Drosophila hormone receptor 38: a second partner for Drosophila USP suggests an unexpected role for nuclear receptors of the nerve growth factor-induced protein B type

In Drosophila the response to the hormone ecdysone is mediated in part by Ultraspiracle (USP) and ecdysone receptor (EcR), which are members of the nuclear receptor superfamily. Heterodimers of these proteins bind to ecdysone response elements (EcREs) and ecdysone to modulate transcription. Herein we describe Drosophila hormone receptor 38 (DHR38) and Bombyx hormone receptor 38 (BHR38), two insect homologues of rat nerve growth factor-induced protein B (NGFI-B). Although members of the NGFI-B family are thought to function exclusively as monomers, we show that DHR38 and BHR38 in fact interact strongly with USP and that this interaction is evolutionarily conserved. DHR38 can compete in vitro against EcR for dimerization with USP and consequently disrupt EcR-USP binding to an EcRE. Moreover, transfection experiments in Schneider cells show that DHR38 can affect ecdysone-dependent transcription. This suggests that DHR38 plays a role in the ecdysone response and that more generally NGFI-B type receptors may be able to function as heterodimers with retinoid X receptor type receptors in regulating transcription.

Microsatellite DNA and isozyme variability in a west African population of Anopheles gambiae

Microsatellites are defined as tracts of tandemly repeated short DNA sequences. Polymorphisms in this class of DNA are currently being used to generate a genetic map of the mosquito Anopheles gambiae. In the present study we explore the potential of microsatellites as a tool for studying the genetic structure of natural populations of this malaria vector. Genetic polymorphism at twenty enzyme coding gene loci and eleven microsatellite DNA loci was surveyed in a population of An. gambiae from Mali, West Africa. All of the microsatellite loci surveyed were polymorphic, as compared to 40% of the isozyme loci. The mean heterozygosity for the isozyme loci was only 0.097 (+/- 0.0035), but for the microsatellite loci it was 0.732 (+/- 0.060). The pattern of variability was very different between isozymes and microsatellites. Typically, at an isozyme locus a single allele occurred at a frequency > or = 0.75, whereas at microsatellite loci the most common allele had a frequency < 0.50. We conclude that microsatellites provide a rich source of genetic polymorphisms for the study of the population genetics of An. gambiae and are in many ways superior to isozymes for this purpose. We discuss the potential for utilizing genetically mapped microsatellite loci to explore the effect of chromosomal inversions on the distribution of genetic polymorphisms in An. gambiae.

BmCF1, a Bombyx mori RXR-type receptor related to the Drosophila ultraspiracle

A PCR approach has been used to obtain an ovarian cDNA clone from the silkmoth Bombyx mori, encoding a 50 kDa protein (BmCF1) that belongs to the RXR subfamily of nuclear hormone receptors and is most similar to the CF1/USP protein (DmCF1) encoded by the ultraspiracle gene of Drosophila. The similarity is high in the DNA-binding and moderately so in the ligand-binding domains, although not in the N-terminal, putatively activator A/B domain. Protein sequence comparisons with the available RXR sequences indicate that although insect USP-like sequences are more related to each other than to vertebrate RXRs, their inter se similarities are lower than in the case of the vertebrate RXRs. Two distinct BmCF1-homologous transcripts are observed consistently, and are indicative either of alternative splicing or of the existence of a second RXR gene in the moth. The transcripts are widely distributed, suggesting functions at multiple developmental stages, as in the case of Drosophila ultraspiracle.

Developmental specificity of a bidirectional moth chorion promoter in transgenic Drosophila

The developmentally specific activity of the 326-bp promoter that is shared by the paired 18 and 401 chorion genes of the silkmoth Antheraea pernyi was studied in transgenic Drosophila melanogaster. Bidirectional activity of this promoter is demonstrated in a novel dual reporter vector, containing lacZ on one side and CAT on the other. The single chorion-specific hexamer of this promoter, TCACGT, is essential for expression in both orientations; even single-base substitutions (at positions 2 or 4 of the hexamer) suppress expression. With both reporter genes, ovarian expression is greater from the 401 side of the promoter, although the difference between the two sides is greater for the CAT gene. The intact promoter fragment (which includes some 5' untranslated sequences) is expressed with a high degree of tissue specificity, but a central 184-bp fragment derived from it is also active ectopically in parts of the internal male genitalia. Strong spatial as well as temporal regulation is evident with both the intact promoter and its 184-bp subfragment, assayed in either orientation.

Functions of the cytoplasmic domain of the beta PS integrin subunit during Drosophila development

Integrins constitute a family of membrane-spanning, heterodimeric proteins that mediate adhesive interactions between cells and surrounding extracellular matrices (or other cells) and participate in signal transduction. We are interested in assessing integrin functions in the context of developing Drosophila melanogaster. This report, using mutants of the beta PS subunit encoded by the myospheroid (mys) locus, analyzes the relationships between integrin protein structure and developmental functions in an intact organism. As a first step in this analysis, we demonstrated the ability of a fragment of wild-type mys genomic DNA, introduced into the germ line in a P-element vector P[mys+], to rescue phenotypes attributed to lack of (or defects in) the endogenous beta PS during several discrete morphogenetic events. We then produced in vitro a series of modifications of the wild-type P[mys+] transposon, which encode beta PS derivatives with mutations within the small and highly conserved cytoplasmic domain. In vivo analysis of these mutant transposons led to the following conclusions. (1) The cytoplasmic tail of beta PS is essential for all developmental functions of the protein that were assayed. (2) An intron at a conserved position in the DNA sequence encoding the cytoplasmic tail is thought to participate in important alternative splicing events in vertebrate beta integrin subunit genes, but is not required for the developmental functions of the mys gene assayed here. (3) Phosphorylation on two conserved tyrosines found in the C terminus of the beta PS cytoplasmic tail is not necessary for the tested developmental functions. (4) Four highly conserved amino acid residues found in the N-terminal portion of the cytoplasmic tail are important but not critical for the developmental functions of beta PS; furthermore, the efficiencies with which these mutant proteins function during different morphogenetic processes vary greatly, strongly suggesting that the cytoplasmic interactions involving PS integrins are developmentally modulated.

Antibody detection of protein complexes bound to DNA

Images

Elements controlling follicular expression of the s36 chorion gene during Drosophila oogenesis

An 84-bp proximal regulatory protein (PRR) of the Drosophila melanogaster s36 chorion gene is sufficient for directing proper temporal and spatial expression of a reporter gene in three domains of the follicle: anterior, posterior, and main body. Here we show that the fidelity of PRR-directed s36 expression is dependent on the proper dorsal-ventral differentiation of the follicular epithelium, which requires the Drosophila epidermal growth factor receptor homolog. Transgenic analysis of site-directed mutants of the PRR suggests that s36 expression is regulated by the concerted action of multiple positive activators. Several cis-acting transcriptional elements have been identified: some appear to function in a quantitative manner, while others either are essential or appear to regulate expression in particular spatial domains. The approximate locations of these regulatory elements have been defined; some map within sequences that are strongly conserved in widely divergent dipteran species. In fact, the PRR analog of the medfly Ceratitis capitata Ccs36 gene directs expression in a manner similar to the D. melanogaster s36 PRR. We propose a model for transcriptional regulation of s36 based on the prechoriogenic polarization of the follicular epithelium that surrounds the developing egg chamber.

A detailed genetic map for the X chromosome of the malaria vector, Anopheles gambiae

Anopheles gambiae, the primary vector of human malaria in Africa, is responsible for approximately a million deaths per year, mostly of children. Despite its significance in disease transmission, this mosquito has not been studied extensively by genetic or molecular techniques. To facilitate studies on this vector, a genetic map has been developed that covers the X chromosome at an average resolution of 2 centimorgans. This map has been integrated with the chromosome banding pattern and used to localize a recessive, sex-linked mutation (white eye) to within 1 centimorgan of flanking markers.

A Drosophila single-strand DNA/RNA-binding factor contains a high-mobility-group box and is enriched in the nucleolus

We have isolated a Drosophila melanogaster cDNA encoding a high-mobility-group (HMG) box-containing protein. This protein shares 50% amino acid identity with the human putative structure-specific recognition protein, hSSRP. The gene encoding the D. melanogaster homolog, DssRP, is developmentally regulated and is expressed most abundantly in ovaries (nurse cells in particular). The protein is localized in nuclei and is particularly abundant in the nucleolus. In vitro binding studies using DssRP produced in bacteria showed that, despite expectation, the protein does not bind to structured DNA. Instead, it binds to single-stranded DNA and RNA, with highest affinity to nucleotides G and U.

Analyses of PS integrin functions during Drosophila development

The Drosophila position-specific (PS) antigens are homologues of the vertebrate integrins, a family of transmembrane proteins that function in cell-matrix and cell-cell adhesion. The common beta subunit of PS integrins (PS beta) is encoded by the lethal(l)myospheroid gene (mys) and is required during wing, eye and muscle development. By expressing PS beta protein at defined developmental periods, we have shown that PS integrins are required throughout pupation, but not earlier, for normal development of wings. In contrast, the key requirement for PS integrins in eye development occurs only in the late pupa. Furthermore, PS integrins are apparently not required for the differentiation of the ommatidial cells; only for their organization. These results are consistent with roles for PS integrins in the interactions between the wing epithelia during the two phases of pupal wing expansion and in maintaining the attachment of a fully formed fenestrated membrane to the basement membrane of the retina. We have also examined the functional significance of alternative splicing of the transcript of the mys gene using P element-mediated transformation to introduce transgenes producing only one of the two spliced forms of PS beta. We find that either form is sufficient to rescue postembryonic mys phenotypes in the wing, eye and muscle but that both of the two splice forms are necessary to rescue the mys embryonic defects. This result indicates a requirement for the alternative splicing of mys during embryogenesis. The location of the alternative exons suggests that the two forms of the PS beta integrin subunit may interact with alternative alpha subunits and/or ligands.

Polytene chromosome microdissection and molecular genome mapping in Drosophila and other dipterans

We are constructing a molecular physical map of the Drosophila melanogaster genome, using microdissection of polytene chromosomes as a source of region specific probes applied to direct mapping work. Chromosome microdissection has been used for a low resolution map of the Anopheles gambiae s.s. genome.

Binding affinity of the Drosophila melanogaster CF1/USP protein to the chorion s15 promoter

The Drosophila chorion factor 1/ultraspiracle transcription factor is a developmentally important member of the nuclear hormone receptor superfamily, and this factor most closely resembles the vertebrate retinoid X receptors. We describe here the expression of the CF1/USP protein in an E. coli T7 RNA polymerase-driven expression system. Using partially purified, bacterially produced CF1/USP, we have determined the equilibrium dissociation constant of this protein to its response element on the chorion s15 promoter. The equilibrium binding activity of CF1/USP to its chorion promoter is equivalent to the binding activities of other nuclear hormone receptors to their cognate elements.

Negative and positive regulators modulate the activity of a silkmoth chorion gene during choriogenesis

We have used in vitro linker substitution mutagenesis and P-element-mediated transformation in Drosophila to define cis-regulatory elements of the bidirectional promoter of the Bombyx mori A/B.L12 chorion genes in an in vivo situation. Within an essential 112 bp part of this promoter, mutations in two non-contiguous approximately 35 base-pair regions (A1 and A2) result in significant reduction of expression at late choriogenesis. The reduction occurs in both promoter orientations, indicating the existence of bidirectionally active positive elements. Mutations in A1, but not in A2, also lead to premature activation at early choriogenesis, suggesting the existence of a negative element essential for the finely tuned expression profile during choriogenesis. Normally, this element apparently prevents early expression; it also acts bidirectionally. The highly specific effects of these mutations suggest that several trans-acting regulators of chorion gene transcription have been conserved between silkmoths and fruitflies.

DNA binding and heteromerization of the Drosophila transcription factor chorion factor 1/ultraspiracle

The Drosophila chorion factor 1/ultraspiracle (CF1/USP) transcription factor, a homologue of the retinoid X receptor, is a developmentally important member of the family of nuclear (steroid) hormone receptors. Using newly developed monoclonal antibodies and a full-length bacterially produced protein, we have studied in detail the in vitro DNA-binding properties of this factor and aspects of its distribution in vivo. During oogenesis, CF1/USP is present both in germline cells and in the somatic follicular epithelium. We have determined the optimal binding site of partially purified bacterially produced CF1/USP by an in vitro selection procedure and also have characterized its binding to the follicular-specific chorion s15 promoter. In vitro this bacterially produced factor is unusual in binding to a single element ("half-site"); simultaneous but noncoordinate binding to a second half-site is possible if these repeated elements are organized in direct orientation and spaced adequately. However, the factor interacts synergistically with several other nuclear hormone receptors: notably, it can form in vitro heteromers with mammalian thyroid and retinoic acid receptors, binding to two half-sites that are organized in either direct or inverted orientation. In vivo the factor most probably functions as a heterodimer, but its partner(s) remains to be determined.

Multiple zinc finger forms resulting from developmentally regulated alternative splicing of a transcription factor gene

Transcripts encoding the Drosophila putative transcription factor CF2 are subject to developmentally regulated alternative splicing, and they encode protein isoforms that differ in the number of zinc fingers. One testis-specific RNA encodes an isoform that includes three zinc fingers and a frame-shifted segment. Two other transcripts encode isoforms with six and seven zinc fingers which bind to distinct promoters and DNA target sequences. Thus, because of alternative splicing, a single gene appears to encode distinct DNA-binding proteins, each capable of regulating different gene sets in different tissues and developmental periods.

Sequence discrimination by alternatively spliced isoforms of a DNA binding zinc finger domain

Two major developmentally regulated isoforms of the Drosophila chorion transcription factor CF2 differ by an extra zinc finger within the DNA binding domain. The preferred DNA binding sites were determined and are distinguished by an internal duplication of TAT in the site recognized by the isoform with the extra finger. The results are consistent with modular interactions between zinc fingers and trinucleotides and also suggest rules for recognition of AT-rich DNA sites by zinc finger proteins. The results show how modular finger interactions with trinucleotides can be used, in conjunction with alternative splicing, to alter the binding specificity and increase the spectrum of sites recognized by a DNA binding domain. Thus, CF2 may potentially regulate distinct sets of target genes during development.

Chorion gene cis-regulatory DNA restricts tissue specificity of reporter gene expression in transformed Drosophila

P element mediated germ-line transformation was used to study the developmental specificity of Drosophila chorion gene regulatory sequences directing expression of the bacterial reporter genes for chloramphenicol acetyltransferase (CAT) and beta-galactosidase (lacZ). DNA fragments containing 5' flanking plus the entire 5' untranslated and the beginning of the coding region of either the s36 or the s15 chorion gene are able to confer on the reporter genes normal tissue as well as temporal specificity of expression, exclusively in the ovary of transformed female flies. However, if 5' untranslated and coding regions are omitted, normal ovarian expression is maintained but tissue specificity is relaxed: expression of the reporter gene is detected both in the ovary and in specific non-ovarian tissues of transformed females and males. The evidence suggests that the missing 5' untranslated and coding sequences may include negative elements that normally suppress expression in non-ovarian tissues, and that these putative elements are distinct from those that prevent premature expression in the ovarian follicles. The exact location of ectopic lacZ expression within the internal male genitalia depends on the constellation of 5' flanking chorion regulatory sequences included in the P element constructs. Ectopic expression of the CAT gene in the male genitalia under s15 promoter control can be abolished by mutating the hexamer TCACGT, a sequence previously shown to be essential for the normal expression of this chorion gene in the ovary.

Positive and negative DNA elements of the Drosophila grimshawi s18 chorion gene assayed in Drosophila melanogaster

Germ line transformation has been used to map the cis regulatory DNA elements responsible for the precise and evolutionarily stable developmental expression of the s18 chorion gene. Constructs containing chimeric combinations of Drosophila melanogaster and D. grimshawi DNA regions, as well as D. grimshawi sequences alone, can direct expression in the follicular epithelium, in an s18-specific temporal and spatial pattern. The results indicate that both positive and negative regulatory elements can function when transferred from D. grimshawi to D. melanogaster. The first ca. 100 bp of the 5'-flanking DNA region constitute a minimal, developmentally regulated promoter, expression of which is inhibited by the next 100-bp DNA segment and activated by positive elements located further upstream. Expression of the minimal promoter can also be enhanced by more distant chorion regulatory elements, provided the inhibitory DNA segment is absent.

FLP-mediated intermolecular recombination in the cytoplasm of Drosophila embryos

We show that when a heat-shock-driven gene that encodes the yeast FLP recombinase is injected into preblastoderm Drosophila embryos, it promotes intermolecular recombination between two coinjected plasmids that bear the specific recombination target sequence, FRT. Minimal, 34-bp FRT sites in the two plasmids are sufficient for their cointegration. The reaction is efficient enough to produce detectable recombinants when one of the plasmids is present in as little as 1000 molecules per embryo. This is comparable to the concentration of unique chromosomal sites, raising the possibility that integration of injected plasmid DNA into FRT-bearing fly chromosomes may also be achievable. Since integrants might be stabilized against the reverse excision reaction if the recombinase could be provided in a sharp pulse, it is encouraging that efficient plasmid cointegration is also achieved when in vitro synthesized FLP RNA rather than DNA is injected into the embryos.

The genome of the Mediterranean fruitfly Ceratitis capitata: localization of molecular markers by in situ hybridization to salivary gland polytene chromosomes

We hybridized cloned DNA segments to salivary gland polytene chromosomes of the medfly, Ceratitis capitata, and thus established molecular markers for 24 sites on 6 out of 10 autosomal arms. An additional marker identified a medfly repetitive element that hybridizes to approximately 100 autosomal sites as well as a granular network that is thought to represent the X chromosome. Some of the markers correspond to 9 characterized transcription units, while 17 remain anonymous; at least 3 of the latter are restriction fragment length polymorphism (RFLP) markers. The characterized transcription units document that chromosomal arm 5L of C. capitata is homologous to the Drosophila melanogaster X chromosome, in agreement with previous inferences based on the extensive conservation of linkage groups in Diptera.

Low-resolution genome map of the malaria mosquito Anopheles gambiae

We have microdissected divisions of the Anopheles gambiae polytene chromosomes, digested the DNAs with a restriction enzyme, and PCR-amplified the DNA fragments to generate a set of pooled probes, each corresponding to approximately 2% of the mosquito genome. These divisional probes were shown to have high complexity. Except for those derived from near the centromeres, they hybridize specifically with their chromosomal sites of origin. Thus, they can be used to map cloned DNAs by a dot blot procedure, which is much more convenient than in situ hybridization to polytene chromosomes. We discuss additional potential uses of these probes, such as easier isolation of molecular markers and genes, including those that cross-hybridize with clones available from other insects. It is expected that the probes will substantially accelerate molecular genetic analysis of this most important malaria vector.

The relative importance of transcriptional and post transcriptional regulation of Drosophila chorion gene expression during oogenesis

To determine the relative roles of transcriptional and post-transcriptional events in establishing the temporal pattern of chorion gene expression in Drosophila, we have examined chorion gene transcription, RNA accumulation, and protein synthesis in follicles of selected pre-, early-, and late-choriogenic stages. Chorion gene transcription was assayed in follicle cell nuclei by nuclear run-on reactions. For the s15, s16, s18, s36, and s38 chorion genes, the periods of intense transcription are as predicted from the dynamics of RNA accumulation and protein synthesis, indicating that these genes are primarily regulated at the transcriptional level. In contrast, gene s19 appears subject to post-transcriptional control at stage 14, when transcription rates are substantially higher than predicted from the observed RNA levels. Transcription of regions between the clustered and tandemly oriented chorion genes was also examined. In contrast to many RNA polymerase II transcribed genes, for the s18 and s36 chorion genes run-on transcription appears to terminate within about 100 base pairs downstream of the polyadenylation sites, corroborating previous reports based on electron microscopy of s36 [Osheim et al., EMBO J 5:3591-3596, 1986].