The reaction with polytene chromosomes of antibodies raised against Drosophila E75A protein

Knockdown of the Halloween Genes spook, shadow and shade Influences Oocyte Development, Egg Shape, Oviposition and Hatching in the Desert Locust

Ecdysteroids are widely investigated for their role during the molting cascade in insects; however, they are also involved in the development of the female reproductive system. Ecdysteroids are synthesized from cholesterol, which is further converted via a series of enzymatic steps into the main molting hormone, 20-hydoxyecdysone. Most of these biosynthetic conversion steps involve the activity of cytochrome P450 (CYP) hydroxylases, which are encoded by the Halloween genes. Three of these genes, spook (spo), phantom (phm) and shade (shd), were previously characterized in the desert locust, Schistocerca gregaria. Based on recent sequencing data, we have now identified the sequences of disembodied (dib) and shadow (sad), for which we also analyzed spatiotemporal expression profiles using qRT-PCR. Furthermore, we investigated the possible role(s) of five different Halloween genes in the oogenesis process by means of RNA interference mediated knockdown experiments. Our results showed that depleting the expression of SchgrSpo, SchgrSad and SchgrShd had a significant impact on oocyte development, oviposition and hatching of the eggs. Moreover, the shape of the growing oocytes, as well as the deposited eggs, was very drastically altered by the experimental treatments. Consequently, it can be proposed that these three enzymes play an important role in oogenesis.

Forward and feedback regulation of cyclic steroid production in Drosophila melanogaster

In most animals, steroid hormones are crucial regulators of physiology and developmental life transitions. Steroid synthesis depends on extrinsic parameters and autoregulatory processes to fine-tune the dynamics of hormone production. In Drosophila, transient increases of the steroid prohormone ecdysone, produced at each larval stage, are necessary to trigger moulting and metamorphosis. Binding of the active ecdysone (20-hydroxyecdysone) to its receptor (EcR) is followed by the sequential expression of the nuclear receptors E75, DHR3 and βFtz-f1, representing a model for steroid hormone signalling. Here, we have combined genetic and imaging approaches to investigate the precise role of this signalling cascade within theprothoracic gland (PG), where ecdysone synthesis takes place. We show that these receptors operate through an apparent unconventional hierarchy in the PG to control ecdysone biosynthesis. At metamorphosis onset, DHR3 emerges as the downstream component that represses steroidogenic enzymes and requires an early effect of EcR for this repression. To avoid premature repression of steroidogenesis, E75 counteracts DHR3 activity, whereas EcR and βFtz-f1 act early in development through a forward process to moderate DHR3 levels. Our findings suggest that within the steroidogenic tissue, a given 20-hydroxyecdysone peak induces autoregulatory processes to sharpen ecdysone production and to confer competence for ecdysteroid biosynthesis at the next developmental phase, providing novel insights into steroid hormone kinetics.

Regulation of Drosophila metamorphosis by xenobiotic response regulators

Mammalian Nrf2-Keap1 and the homologous Drosophila CncC-dKeap1 protein complexes regulate both transcriptional responses to xenobiotic compounds as well as native cellular and developmental processes. The relationships between the functions of these proteins in xenobiotic responses and in development were unknown. We investigated the genes regulated by CncC and dKeap1 during development and the signal transduction pathways that modulate their functions. CncC and dKeap1 were enriched within the nuclei in many tissues, in contrast to the reported cytoplasmic localization of Keap1 and Nrf2 in cultured mammalian cells. CncC and dKeap1 occupied ecdysone-regulated early puffs on polytene chromosomes. Depletion of either CncC or dKeap1 in salivary glands selectively reduced early puff gene transcription. CncC and dKeap1 depletion in the prothoracic gland as well as cncC^K6/K6 and dKeap1^EY5/EY5 loss of function mutations in embryos reduced ecdysone-biosynthetic gene transcription. In contrast, dKeap1 depletion and the dKeap1^EY5/EY5 loss of function mutation enhanced xenobiotic response gene transcription in larvae and embryos, respectively. Depletion of CncC or dKeap1 in the prothoracic gland delayed pupation by decreasing larval ecdysteroid levels. CncC depletion suppressed the premature pupation and developmental arrest caused by constitutive Ras signaling in the prothoracic gland; conversely, constitutive Ras signaling altered the loci occupied by CncC on polytene chromosomes and activated transcription of genes at these loci. The effects of CncC and dKeap1 on both ecdysone-biosynthetic and ecdysone-regulated gene transcription, and the roles of CncC in Ras signaling in the prothoracic gland, establish the functions of these proteins in the neuroendocrine axis that coordinates insect metamorphosis.

Human Nrf2-Keap1 and the fruit fly CncC-dKeap1 protein complexes function both in response to foreign chemicals and in development. We found that CncC and dKeap1 control fruit fly development by regulating the production and actions of the principal hormone that controls the transformation of larvae into pupae. In hormone-responsive cells, CncC and dKeap1 bound to the genes that are activated by the hormone. When the amount of CncC or dKeap1 in these cells was reduced, the genes were not activated efficiently. When the amount of CncC or dKeap1 was reduced in the organ where the hormone is made, the genes whose products make the hormone were not activated efficiently. Because less hormone was made, it took longer for the larvae to turn into pupae, and the resulting pupae were bigger. Reduction of the amount of CncC intercepted previously identified signals for pupation. Nrf2 is required for the same signals to cause cancer in mice. The effects of CncC and dKeap1 both on genes that control hormone production and on genes that are switched on by the hormone in different organs indicate that they have multiple roles in the transformation of fruit fly larvae into pupae.

Translational control by the DEAD Box RNA helicase belle regulates ecdysone-triggered transcriptional cascades

Steroid hormones act, through their respective nuclear receptors, to regulate target gene expression. Despite their critical role in development, physiology, and disease, however, it is still unclear how these systemic cues are refined into tissue-specific responses. We identified a mutation in the evolutionarily conserved DEAD box RNA helicase belle/DDX3 that disrupts a subset of responses to the steroid hormone ecdysone during Drosophila melanogaster metamorphosis. We demonstrate that belle directly regulates translation of E74A, an ets transcription factor and critical component of the ecdysone-induced transcriptional cascade. Although E74A mRNA accumulates to abnormally high levels in belle mutant tissues, no E74A protein is detectable, resulting in misregulation of E74A-dependent ecdysone response genes. The accumulation of E74A mRNA in belle mutant salivary glands is a result of auto-regulation, fulfilling a prediction made by Ashburner nearly 40 years ago. In this model, Ashburner postulates that, in addition to regulating secondary response genes, protein products of primary response genes like E74A also inhibit their own ecdysone-induced transcription. Moreover, although ecdysone-triggered transcription of E74A appears to be ubiquitous during metamorphosis, belle-dependent translation of E74A mRNA is spatially restricted. These results demonstrate that translational control plays a critical, and previously unknown, role in refining transcriptional responses to the steroid hormone ecdysone.

Pulses of steroid hormones regulate a variety of biological processes, but how these simple global cues are converted into specific local responses remains unclear. While steroid responses have traditionally been thought to be regulated at the transcriptional level, here we demonstrate that translational control plays a novel role in refining steroid signals. The DEAD box RNA helicase belle directly regulates the translation of E74A mRNA, which encodes a transcription factor that is induced by the fly steroid hormone ecdysone and then rapidly repressed. This process is disrupted in belle mutant tissues, where E74A mRNA accumulates to abnormally high levels but is not translated. We demonstrate that Belle-dependent translation of E74A is required to both repress its own transcription and to induce tissue-specific target genes. These findings confirm the prediction that auto-regulation is important for the self-limiting behavior of steroid responses and demonstrate a critical role for translational control in refining a global hormonal signal into specific biological responses.

Ecdysone receptors: from the Ashburner model to structural biology

In 1974, Ashburner and colleagues postulated a model to explain the control of the puffing sequence on Drosophila polytene chromosomes initiated by the molting hormone 20-hydroxyecdysone. This model inspired a generation of molecular biologists to clone and characterize elements of the model, thereby providing insights into the control of gene networks by steroids, diatomic gases, and other small molecules. It led to the first cloning of the EcR subunit of the heterodimeric EcR-USP ecdysone receptor. X-ray diffraction studies of the ligand-binding domain of the receptor are elucidating the specificity of receptor-ecdysteroid interactions, the selectivity of some environmentally friendly insecticides, the evolution of the EcR-USP heterodimer, and indeed Ashburner's classical biochemical evidence for the central role of the ecdysone receptor in his model.

Nitric oxide coordinates metabolism, growth, and development via the nuclear receptor E75

Nitric oxide gas acts as a short-range signaling molecule in a vast array of important physiological processes, many of which include major changes in gene expression. How these genomic responses are induced, however, is poorly understood. Here, using genetic and chemical manipulations, we show that nitric oxide is produced in the Drosophila prothoracic gland, where it acts via the nuclear receptor ecdysone-induced protein 75 (E75), reversing its ability to interfere with its heterodimer partner, Drosophila hormone receptor 3 (DHR3). Manipulation of these interactions leads to gross alterations in feeding behavior, fat deposition, and developmental timing. These neuroendocrine interactions and consequences appear to be conserved in vertebrates.

Proteomic analysis of steroid-triggered autophagic programmed cell death during Drosophila development

Two morphological forms of programmed cell death, apoptosis and autophagic cell death, remove unneeded or damaged cells during animal development. Although the mechanisms that regulate apoptosis are well studied, little is known about autophagic cell death. A shotgun proteome analysis of purified dying larval salivary glands in Drosophila was used to identify proteins that are expressed during autophagic programmed cell death. A total of 5661 proteins were identified from stages before and after the onset of cell death. Analyses of these data enabled us to identify proteins from a number of interesting categories including regulators of transcription, the apoptosis, autophagy, lysosomal, and ubiquitin proteasome degradation pathways, and proteins involved in growth control. Several of the identified proteins, including the serine/threonine kinase warts (Wts), were not detected using whole-genome DNA microarrays, providing support for the importance of such high-throughput proteomic technology. Wts regulates cell-cycle arrest and apoptosis, and significantly, mutations in wts prevent destruction of salivary glands.

Dynamic localisation of KR-H during an ecdysone response in Drosophila

The propagation of a hormonal response following an increase in titre involves intensive cross-talk between the products of the key regulatory genes. The Kr-h gene of Drosophila is a modulator of both the embryonic and metamorphic hierarchies of ecdysone responsive genes, but its mode of action is puzzling as mutants have both quantitative and qualitative (timing) effects on the ecdysone responses. We have used an antibody against KR-H to follow its distribution in larval tissues as they prepare for metamorphosis. While in most tissues protein levels remain stable, its distribution within salivary gland cells changes throughout the late larval ecdysone response and the ensuing prepupal period. We show that, at the chromosomal level, KR-H localisation is dynamic and that the protein is recruited to, and released from, loci harbouring an important subset of the known regulatory genes as the response advances. Such behaviour is most likely a conserved characteristic of hormonal responses.

Hormonal regulation and functional role of Drosophila E75A orphan nuclear receptor in the juvenile hormone signaling pathway

Ecdysone and juvenile hormone (JH) are important regulators of insect growth and development. While ecdysone initiates a transition from one developmental stage to another, JH determines the nature of the transition. How these two hormones interact at the molecular level is not known. Here we report the JH inducibility of the E75A nuclear receptor encoded by the E75 early ecdysone-inducible gene. In Drosophila S2 cells, E75A transcription is specifically activated by JH at concentrations well within the physiological range found in larvae and adults. The induction is rapid and does not require a concurrent protein synthesis, and thus represents a primary hormone response. Consistent with JH regulation, E75A mRNA levels are reduced in ovaries of apterous(4) mutant adults defective in JH secretion. Expression is rescued by topical methoprene application. We further provide evidence that ectopic E75A is sufficient to perform several functions in the JH signaling pathway. First, it can down-regulate its own transcription. Second, E75A can potentiate the JH inducibility of a secondary response gene, JhI-21. Finally, in the presence of JH, E75A can repress ecdysone activation of early genes including Broad-Complex. Based on these data, we propose a model for the role of E75A in the ecdysone-JH regulatory interplay.

Loss of the ecdysteroid-inducible E75A orphan nuclear receptor uncouples molting from metamorphosis in Drosophila

Isoform-specific null mutations were used to define the functions of three orphan members of the nuclear receptor superfamily, E75A, E75B, and E75C, encoded by the E75 early ecdysteroid-inducible gene. E75B mutants are viable and fertile, while E75C mutants die as adults. In contrast, E75A mutants have a reduced ecdysteroid titer during larval development, resulting in developmental delays, developmental arrests, and molting defects. Remarkably, some E75A mutant second instar larvae display a heterochronic phenotype in which they induce genes specific to the third instar and pupariate without undergoing a molt. We propose that ecdysteroid-induced E75A expression defines a feed-forward pathway that amplifies or maintains the ecdysteroid titer during larval development, ensuring proper temporal progression through the life cycle.

Isolation and characterization of novel mutations of the Broad-Complex, a key regulatory gene of ecdysone induction in Drosophila melanogaster

Seven new alleles of the Broad-Complex gene of Drosophila melanogaster, which encodes a family of four zinc finger protein isoforms BR-C Z1, Z2, Z3 and Z4, were generated by transposase-induced mobilization of a P[Zw] element inserted in either the first intron downstream from the P165 promoter or the exon encoding the Z2-specific zinc finger domain. They were characterized by genetic complementation tests, molecular mapping and cytogenetic analysis of their effect on ecdysone-induced puffing and BR-C proteins binding to polytene chromosomes. Four mutations that correspond to three overlapping deletions and one tandem insertion of the P[Zw] element are located in the intron. They provide evidence that regulatory elements essential for a correct expression of the BR-C Z2 and BR-C Z3 transcripts are located within the intron downstream from the P165 promoter. Three mutations correspond to internal deletions of the locus and exhibit a complete loss of all BR-C(+) genetic functions in the complementation and cytogenetic tests. They thus provide well characterized new amorphic reference alleles of the BR-C gene. The precise cytogenetic location of more than 300 binding sites of BR-C proteins on larval salivary gland polytene chromosomes was determined by immunostaining using specific antibodies. Sites were found in big ecdysone inducible puffs, constitutively active small puffs as well as interbands. A complete list of the major sites on all four salivary gland polytene chromosomes of BR-C(+) larvae is presented.

Selective binding of Drosophila BR-C isoforms to a distal regulatory element in the hsp23 promoter

The Broad-Complex (BR-C) gene plays a key role in the ecdysone regulatory hierarchy. Together with other early ecdysone-inducible genes BR-C transmits the hormonal signal to a set of secondary response genes in a tissue-specific manner. Among its targets is the hsp23 gene. Previously we showed that expression of the hsp23 gene in late third instar is BR-C-dependent, and accompanied by the appearance of a BR-C-dependent DNase I hypersensitive site at position -1400 (DHS-1400). BR-C encodes a family of transcription factors, and we show here that at least three BR-C protein isoforms--Z1, Z2, and Z3--bind to the sequences around DHS-1400 in vitro. A DNase I footprinting assay reveals five protected regions, designated site 1 to site 5, each of which specifically associates with one or several BR-C protein isoforms. We also show that a 100 bp region overlapping site 5, which binds all three isoforms in vitro, is required for hsp23 activity in vivo. The deletion of binding site 5 in a reporter gene construct reproduced the effect of the npr class mutations, that is, hsp23 is no longer expressed in any tissue tested except brain. Thus, BR-C regulates hsp23 expression via direct interaction of the predominant isoform with the distal regulatory element.

Steroid regulation of programmed cell death during Drosophila development

Steroid hormones play an important role in the regulation of numerous physiological responses, but the mechanisms that enable these systemic signals to trigger specific cell changes remain poorly characterized. Recent studies of Drosophila illustrate several important features of steroid-regulated programmed cell death. A single steroid hormone activates both cell differentiation and cell death in different tissues and at multiple stages during development. While several steroid-regulated genes are required for cell execution, most of these genes function in both cell differentiation and cell death, and require more specific factors to kill cells. Genes that regulate apoptosis during Drosophila embryogenesis are induced by steroids in dying cells later in development. These apoptosis genes likely function downstream of hormone-induced factors to serve a more direct role in the death response. This article reviews the current knowledge of steroid signaling and the regulation of programmed cell death during development of Drosophila.

Cloning and development expression of Choristoneura hormone receptor 75: a homologue of the Drosophila E75A gene

Cloning and characterization of a cDNA of the spruce budworm, Choristoneura fumiferana, that showed high amino acid similarity with the deduced amino acid sequences of E75 cDNAs cloned from Manduca sexta, Galleria melonella, and Drosophila melanogaster are described. Initially, a cDNA fragment and then a full length cDNA were cloned from C. fumiferana. The longest open reading frame of this cDNA had 690 codons and its deduced amino acid sequence had all five domains typical of a steroid hormone nuclear receptor. The deduced amino acid sequence of this cDNA showed the highest identity with the deduced amino acid sequence of E75A cDNAs cloned from M. sexta, G. melonella, and D. melanogaster, and is therefore named Choristoneura hormone receptor 75A (CHR75A). The CHR75A cDNA probe detected a 2.6 kb mRNA that was abundant at the time of the ecdysteroid peaks during molting in the embryonic, larval and pupal stages. In the sixth instar larvae, CHR75 mRNA was detected in the epidermis, fat body and midgut, and maximum expression was observed during the prepupal peak of ecdysteroids in the hemolymph. CHR75 mRNA was induced in ecdysone treated CF-203 cells and in the midgut, fat body and epidermis of larvae that were fed the non-steroidal ecdysteroid agonist, RH-5992. In vitro transcription and translation of the CHR75A cDNA yielded a 79 kDa protein that bound to the retinoic acid receptor related orphan receptor response element (RORE).

The 82F late puff contains the L82 gene, an essential member of a novel gene family

Metamorphosis in Drosophila results from a hierarchy of ecdysone-induced gene expression initiated at the end of the third larval instar. A now classical model of this hierarchy was proposed based on observations of the activity of polytene chromosome "puffs" which distinguished "early" puffs as those directly induced by ecdysone and "late" puffs as those which become active as a secondary response to the hormone. We report here the isolation and characterization of the L82 gene corresponding to the extensively characterized late puff at 82F. L82 is a complex gene that spans at least 50 kb of genomic DNA, produces at least seven different nested mRNAs, and has homology to a novel gene family. In contrast to most previously characterized puff genes, the broad developmental expression pattern of L82 suggests that it is controlled by both ecdysone-dependent and ecdysone-independent regulatory mechanisms. L82 mutations were identified by transgene rescue of developmental delay and eclosion lethal phenotypes.

E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction

The steroid hormone ecdysone controls genetic regulatory hierarchies underlying insect molting, metamorphosis and, in some insects, reproduction. Cytogenetic and molecular analysis of ecdysone response in Drosophila larval salivary glands has revealed regulatory hierarchies including early genes which encode transcription factors controlling late ecdysone response. In order to determine whether similar hierarchies control reproductive ecdysone response, we have investigated ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. Here, we identify the homologue of the Drosophila E75 early ecdysone inducible gene in the yellow fever mosquito Aedes aegypti, and show that, as in Drosophila, the mosquito homologue, AaE75, consists of three overlapping transcription units with three mRNA isoforms, AaE75A, AaE75B, and AaE75C, originating as a result of alternative splicing. All three AaE75 isoforms are induced at the onset of vitellogenesis by a blood meal-activated hormonal cascade, and highly expressed in the mosquito ovary and fat body, suggesting their involvement in the regulation of oogenesis and vitellogenesis, respectively. Furthermore, in vitro fat body culture experiments demonstrate that AaE75 isoforms are induced by 20-hydroxyecdysone, an active ecdysteroid in the mosquito. These findings suggest that related ecdysone-triggered regulatory hierarchies may be used reiteratively during developmental and reproductive ecdysone responses.

Ultrastructural changes of Drosophila larval and prepupal salivary glands cultured in vitro with ecdysone

Alterations in the ultrastructure of in vitro cultured larval salivary glands of Drosophila melanogaster in response to the steroid hormone ecdysone were studied in relation to complex changes in puffing patterns. We found that the changes in the fine structure of cultured glands reflected progression of the puffing pattern, and they paralleled those seen in vivo. We observed that glue secretion by exocytosis, the main function of salivary glands, took place between puff stage 5 (PS5) and PS7. Glue could not be expectorated under culture conditions but was slowly released from the lumen through a duct into the medium. After the cultured glands reached PS13/PS14, further progress of puffing and fine structural alterations required that the ecdysteroid titer be transiently extremely low or absent. Under in vitro conditions we did not observe the putative new secretory program(s) described for glands in vivo after PS12. However, ultrastructural changes which unambiguously indicated that an autohistolytic process had begun in vitro started to appear after PS17. Many salivary gland cells developed numerous features of progressive self-degradation between PS18 and PS21. Actual degradation of salivary glands in vivo seemed to be rapid, but in vitro degradation was never completed, probably due to a lack of exogenous factors from the hemolymph. Manipulations of ecdysone titer in vitro in the culture medium, known during the larval puffing cycle to cause premature induction of developmentally specific puffing patterns, did not affect the normal development of ultrastructural features of the cytoplasm and nucleus.

The Broad-Complex gene is a tissue-specific modulator of the ecdysone response of the Drosophila hsp23 gene

The steroid hormone ecdysone causes dramatic changes in the genetic programs leading to the pupariation of Drosophila melanogaster, and the Broad-Complex (BR-C) gene is known to play a key role in this process. Previously we showed that BR-C regulates developmental changes in transcription and chromatin structure of the 67B heat shock gene cluster, which contains four small hsp genes. Importantly, the downregulation of the hsp23 gene in the BR-C mutants correlates with the absence of a DNase I-hypersensitive site (DHS) at position -1400. To study the functional importance of the DHS-1400, we have introduced genomic fragments containing a modified hsp23 gene into the Drosophila germ line. Our analysis shows that the ecdysone response element is necessary but not sufficient for full developmental expression of hsp23 in the late third instar and that there is, indeed, another regulatory element, in the vicinity of DHS-1400. We also show that hsp23 developmental expression is not tissue specific. A construct lacking the ecdysone response element is unable to direct normal hsp23 expression in all tissues except the brain. Similarly, brain-specific expression is BR-C independent, although in the other tissues we find different requirements for BR-C genetic functions. The effect of the br mutations is restricted to wing imaginal discs and midgut tissue, while that of 2Bc is restricted to the fat body and Malpighian tubules, and mutations in the rbp group have no effect in any of the tissues studied. Thus, BR-C regulatory action is mediated through different genetic functions in a tissue-specific manner.

The Afrotropical Drosophila montium subgroup: Balbiani ring 1, polytene chromosomes, and heat shock response of Drosophila vulcana

A detailed photographic map of the salivary gland polytene chromosomes of Drosophila vulcana, an Afrotropical species of the montium subgroup of the melanogaster group, is presented, along with chromosomal rearrangements, such as reverse tandem duplications and inversions, the well-formed Balbiani ring 1, and the most prominent puffs during normal larval and white prepupal development and after ecdysone treatment. In addition, the heat inducible protein and puffing pattern and the loci of the major heat shock genes, namely, hsp70, hsp83, the "small" hsps, and a putative hsp68, of this species were studied. In the light of the data revealed by the above studies, phylogenetic relationship among the montium subgroup species are attempted.

Ecdysone signaling cascade and regulation of Drosophila metamorphosis

Pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) regulate diverse biological responses during the life history of insects. Studies of the fruit fly, Drosophila melanogaster, have provided significant insights into the mechanisms underlying ecdysone mediated regulation of development. During the dramatic metamorphosis of Drosophila, ecdysone induces the histolysis of nearly all of the larval tissues and differentiation and morphogenesis of the structures composing the adult fly. These changes are mediated by a genetic signaling cascade that was first recognized as puffs in the giant polytene chromosomes of the salivary gland. This genetic regulatory cascade is composed of early and late genes that are intricately coordinated by changes in hormone titer. Early genes encode regulatory proteins that are involved in the proper regulation of late genes, which are thought to play a more direct role in development. The regulation and function of these genes is discussed in the context of the cell- and tissue-specific changes required for the reorganization of a larva to form an adult fly.

Hormonal induction of Dopa decarboxylase in the epidermis of Drosophila is mediated by the Broad-Complex

The 2B5 early puff locus corresponds to the Broad-Complex BR-C) and encodes a family of transcription factors whose members are induced by the molting hormone ecdysone. Mutations in the br subcomplementation group substantially reduce the levels of Dopa decarboxylase (DDC) in the epidermis of mature third instar larvae but not in mature second instar organisms. Enzyme levels are normal in the central nervous system of the two mutants examined. The specificity of these effects suggests that a product of the BR-C locus mediates the rapid appearance of DDC in mature third instar larvae experiencing an elevated titer of ecdysone. The likely identity of this protein has been confirmed by pursuing the observation that the br28 allele caused by the insertion of a Pelement into the Z2 DNA-binding domain. Both the transcript and a protein carrying this domain are present in the epidermis and a BR-C recombinant protein carrying the Z2 finger binds to the first intron of the Ddc gene. Five binding sites have been identified within the intron by DNAase I footprinting and a core consensus sequence has been derived which shares some identity with the consensus binding site of the Z2 protein to the Sgs-4 regulatory region. Our demonstration that Ddc is a target of BR-C in the epidermis is the first direct evidence of a role for this early gene in a tissue other than the salivary glands. The data reinforce the idea that BR-C, which clearly mediates a salivary gland-specific response to ecdysone, may play a widespread role in the hormone's activation of gene cascades in other target tissues.

The Drosophila E74 gene is required for the proper stage- and tissue-specific transcription of ecdysone-regulated genes at the onset of metamorphosis

The steroid hormone ecdysone directly induces a small set of early genes, visible as puffs in the larval salivary gland polytene chromosomes, as it signals the onset of Drosophila metamorphorsis. The products of these genes appear to function as regulators that both repress their own expression and induce a large set of secondary-response late genes. We have identified recessive loss-of-function mutations in the early gene E74, a member of the ets protooncogene family that encodes two related DNA-binding proteins, E74A and E74B. These mutations cause defects in pupariation and pupation, and result in lethality during metamorphosis. Here we extend our phenotypic characterization of the E74A and E74B mutant alleles to the molecular level by examining their effects on the transcription of over 30 ecdysone-regulated genes. We show that the transcription of most ecdysone primary-response genes during late larval and prepupal development is unaffected by the E74 mutations. Rather, we find that E74 is necessary for the appropriate regulation of many ecdysone secondary-response genes. E74B is required for the maximal induction of glue genes in mid third instar larval salivary glands, while E74A is required in early prepupae for the proper timing and maximal induction of a subset of late genes. E74 activity is also necessary for the correct regulation of genes expressed predominantly in the fat body, epidermis or imaginal discs. These observations confirm that E74 plays a critical role in regulating transcription during the early stages of Drosophila metamorphosis. In addition, the widespread effects of the E74 mutations on transcription indicate that E74 functions in regulatory hierarchies not only in the larval salivary gland, but throughout the entire organism.

Differential expression of Broad-Complex transcription factors may forecast tissue-specific developmental fates during Drosophila metamorphosis

The steroid hormone ecdysone initiates metamorphosis in Drosophila melanogaster by activating a cascade of gene activity that includes primary response transcriptional regulators and secondary response structural genes. The Broad-Complex (BR-C) primary response gene is composed of several distinct genetic functions and encodes a family of related transcription factor isoforms. Our objective was to determine whether BR-C isoforms were components of the primary ecdysone response in all tissues and whether tissue-specific isoform expression is associated with tissue-specific metamorphic outcomes. We used specific antibody reagents that recognize and distinguish among the Z1, Z2 and Z3 BR-C protein isoforms to study protein expression patterns during the initial stages of metamorphosis. Western blot analyses demonstrated that BR-C isoforms are induced at the onset of metamorphosis, each with unique kinetics of induction and repression. Whole-mount immunostaining showed that the BR-C proteins accumulate in the nuclei of all larval and imaginal tissues indicating that the BR-C is induced as a primary response in many tissues. Several tissues express different levels and combinations of the BR-C isoforms suggesting that the BR-C is important in determining the tissue-specific outcome of many parallel ecdysone response cascades. For example, prepupal salivary glands (destined for histolysis during metamorphosis) express Z1 isoforms while imaginal discs (destined for cell differentiation and morphogenesis) shift from the synthesis of Z2 isoforms to the synthesis of Z1 isoforms. The prepupal central nervous system (destined for tissue remodeling) expresses all isoforms, with Z3 predominating. Salivary gland chromosome immunostaining indicated that BR-C proteins interact directly with numerous loci in the polytene genome. Finally, western blot analyses showed that distinct BR-C genetic functions can be correlated with single and specific BR-C protein isoforms.

Isolation, characterization and developmental expression of the ecdysteroid-induced E75 gene of the wax moth Galleria mellonella

Using the cDNA for the Drosophila ecdysteroid-induced member of the steroid-hormone-receptor superfamily, E75A, we isolated a genomic clone from Galleria mellonella that revealed 77% similarity with the region of E75A cDNA encoding the C-terminal zinc-finger motif. A Galleria cDNA clone was isolated that encoded a complete DNA-binding domain composed of two zinc fingers and designated GmE75A. Its deduced amino acid sequence showed 100% and 85% identities within the DNA-binding and ligand-binding domains of Drosophila E75A, respectively. The Galleria genomic clone did not encode the N-terminal zinc finger, but included a sequence similar to the B1 exon, which is unique to the B isoform of E75. Thus, the cDNA and genomic DNA sequences indicated that the Galleria gene E75 encoded at least two isoforms, GmE75A and GmE75B, which differed in their N-termini. Probes specific for GmE75A and B hybridized to two distinct transcripts of 2.6 kb. Both GmE75A and B mRNA levels correlated closely with the ecdysteroid titer during development. At the onset of larval/pupal transformation, both transcripts appeared in high amounts within 4 h of the ecdysteroid rise, then declined concurrently with the hormone titer decline. At the time of pupal ecdysis, there was another peak of GmE75A expression but not GmE75B expression, coincident with a minor ecdysteroid pulse. In isolated abdomens of final instar larvae, GmE75A mRNA was induced by 20-hydroxyecdysone within 20 min of the injection; the mRNA levels were maximal at 1 h and declined by 3 h following the treatment.

The Drosophila 78C early late puff contains E78, an ecdysone-inducible gene that encodes a novel member of the nuclear hormone receptor superfamily

We report the molecular definition of an early late puff locus, at position 78C, that is inducible by ecdysone at the onset of Drosophila metamorphosis. This puff contains a single ecdysone-inducible gene consisting of two nested transcription units, E78A and E78B. E78A mRNA is expressed during a brief interval in mid-pupal development and encodes a novel member of the nuclear hormone receptor superfamily. E78B encodes a truncated receptor isoform that lacks the DNA-binding domain and is predominantly expressed at puparium formation and immediately following E78A in pupae. E78B is directly inducible by ecdysone in late third instar larvae and depends on ecdysone-induced protein synthesis for its maximal level of expression. These observations indicate that E78 represents a distinct subset of early ecdysone-inducible regulatory genes.