatiotemporal progression of string transcription is largely unaffected in mutant embryos specifically arrested in G2 of cycles 14, 15, or 16, or G1 of cycle 17. Thus, there is a regulatory hierarchy in which developmental inputs, not cell cycle inputs, control the timing of string transcription and hence cell cycle progression.

Synergy between the hunchback and bicoid morphogens is required for anterior patterning in Drosophila

Anterior patterning of the Drosophila embryo is specified by the localized expression of the gap genes, which is controlled by the gradient of the maternal morphogen bicoid (bcd). Another maternal component, hunchback (hb), can substitute for bcd in the thorax and abdomen. Here we show that hb is required for bcd to execute all of its functions. Removal of both maternal and zygotic hb produces embryos with disrupted polarity that fail to express all known bcd target genes correctly. Proper expression of hb and the head gap genes requires synergistic activation by hb and bcd. We propose that it is the combined activity of bcd and hb, and not bcd alone, that forms the morphogenetic gradient that specifies polarity along the embryonic axis and patterns the embryo. bcd may be a newly acquired Drosophila gene, which is gradually replacing some of the functions performed by maternal hb in other species.

Number, identity, and sequence of the Drosophila head segments as revealed by neural elements and their deletion patterns in mutants

The development of the insect head tagma involves massive rearrangements and secondary fusions of segment anlagen during embryogenesis. Due to the lack of reliable morphological markers, the number, identity, and sequence of the head segments, particularly in the pregnathal region, are still a matter of ongoing debates. We examined the complex array of internal structures of the embryonic Drosophila melanogaster head such as the sensory structures and nerves of the peripheral and stomatogastric nervous systems, and we used embryonic head mutations causing a lack of overlapping segment anlagen to unravel the segmental identity and the sequence of the neural elements. Our results provide evidence for seven distinct segments in the Drosophila head, each characterized by a specific set of sensory neurons, consistent with the proposal that insects, myriapods, and crustaceans share a monophyletic evolutionary tree from a common annelid-like ancestor.

Perspectives on zinc finger protein function and evolution--an update

Complexity is one of the hallmarks that applies to C2H2 type zinc finger proteins (ZFPs). Structurally distinct clusters of zinc finger modules define an extremely large superfamily of nucleic acid binding proteins with several hundred, perhaps thousands of different members in vertebrates. Recent discoveries have provided new insights into the biochemistry of RNA and DNA recognition, into ZFP evolution and genomic organization, and also into basic aspects of their biological function. However, as much as we have learned, other fundamental questions about ZFP function remain highly enigmatic. This essay is meant to define what we personally feel are important questions, rather than trying to provide a comprehensive, encyclopaedic review.

Sp1/egr-like zinc-finger protein required for endoderm specification and germ-layer formation in Drosophila

Much of our present knowledge of the biological processes involved in pattern formation in Drosophila is derived from segmentation analysis. Comparatively little is known about the genetic requirement and mechanisms underlying the formation and separation of germ layers by morphogenetic movements during gastrulation. Here we show that the Drosophila gene huckebein (hkb), a member of the gap-gene class of segmentation genes, is required for germ-layer formation at blastoderm. Absence of the hkb product, an Sp1/egr-like zinc-finger protein, causes the ectodermal and mesodermal primordia to expand at the expense of endoderm anlagen. Conversely, ectopic expression of hkb inhibits the formation of the major gastrulation fold which gives rise to the mesoderm and prevents normal segmentation in the ectoderm. Thus, hkb is necessary for endoderm development and its activity defines spatial limits within the blastoderm embryo in which the germ layers are established.

Identical transacting factor requirement for knirps and knirps-related Gene expression in the anterior but not in the posterior region of the Drosophila embryo

The Drosophila genes knirps (kni) and knirps-related (knrl) are located within the 77E1,2 region on the left arm of the third chromosome. They encode nuclear hormone-like transcription factors containing almost identical Cys2/Cys2 DNA-binding zinc finger motifs which bind to the same target sequence. kni is a member of the gap class of segmentation genes, and its activity is required for the normal establishment of the abdomen. The function of knrl is still unknown; however, a possible gap gene function in the abdominal region of the embryo can be excluded. Both genes are initially expressed in three identical regions of the blastoderm embryo: in an anterior cap domain, in an anterior stripe and in a posterior broad band linked to the kni gap gene function. The transacting factor requirement for the expression of kni and knrl is identical for the two anterior domains but different, although similar, for the posterior domain of expression in the blastoderm. Both the anteroposterior morphogen bicoid and the dorsoventral morphogen dorsal are necessary but not sufficient for the activation of the two genes in the anterior cap domain, suggesting they act together to bring about its normal spatial limits.

Localized expression of a novel micropia-like element in the blastoderm of Drosophila melanogaster is dependent on the anterior morphogen bicoid

We have identified a novel transposon-like element of Drosophila melanogaster that is present in approximately 20 copies in the genome. It codes for a polyprotein containing the diagnostic sequence motifs for a nucleic acid binding CCHC protein, a proteinase, a reverse transcriptase and an integrase as typically found in retroviruses. Owing to its early expression in the blastoderm embryo, and its close relationship to micropia, a previously identified Drosophila retrotransposon, we termed the novel element "blastopia". The spatially restricted expression of blastopia transcripts in head anlagen of the blastoderm embryo is under the direct or indirect control of the Drosophila morphogen bicoid, which is normally required to establish the anterior pattern elements in the embryo. Our results suggest that a blastopia element acts as an "enhancer trap", and thereby participates in the control of an as yet unidentified gene normally expressed in the head anlagen of the embryo.