DNA sequence of a 3.8 kilobase pair region controlling Drosophila chorion gene amplification

Chorion genes: a landscape of their evolution, structure, and regulation

Differential regulation at the level of transcription provides a means for controlling gene expression in eukaryotes, especially during development. Insect model systems have been extensively used to decipher the molecular basis of such regulatory cascades, and one of the oldest such model systems is the regulation of chorion gene expression during ovarian follicle maturation. Recent experimental and technological advances have shed new light onto the system, allowing us to revisit it. Thus, in this review we try to summarize almost 40 years' worth of studies on chorion gene regulation while-by comparing Bombyx mori and Drosophila melanogaster models-attempting to present a comprehensive, unified model of the various regulatory aspects of choriogenesis that takes into account the evolutionary conservation and divergence of the underlying mechanisms.

Aplysia capsulin is localized to egg capsules and egg cordon sheaths and shares sequence homology with Drosophila dec-1 gene products

Differential library screening of an albumen gland cDNA library, Western blot analysis, protein expression, immunolocalization studies, comparative genomics, and secretion assays identified a major Aplysia californica albumen gland protein ('capsulin') that is localized to egg capsules and to the sheaths of the egg cordon. Capsulin shared sequence homology with eggshell proteins encoded by the Drosophila dec-1 gene. The 1790-amino acid A. californica precursor contains 17 repeat sequences that are flanked by basic residue processing sites. The numerous proteolytic processing sites may facilitate the breakdown of capsulin prior to when veliger larvae break out of egg capsules as free-swimming larvae. An Aplysia brasiliana capsulin repeat sequence was 97% identical to its A. californica homolog. Capsulin fragments were not detected in the eluates of egg cordons, suggesting that capsulin is not a candidate water-borne pheromone precursor.

Developmental gene amplification and origin regulation

Developmentally regulated gene amplification serves to increase the number of templates for transcription, yielding greatly increased protein and/or RNA product for gene(s) at the amplified loci. It is observed with genes that are very actively transcribed and during narrow windows of developmental time where copious amounts of those particular gene products are required. Amplification results from repeated firing of origins at a few genomic loci, while the rest of the genome either does not replicate, or replicates to a lesser extent. As such, amplification is a striking exception to the once-and-only-once rule of DNA replication and may be informative as to that mechanism. Drosophila amplifies eggshell (chorion) genes in the follicle cells of the ovary to allow for rapid eggshell synthesis. Sciara amplifies multiple genes in larval salivary gland cells that encode proteins secreted in the saliva for the pupal case. Finally, Tetrahymena amplifies its rRNA genes several thousand-fold in the creation of the transcriptionally active macronucleus. Due to the ease of molecular and genetic analysis with these systems, the study of origin regulation has advanced rapidly. Comparisons reveal an evolutionarily conserved trans-regulatory apparatus and a similar organization of sequence-specific cis-regulatory replicator and origin elements. The studies indicate a regulatory role for chromatin structure and transcriptionally active genes near the origins.

The functions of the multiproduct and rapidly evolving dec-1 eggshell gene are conserved between evolutionarily distant species of Drosophila

The Drosophila dec-1 gene encodes multiple proteins that are required for female fertility and proper eggshell morphogenesis. Genetic and immunolocalization data suggest that the different DEC-1 proteins are functionally distinct. To identify regions within the proteins with potential biological significance, we cloned and sequenced the D. yakuba and D. virilis dec-1 homologs. Interspecies comparisons of the predicted translation products revealed rapidly evolving sequences punctuated by blocks of conserved amino acids. Despite extensive amino acid variability, the proteins produced by the different dec-1 homologs were functionally interchangeable. The introduction of transgenes containing either the D. yakuba or the D. virilis dec-1 open reading frames into a D. melanogaster DEC-1 protein null mutant was sufficient to restore female fertility and wild-type eggshell morphology. Normal expression and extracellular processing of the DEC-1 proteins was correlated with the phenotypic rescue. The nature of the conserved features highlighted by the evolutionary comparison and the molecular resemblance of some of these features to those found in other extracellular proteins suggests functional correlates for some of the multiple DEC-1 derivatives.

The chorion genes of the medfly. II. DNA sequence evolution of the autosomal chorion genes s18, s15, s19 and s16 in Diptera

We present a total of approximately 15 kb of DNA sequences, encompassing four chorion genes Ccs18, Ccs15, Ccs19, Cc16 and their flanking DNA in the medfly C. capitata. Comparison of coding regions, introns and intergenic sequences in five Dipteran species, D. melanogaster, D. subobscura, D. virilis, D. grimshawi and C. capitata documented an extensive divergence in introns and coding regions, but few well conserved elements in the proximal 5' flanking regions in all species. These elements are related to conserved regulatory features of three of the genes, including tissue- and temporal regulation. In the fourth, gene s15, significant alterations in the 5' flanking region may be responsible for its changed temporal regulation in C. capitata. One long intergenic sequence, located in the distal 5' flanking region of gene s18, is homologous to ACE3, a major amplification control element and contains an 80-bp A/T-rich sequence, known to stimulate strong binding of the origin recognition complex (ORC) in D. melanogaster. Analysis of the nucleotide composition of all chorion genes in C. capitata and D. melanogaster showed that C. capitata exhibit less biased representation of synonymous codons than does D. melanogaster.

Functionally distinct, sequence-specific replicator and origin elements are required for Drosophila chorion gene amplification

To meet the demand for the rapid synthesis of chorion (eggshell) proteins, Drosophila ovarian follicle cells amplify the chromosomal loci containing the chorion gene clusters up to 60-fold. Amplification occurs by repeated firing of one or more origins located within each gene cluster. Deletion analyses of transgenic constructs derived from the third chromosome cluster have identified a 320-bp amplification control element (ACE3) required for amplification, as well as several stimulatory amplification enhancing regions (AERs). Two-dimensional (2D) gel analyses have identified multiple DNA replication initiation sites (origins) that partially overlap in location with ACE3 and the AERs. To further study sequence requirements for amplification, a vector was used in which transgenic constructs are protected from chromosomal position effects by flanking insulator elements, the suppressor Hairy-wing protein binding site (SHWBS). Using the buffered vector, the 320-bp ACE3 and an 884-bp element designated ori-beta were found to be necessary and sufficient for amplification. Two-dimensional gels revealed that ori-beta was acting as the origin. In contrast, origin activity could not be detected for ACE3. An insulator placed between ACE3 and ori-beta inhibited amplification, indicating that ACE3 activates ori-beta in cis. The results suggest that ACE3 acts as a replicator and support and extend the replicator model for the organization of metazoan chromosomal replicons.

Morphogenesis of the eggshell in Drosophila

The Drosophila eggshell is a specialized extracellular matrix that forms between the oocyte and overlaying somatic follicle cells during the latter stages of oogenesis. Largely proteinaceous, the eggshell is a highly organized multilayered structure with regional specializations designed to perform a variety of functions. Production of a functional eggshell features: (1) the differentiation of subsets of follicle cells in response to ovarian signals, (2) directed migrations of the follicle cells within the developing egg chamber, (3) expression of eggshell structural genes by the follicle cells in a defined temporal and spatial order, (4) postdepositional modifications of the eggshell proteins including several temporally regulated proteolytic cleavage events, and (5) regulated trafficking of several eggshell proteins in the assembling structure. By exploiting the genetic advantages of Drosophila and using evolution as a guide, the eggshell provides an excellent experimental system to study, in vivo, molecular mechanisms used to regulate protein-protein interactions throughout the assembly of a complex extracellular architecture in a developing organism.

Modular sequence elements associated with origin regions in eukaryotic chromosomal DNA

We have postulated that chromosomal replication origin regions in eukaryotes have in common clusters of certain modular sequence elements (Benbow, Zhao, and Larson, BioEssays 14, 661-670, 1992). In this study, computer analyses of DNA sequences from six origin regions showed that each contained one or more potential initiation regions consisting of a putative DUE (DNA unwinding element) aligned with clusters of SAR (scaffold associated region), and ARS (autonomously replicating sequence) consensus sequences, and pyrimidine tracts. The replication origins analyzed were from the following loci: Tetrahymena thermophila macronuclear rDNA gene, Chinese hamster ovary dihydrofolate reductase amplicon, human c-myc proto-oncogene, chicken histone H5 gene, Drosophila melanogaster chorion gene cluster on the third chromosome, and Chinese hamster ovary rhodopsin gene. The locations of putative initiation regions identified by the computer analyses were compared with published data obtained using diverse methods to map initiation sites. For at least four loci, the potential initiation regions identified by sequence analysis aligned with previously mapped initiation events. A consensus DNA sequence, WAWTTDDWWWDHWGWHMAWTT, was found within the potential initiation regions in every case. An additional 35 kb of combined flanking sequences from the six loci were also analyzed, but no additional copies of this consensus sequence were found.

Comparative analysis of the sequence and structure of two Drosophila melanogaster genes encoding vitelline membrane proteins

Two Drosophila melanogaster vitelline membrane protein-encoding genes (VM), located at polytene band positions 26A and 34C, have been cloned and comparatively characterized at the nucleotide level. Sequence analysis of genomic and cDNA clones for the two genes, VM26A.1 and VM34C.1, indicates that both are similarly organized with a central highly conserved domain [Scherer et al., Dev. Biol. 130 (1988) 786-788] which is flanked by unrelated regions, and that both genes lack introns. Comparison of the upstream regions reveals that both VM genes contain a hepatmeric element identical to one associated with the D. melanogaster yolk protein-encoding genes (YP). This heptamer occurs in the specific 5' flanking region responsible for ovarian temporal- and tissue-specific control in both VM and YP genes. A putative chorion transcription factor 2 site is also associated with an upstream control element of VM26A.1, but not with any sequenced portion of VM34C.1.

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.

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 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.

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.

The Drosophila ACE3 chorion element autonomously induces amplification

The Drosophila chorion genes amplify in the follicle cells by repeated rounds of reinitiation of DNA replication. ACE3 (amplification control element from the third chromosome) has been identified by a series of deletion experiments as an important control element for amplification of the third-chromosome chorion cluster. Several elements that quantitatively enhance amplification also have been defined. We show that a single 440-bp ACE3 sequence is sufficient to regulate amplification with proper developmental specificity autonomously from other chorion DNA sequences and regulatory elements. Although ACE3 is sufficient for amplification, the levels of amplification are low even when ACE3 is present in multiple copies. When controlled solely by ACE3, amplification initiates either at ACE3 or within closely linked sequences. Amplification of an ACE3 transposon insertion produces a gradient of amplified DNA that extends into flanking sequences approximately the same distance as does the amplification gradient at the endogenous chorion locus. The profile and extent of the amplified gradient imply that the low levels of amplification observed are the result of limited rounds of initiation of DNA replication. Transposon inserts containing multiple copies of ACE3 in a tandem, head-to-tail array are maintained stably in the chromosome. However, mobilization of the P-element transposons containing ACE3 multimers results in deletions within the array at a high frequency.

The Drosophila ACE3 chorion element autonomously induces amplification

The Drosophila chorion genes amplify in the follicle cells by repeated rounds of reinitiation of DNA replication. ACE3 (amplification control element from the third chromosome) has been identified by a series of deletion experiments as an important control element for amplification of the third-chromosome chorion cluster. Several elements that quantitatively enhance amplification also have been defined. We show that a single 440-bp ACE3 sequence is sufficient to regulate amplification with proper developmental specificity autonomously from other chorion DNA sequences and regulatory elements. Although ACE3 is sufficient for amplification, the levels of amplification are low even when ACE3 is present in multiple copies. When controlled solely by ACE3, amplification initiates either at ACE3 or within closely linked sequences. Amplification of an ACE3 transposon insertion produces a gradient of amplified DNA that extends into flanking sequences approximately the same distance as does the amplification gradient at the endogenous chorion locus. The profile and extent of the amplified gradient imply that the low levels of amplification observed are the result of limited rounds of initiation of DNA replication. Transposon inserts containing multiple copies of ACE3 in a tandem, head-to-tail array are maintained stably in the chromosome. However, mobilization of the P-element transposons containing ACE3 multimers results in deletions within the array at a high frequency.

Oocyte-specific factors bind a conserved upstream sequence required for mouse zona pellucida promoter activity

The zona pellucida of mouse oocytes, composed of three major glycoproteins (ZP1, ZP2, and ZP3), performs crucial functions at fertilization and in early development. The transcripts encoding mouse ZP2 and ZP3 are coordinately expressed and accumulate in oocytes during a 2-week growth phase prior to ovulation. The 5'-flanking regions of mouse Zp-2 and Zp-3 genes and their human homologs contain five short DNA sequences (4 to 12 bp) that are 60 to 100% identical and are approximately equidistant upstream of the TATAA box in the four genes. Mutation of these five elements (I, IIA, IIB, III, and IV) in Zp-luciferase constructs demonstrates that the 12-bp element IV, positioned approximately 200 bp upstream from the TATAA box, is necessary for high-level expression from the mouse Zp-2 and Zp-3 promoters after microinjection into the nuclei of 50-microns-diameter oocytes. Injection of minimal Zp-3 promoter constructs containing element IV in either orientation also resulted in high levels of reporter gene activity, suggesting that the element is not only necessary but also sufficient for expression from zona pellucida promoters. Oligonucleotides containing the conserved element from either Zp-2 or Zp-3 form DNA-protein complexes of identical mobility in gel retardation assays using extracts of oocytes but not other tissues. These data are consistent with the hypothesis that common factors binding to conserved element IV are involved in coordinate expression of the oocyte-specific Zp-2 and Zp-3 zona pellucida genes.

Oocyte-specific factors bind a conserved upstream sequence required for mouse zona pellucida promoter activity

The zona pellucida of mouse oocytes, composed of three major glycoproteins (ZP1, ZP2, and ZP3), performs crucial functions at fertilization and in early development. The transcripts encoding mouse ZP2 and ZP3 are coordinately expressed and accumulate in oocytes during a 2-week growth phase prior to ovulation. The 5'-flanking regions of mouse Zp-2 and Zp-3 genes and their human homologs contain five short DNA sequences (4 to 12 bp) that are 60 to 100% identical and are approximately equidistant upstream of the TATAA box in the four genes. Mutation of these five elements (I, IIA, IIB, III, and IV) in Zp-luciferase constructs demonstrates that the 12-bp element IV, positioned approximately 200 bp upstream from the TATAA box, is necessary for high-level expression from the mouse Zp-2 and Zp-3 promoters after microinjection into the nuclei of 50-microns-diameter oocytes. Injection of minimal Zp-3 promoter constructs containing element IV in either orientation also resulted in high levels of reporter gene activity, suggesting that the element is not only necessary but also sufficient for expression from zona pellucida promoters. Oligonucleotides containing the conserved element from either Zp-2 or Zp-3 form DNA-protein complexes of identical mobility in gel retardation assays using extracts of oocytes but not other tissues. These data are consistent with the hypothesis that common factors binding to conserved element IV are involved in coordinate expression of the oocyte-specific Zp-2 and Zp-3 zona pellucida genes.

EM analysis of Drosophila chorion genes: amplification, transcription termination and RNA splicing

We have used the electron microscope to examine ultrastructurally several events occurring during the biogenesis of two very abundant chorion (eggshell) mRNA molecules in the follicle cells of Drosophila melanogaster--namely, selective gene amplification, transcription initiation and termination, and RNA processing. We find that the highly transcribed s36 and s38 genes are positioned in the central region of large, multi-forked amplified DNA structures. Transcript morphology is consistent with the known presence of a small intron at the 5' end of each gene. Mature transcripts are associated with spliceosomes, demonstrating that splice site selection occurs co-transcriptionally but that splicing is completed after transcript release from the template. We have also mapped the termination sites for the genes. The two genes exhibit efficient termination very near their poly(A) sites--within a 210 bp region for s36 and a 360 bp region for s38.

Genomic organization and polypeptide primary structure of zona pellucida glycoprotein hZP3, the hamster sperm receptor

During the course of fertilization in mammals, free-swimming sperm bind tightly to receptors located in the egg extracellular coat, or zona pellucida. Recently, the hamster sperm receptor, a 56,000 Mr zona pellucida glycoprotein called hZP3, was identified and partially characterized (C. C. Moller et al., (1990). Dev. Biol. 137, 276-286). Here, we describe genomic cloning of hZP3, certain organizational features of the hZP3 gene, and primary structures of hZP3 mRNA and polypeptide. The findings are compared with reported results of comparable analyses of the mouse sperm receptor, an 83,000 Mr zona pellucida glycoprotein called mZP3. Such comparisons reveal a high degree of conservation of genomic organization and polypeptide structure for the two mammalian sperm receptors, despite the considerable difference in their Mrs. These findings are of interest in view of the extremely restricted expression of the ZP3 gene during development and the important role of ZP3 oligosaccharides in gamete adhesion.

Multiple proteins are produced from the dec-1 eggshell gene in Drosophila by alternative RNA splicing and proteolytic cleavage events

The defective chorion-1 gene (dec-1) in Drosophila encodes follicle cell proteins necessary for proper eggshell assembly. A distinctive feature of the gene is the production of multiple products by both alternative RNA splicing and proteolytic processing events. DNA and protein sequencing studies have revealed several dec-1 protein products. The predominant translation product, fc106, has a vitelline membrane-like N-terminal domain followed by a glutamine, methionine-rich central region, largely in the form of 26 amino acid repeats. During late stage 10 the N-terminal portion of fc106 is cleaved, yielding s80, a major eggshell protein. Conceptual translation of the DNA sequence as well as molecular analyses of several dec-1 mutants suggest that the less abundant alternatively spliced RNAs encode primary translation products with different carboxy terminal ends. These results are discussed with respect to previous genetic analyses of dec-1 mutants as well as with respect to potential protein-protein interactions which may underlie stabilization of this complex extracellular structure.

Evolution of the autosomal chorion cluster in Drosophila. IV. The Hawaiian Drosophila: rapid protein evolution and constancy in the rate of DNA divergence

Autosomal chorion genes s18, s15, and s19 are shown to diverge at extremely rapid rates in closely related taxa of Hawaiian Drosophila. Their nucleotide divergence rates are at least as fast as those of intergenic regions that are known to evolve more extensively between distantly related species. Their amino acid divergence rates are the fastest known to date. There are two nucleotide replacement substitutions for every synonymous one. The molecular basis for observed length and substitution mutations is analyzed. Length mutations are strongly associated with direct repeats in general, and with tandem repeats in particular, whereas the rate for an average transition is twice that for an average transversion. The DNA sequence of the cluster was used to construct a phylogenetic tree for five taxa of the Hawaiian picture-winged species group of Drosophila. Assignment of observed base substitutions occurring in various branches of the tree reveals an excess of would-be homoplasies in a centrally localized 1.8-kb segment containing the s15 gene. This observation may be a reflection of ancestral excess polymorphisms in the segment. The chorion cluster appears to evolve at a constant rate regardless of whether the central 1.8-kb segment is included or not in the analysis. Assuming that the time of divergence of Drosophila grimshawi and the planitibia subgroup coincides with the emergence of the island of Kauai, the overall rate of base substitution in the cluster is estimated to be 0.8% million years, whereas synonymous sites are substituted at a rate of 1.2% million years.

Evolution of the autosomal chorion cluster in Drosophila. III. Comparison of the s18 gene in evolutionarily distant species and heterospecific control of chorion gene amplification

We present a total of 6.1 x 10(3) base-pairs of DNA sequences, encompassing the s18 gene and flanking regions within the autosomal chorion cluster of three Drosophila species. Against a background of extensive divergence in the intron and even in parts of the coding region, islands of strong sequence conservation are evident. These are particularly notable in the 5' flanking DNA where they extend to approximately -600 base-pairs from the transcription start site. The most conserved segment of the entire chorion cluster is 71 base-pairs in the s18 5' flanking DNA, which in D. melanogaster is part of a region defined functionally as containing amplification control elements (ACE3 region). Transformation analysis, using chimeric transposons of D. melanogaster and D. grimshawi DNA, revealed that amplification control elements of D. grimshawi can support amplification in D. melanogaster. The functionally defined ACE3 region of D. grimshawi includes the conserved 71 base-pair segment, but also non-conserved sequences further upstream, which apparently enhance amplification.

An upstream region of the mouse ZP3 gene directs expression of firefly luciferase specifically to growing oocytes in transgenic mice

The gene encoding the mouse egg primary receptor for sperm, a zona pellucida glycoprotein called ZP3, is expressed exclusively in growing oocytes within ovaries of sexually immature and mature female mice. We have constructed a transgene in which 6.5 kilobases of ZP3 gene 5'-flanking sequence is fused to the coding region of the firefly luciferase gene, and we have generated four independent lines of transgenic mice. In these animals, the transgene is expressed exclusively in ovaries. Furthermore, within ovaries, expression is confined to growing oocytes, and luciferase activity can be detected by assaying individual, isolated oocytes. The pattern of firefly luciferase expression during oocyte growth is similar to that observed in previous studies of ZP3 expression during oogenesis in mice. Observations reported here strongly suggest that cis-acting elements present in the ZP3 gene 5'-flanking region regulate oocyte-specific and, therefore, sex-specific expression of the sperm receptor gene during mouse development. They also suggest that such elements can be used to direct expression of cloned genes specifically to oocytes of transgenic mice and to evaluate the effects of such expression on various aspects of early mammalian development.

Proteins that bind to Drosophila chorion cis-regulatory elements: a new C2H2 zinc finger protein and a C2C2 steroid receptor-like component

Gel mobility-shift assays have been used to identify proteins that bind specifically to the promoter region of the Drosophila s15 chorion gene. These proteins are present in nuclear extracts of ovarian follicles, the tissue where s15 is expressed during development, and bind to specific elements of the promoter that have been shown by transformation analysis to be important for in vivo expression. The DNA binding specificity has been used for molecular cloning of two components from expression cDNA libraries and for their tentative identification with specific DNA-binding proteins of the nuclear extracts. The mRNAs for both of these components, CF1 and CF2, are differentially enriched in the follicles. DNA sequence analysis suggests that both CF1 and CF2 are novel Drosophila transcription factors. CF2 is a member of the C2H2 family of zinc finger proteins, whereas CF1 is a member of the family of steroid hormone receptors. The putative DNA-binding domain of CF1 is highly similar to the corresponding domains of certain vertebrate hormone receptors and recognizes a region of DNA with similar, hyphenated palindromic sequences. The nature of CF1 raises the possibility of hormonal control of choriogenesis in Drosophila.

The chorion genes of the medfly, Ceratitis capitata, I: Structural and regulatory conservation of the s36 gene relative to two Drosophila species

We have used low stringency screening with the Drosophila melanogaster s36 chorion gene to recover its homologue from genomic and cDNA libraries of the medfly, Ceratitis capitata. The same gene has also been recovered from a genomic library of D. virilis. The medfly s36 gene shows similar developmental specificity as in Drosophila (early choriogenesis). It is also specifically amplified in ovarian follicles; this is the first report of chorion gene amplification outside the genus Drosophila. Alignments of s36 sequences from three species show that, in addition to its regulatory conservation, the s36 gene is extensively conserved in sequence, in a region corresponding to a central protein domain, and in short regions of 5' flanking DNA that might correspond to cis-regulatory elements.

Multiple replication origins are used during Drosophila chorion gene amplification

DNA from Drosophila egg chambers undergoing chorion gene amplification was analyzed using the two-dimensional gel technique of Brewer and Fangman. At stage 10, 34% of DNA molecules from the maximally amplified region of the third chromosome chorion gene cluster contained replication forks or bubbles. These nonlinear forms were intermediates in the process of amplification; they were confined to follicle cells, and were found only within the replicating region during the time of amplification. Multiple origins gave rise to these intermediates, since three separate regions of the third chromosome chorion locus contained replication bubbles. However, initiation was nonrandom; the majority of initiations appeared to occur near the Bgl II site located between the s18 and s15 chorion genes. The P[S6.9] chorion transposon also contained abundant replication intermediates in follicle cells from a transformed line. Initiation within P[S6.9] occurred near two previously defined cis-regulatory elements, one near the same Bgl II site (in the AER-d region) and one near the ACE3 element.

Transgenic regulation of moth chorion gene promoters in Drosophila: tissue, temporal, and quantitative control of four bidirectional promoters

Bidirectional chorion gene promoter regions from three silkmoth species, Bombyx mori, Antheraea pernyi, or Antheraea polyphemus (members of two different moth families), were tested for their ability to transcriptionally activate a bacterial marker gene (chloramphenicol acetyltransferase) in transformant Drosophila. Relatively short 5' flanking DNA fragments (272-367 bp) of chorion gene pairs are sufficient to confer a high degree of tissue and choriogenic stage specificity of expression to the marker gene. Thus, significant conservation of molecular interactions controlling transcription during choriogenesis is observed between the distantly related orders, Lepidoptera and Diptera. However, quantitative and fine temporal regulation in the Drosophila host does not fully parallel the in situ regulation in moths, indicating that some regulatory protein-DNA interactions have diversified in the approximately 250 million years since the last common ancestor of these insect groups. Limited in vitro mutagenesis of a B. mori promoter DNA has shown that a central 189-bp region includes elements sufficient for the qualitative specificity of chorion-specific expression. The same experiments have shown that a previously identified essential element, centered on the TCACGT hexamer, is not sufficient for chorion-specific expression: an additional essential element or elements are found farther upstream, within a 112-bp DNA region. Comparisons of silkmoth and Drosophila chorion gene promoter sequences have identified some candidates for cis-acting elements involved in the developmental regulation of chorion gene expression.

Nonuniform evolution of duplicated, developmentally controlled chorion genes in a silkmoth

We report the sequence of A/B.L1, a tightly linked pair of genes from the A and B chorion families in Bombyx mori. Comparison with the previously characterized A/B.L11 and A/B.L12 pairs from the same species reveals moderate sequence divergence, which is clearly nonuniform. Although the average divergence of A/B.L12 from the other two pairs is more than double that between A/B.L11 and A/B.L1, the ratio differs by more than 30-fold in different DNA regions. One domain of the A gene is highly divergent between A/B.L12 and A/B.L1 or A/B.L11, but essentially invariable in the latter two. In well-aligned DNA segments, nearly all mutated sites (111/112) show variants shared by two of the three sequences, in 42% of the cases between the more distantly related pairs (A/B.L12 and either A/B.L1 or A/B.L11). Eight of the variants shared by distantly related pairs are clustered within 51 bp, suggesting the possibility that they arose through sequence transfers between gene pairs, rather than being primitive or resulting from independent mutations. The short intergenic, putatively regulatory DNA of the developmentally middle A/B.L1 and A/B.L11 pairs resembles that of the late HcA/HcB pairs, particularly in patches that may correspond to cis-regulatory elements.

Molecular analysis and rescue of a vitelline membrane mutant in Drosophila

The eggshell in Drosophila is produced by ovarian follicle cells during the later stages of oogenesis. Eggshell formation involves the ordered synthesis and assembly of several protein components. Genes encoding the most abundant eggshell proteins have been identified by molecular cloning studies. Morphological examination of eggs produced by females carrying female sterile mutations on the X and third chromosomes have revealed additional loci involved in chorion formation. In this study we screened a collection of female sterile mutants carrying EMS-induced mutations on the second chromosome for eggshell mutants. A class of six mutants with potential vitelline membrane defects was identified on the basis of the response of mutant eggs to hypochlorite solutions. Biochemical analysis showed that one mutant, fs(2)QJ42, failed to produce a major vitelline membrane protein, sV23. The mutation was mapped cytogenetically to 26A, a region previously implicated in vitelline membrane formation by molecular cloning studies. Northern blot analysis using a cloned copy of the sV23 gene as probe showed a 10- to 15-fold reduction of sV23 RNA levels in the mutant. sV23 synthesis and fertility were restored when a normal copy of the sV23 gene was introduced into the mutant via germ line transformation. Transposons carrying the sV23 gene with as little as 147 bp of 5' flanking DNA were capable of restoring fertility and sV23 protein to wild type levels.

Evolution of the autosomal chorion cluster in Drosophila. II. Chorion gene expression and sequence comparisons of the s16 and s19 genes in evolutionarily distant species

We present a total of 13 kb of DNA sequences, encompassing autosomal chorion genes and their flanking DNA in four species of the genus Drosophila. Against a background of extensive divergence in introns and even in parts of the coding regions, islands of strong conservation are evident in the proximal 5' flanking and 5' untranslated sequences. An extragenic region of strong conservation is seen downstream of the last chorion gene in the autosomal cluster. The conserved DNA elements may be related to the conserved regulatory features of this cluster, including gene amplification and tissue- and temporally regulated transcription.

Small gene family encoding an eggshell (chorion) protein of the human parasite Schistosoma mansoni

We have isolated six independent genomic clones encoding schistosome chorion or eggshell proteins from a Schistosoma mansoni genomic library. A linkage may of five of the clones spanning 35 kilobase pair (kbp) of the S. mansoni genome was constructed. The region contained two eggshell protein genes closely linked, separated by 7.5 kbp of intergenic DNA. The two genes of the cluster were arranged in the same orientation, that is, they were transcribed from the same strand. The sixth clone probably represents a third copy of the eggshell gene that is not contained within the 35-kbp region. The 5' end of the mRNA transcribed from these genes was defined by primer extension directly off the RNA. The ATCAT cap site sequence was homologous to a silkmoth chorion PuTCATT cap site sequence, where Pu indicates any purine. DNA sequence analysis showed that there were no introns in these genes. The DNA sequences of the three genes were very homologous to each other and to a cDNA clone, pSMf61-46, differing only in three or four nucleotides. A multiple TATA box was located at positions -23 to -31, and a CAAAT sequence was located at -52 upstream of the eggshell transcription unit. Comparison of sequences in regions further upstream with silkmoth and Drosophila sequences revealed several very short elements that were shared. One such element, TCACGT, recently shown to be an essential cis-regulatory element for silkmoth chorion gene promoter function, was found at a similar position in all three organisms.

Sex-, tissue-, and stage-specific expression of a vitelline membrane protein gene from region 32 of the second chromosome of Drosophila melanogaster

This study isolated cDNA clones from egg-chamber and adult female Drosophila cDNA libraries using as probe a DNA fragment from a 200-kb "chromosome walk" in region 32E of the second chromosome of D. melanogaster. The present authors believe that these clones correspond to a new vitelline membrane protein (VMP) gene because 1) cDNA clones in Northern blots identify a transcript expressed in a tissue- and stage-specific manner: stage 10 egg-chambers; 2) the sequence of cDNAs and of the genomic subclone shows homology with the other VMP genes that have been identified to date; 3) the amino acid composition of the translational product has the high content of proline and alanine characteristic of VMPs. Two aspects emerging from this study are worth stressing: 1) the presence of a hydrophobic domain that is highly conserved in all the VMP genes; and 2) the particularly narrow period of expression of the isolated gene, which could be related to the mechanism of vitelline membrane assembly.

Visualization of Drosophila melanogaster chorion genes undergoing amplification

We visualized by electron microscopy the preferential amplification of Drosophila chorion genes in late-stage follicle cells. Chromatin spreads revealed large clusters of actively transcribed genes of the appropriate size, spacing, and orientation for chorion genes that were expressed with the correct temporal specificity. Occasionally the active genes were observed within or contiguous with intact replicons and replication forks. In every case, our micrographs are consistent with the hypothesis that the central region of each chorion domain contains a replication origin(s) used during the amplification event. In one case, a small replication bubble was observed precisely at the site of the essential region of the X chromosome amplification control element. The micrographs also suggest that forks at either end of a replicon frequently progress very different distances, presumably due to different times in initiation or different rates of movement. It appears that all chorion genes (even those coding for minor proteins) are transcribed in a "fully on" condition, albeit for varied durations, and that if replication fork passage does inactivate a promoter, it does so very transiently. Furthermore, a DNA segment containing one active gene is likely to have an additional active gene(s). Surprisingly, during the time frame of expected maximum activity, approximately half of the chorion sequences appear transcriptionally inactive.

Small gene family encoding an eggshell (chorion) protein of the human parasite Schistosoma mansoni

We have isolated six independent genomic clones encoding schistosome chorion or eggshell proteins from a Schistosoma mansoni genomic library. A linkage may of five of the clones spanning 35 kilobase pair (kbp) of the S. mansoni genome was constructed. The region contained two eggshell protein genes closely linked, separated by 7.5 kbp of intergenic DNA. The two genes of the cluster were arranged in the same orientation, that is, they were transcribed from the same strand. The sixth clone probably represents a third copy of the eggshell gene that is not contained within the 35-kbp region. The 5' end of the mRNA transcribed from these genes was defined by primer extension directly off the RNA. The ATCAT cap site sequence was homologous to a silkmoth chorion PuTCATT cap site sequence, where Pu indicates any purine. DNA sequence analysis showed that there were no introns in these genes. The DNA sequences of the three genes were very homologous to each other and to a cDNA clone, pSMf61-46, differing only in three or four nucleotides. A multiple TATA box was located at positions -23 to -31, and a CAAAT sequence was located at -52 upstream of the eggshell transcription unit. Comparison of sequences in regions further upstream with silkmoth and Drosophila sequences revealed several very short elements that were shared. One such element, TCACGT, recently shown to be an essential cis-regulatory element for silkmoth chorion gene promoter function, was found at a similar position in all three organisms.

Visualization of Drosophila melanogaster chorion genes undergoing amplification

We visualized by electron microscopy the preferential amplification of Drosophila chorion genes in late-stage follicle cells. Chromatin spreads revealed large clusters of actively transcribed genes of the appropriate size, spacing, and orientation for chorion genes that were expressed with the correct temporal specificity. Occasionally the active genes were observed within or contiguous with intact replicons and replication forks. In every case, our micrographs are consistent with the hypothesis that the central region of each chorion domain contains a replication origin(s) used during the amplification event. In one case, a small replication bubble was observed precisely at the site of the essential region of the X chromosome amplification control element. The micrographs also suggest that forks at either end of a replicon frequently progress very different distances, presumably due to different times in initiation or different rates of movement. It appears that all chorion genes (even those coding for minor proteins) are transcribed in a "fully on" condition, albeit for varied durations, and that if replication fork passage does inactivate a promoter, it does so very transiently. Furthermore, a DNA segment containing one active gene is likely to have an additional active gene(s). Surprisingly, during the time frame of expected maximum activity, approximately half of the chorion sequences appear transcriptionally inactive.

Persistence and replication of plasmid DNA microinjected into early embryos of Xenopus laevis

The persistence and replication of defined circular and linear plasmid DNA molecules microinjected into fertilized eggs of Xenopus laevis were analyzed. For all plasmids tested, a small fraction of microinjected circular molecules was replicated; however, the overall copy numbers of either free form I or form II molecules usually did not increase through blastulation. In contrast, extensive amplification of input DNA sequences was seen whenever the microinjected DNA was assembled into high molecular weight concatemers. Moreover, the appearance and subsequent replication of injected sequences in high molecular weight DNA were enhanced when linear (form III), rather than circular, molecules were microinjected. The injected form III DNA was rapidly converted into long linear concatemers. All possible orientations of monomeric molecules within the concatemers were observed although, on occasion, head-to-tail orientations were favored. Long linear concatemers were replicated very efficiently, irrespective of the sequence of the input DNA. Form I and form II DNA molecules were also formed in the embryo from microinjected form III DNA. A small fraction of these circular forms was replicated, although overall copy numbers did not increase significantly. Form III molecules that remained monomeric were not observed to be replicated at all within our limits of detection. In some batches of embryos, form I and form II DNA molecules were replicated to the extent that overall copy number increased. Even in these cases, however, the amplification of long linear concatemers of the input DNA sequences was more efficient.

Organization and expression of a second chromosome follicle cell gene cluster in Drosophila

Four genes expressed during the period of vitelline membrane formation are clustered within 8 kb of DNA in region 26A of the second chromosome. Temporal and quantitative difference in the profiles of accumulated RNA suggest that the genes are independently regulated although they are selectively expressed during the stages of vitelline membrane biosynthesis. In situ hybridization and S1 analyses of RNAs from fractionated eggchambers established that these genes are active only in the follicle cells. S1 mapping with in vitro synthesized RNA probes shows that three of the genes are tandemly oriented. All four appear to be intronless. In vitro translation products from hybrid-selected RNAs indicate that two of these genes code for major vitelline membrane proteins. Sequence analysis of these two genes support this conclusion. The cell- and stage-specific expression of the other two genes, encoding less abundant RNAs, suggests that they also play a role in early eggshell production.

Organization and expression of three genes from the silkmoth early chorion locus

Chorion genes in the silkmoth Bombyx mori are clustered in a small region of one chromosome. Genes are grouped within this locus according to their expression in either early, middle, or late stages of choriogenesis. The entire set of late genes and a large fraction of the middle genes have been cloned and extensively characterized. We report here the first molecular characterization of a set of early chorion genes clustered within a 22.5-kb region. Transcripts homologous to these genes appear in the very first choriogenic follicle but disappear with different kinetics. One of the three early genes, 5H4, is the first example of a chorion gene which is not a member of a multigene family, or divergently paired with another chorion gene. The other two early genes, (ErA.1 and ErB.1) are divergently paired with each other in the manner of the middle and late chorion genes. The common 5' region between ErA.1 and ErB.1 is significantly shorter and does not contain certain sequence elements shared by the highly conserved 5' regions of middle and late chorion gene pairs. This evidence taken together with the fact that 5H4 is unpaired indicates that the early genes may utilize a regulatory mechanism somewhat different from the middle and late gene families.

Characterization and sequence of follicle cell genes selectively expressed during vitelline membrane formation in Drosophila

To isolate genes involved in vitelline membrane production, an ovarian cDNA library was screened with eggchamber RNAs labeled in vivo. Two cDNA clones encoding RNAs that are selectively expressed in follicle cells during the period of vitelline membrane formation were isolated. Following isolation of homologous genomic clones from a Drosophila library, one gene was localized by in situ hybridization to chromosomal region 26A, and the other to 3C. Developmental Northern blots demonstrated that both genes produce 700-800 nucleotide transcripts that accumulate during the stages of vitelline membrane synthesis. In vitro translation products from hybrid selected RNAs and DNA sequence analysis both indicate that the 26A region gene encodes a major protein component of the vitelline membrane. The structural properties of the 3C region follicle cell gene seem more compatible with an intracellular function.

Amplification of a chorion gene cluster in Drosophila is subject to multiple cis-regulatory elements and to long-range position effects

We have used P-element transformation to study cis-acting elements involved in the control of amplification of the third chromosome chorion gene cluster (66D12-15) in Drosophila melanogaster. To reduce position effects large fragments (5.7 to 12 kb; kb = 10(3) bases) of chorion DNA and the 7.2 kb ry+ fragment were used to "buffer" these putative elements from sequences at the insertion site. Nevertheless, even the longest constructs were profoundly affected by the insertion sites and showed amplification levels ranging from undetectable to higher than in the endogenous locus. Any amplification was tissue and temporally correct and extended into the neighboring ry+ sequences. Analysis of amplification levels at various points along two constructs bearing the same 10 kb chorion insert in opposite orientations showed maximal levels occurring at one end of the chorion fragment, irrespective of whether that end was buffered at the middle of the transposon or exposed close to the insertion site. The maximally amplifying region encompasses the amplification control element (ACE), which has been shown to be necessary for amplification, in agreement with its putative role as a replication origin. We have additionally identified amplification-enhancing elements present elsewhere in the 10 kb chorion fragment, which are needed for attainment of high copy number. These elements, distinct from the ACE, have been only coarsely localized within two 2.25 to 2.3 kb regions. Some interesting sequence similarities between these two regions and the ACE element are pointed out.

Comparison of the consensus sequence flanking translational start sites in Drosophila and vertebrates

The previously presented consensus sequence for eukaryotic translation initiation sites by Kozak was derived substantially from vertebrate mRNA sequences. Drosophila nuclear genes exhibit a significantly different translation start consensus sequence. These differences probably do not represent mechanistic differences in translation initiation inasmuch as both taxa exhibit identical preferences and restrictions at the crucial -3 position. Using more conservative criteria for the assignment of consensus the following consensus sequences were derived: vertebrate--CANCAUG and Drosophila--CAAAACAUG.

Drosophila chorion gene amplification requires an upstream region regulating s18 transcription

A cluster of Drosophila melanogaster chorion genes at locus 66D on the third chromosome amplifies 60-fold in the ovarian follicle cells prior to the onset of gene expression. A 3.8-kilobase (kb) region of the gene cluster can induce tissue-specific amplification in transformants. Previous models postulated that amplification is activated in follicle cells by transcription of one of the two chorion genes (s15 and s18) located within the 3.8-kb essential region. In this study, we showed that neither s15 nor s18 chorion gene transcription was required for amplification. However, a 510-bp region upstream from s18 contained sequences essential for both amplification and s18 transcription. No other region within the 3.8-kb fragment was required for amplification. We propose that upstream transcription control elements rather than transcription per se are involved in controlling amplification during development.

["Artificial" chromosomes]

The structural elements required for chromosome replication, segregation, and stability are replication origins, centromeres, and telomeres. DNA sequences capable of organizing these three elements have been isolated from yeast chromosomal DNA by means of recombinant DNA techniques and yeast cell transformation. It is now possible to combine these sequences into "artificial" chromosomes for yeast cells to obtain more insight into chromosome structure and function. Evidence is presented that the construction of artificial chromosomes functional in higher eukaryotes will be possible in the near future.

Patterns of region-specific chorion gene expression in the silkmoth and identification of shared 5' flanking genomic elements

Time-dependent and cell-specific changes in concentrations of total RNA, of poly A+ RNA, and of specific mRNAs have been measured throughout silkmoth choriogenesis. Levels of total RNA and of poly A+ RNA are maintained throughout much of choriogenesis, but decrease at least fourfold during the very late period, in parallel with a decrease in overall protein synthesis. Very late period changes in total RNA and in poly A+ RNA are less pronounced in the aeropyle crown region, where a subset of chorion proteins is preferentially synthesized, than in the flat region. Maximal accumulation of the E1 and E2 chorion mRNAs occurs preferentially in the aeropyle crown region during the very late period. Uridine pulse-labeling studies suggest that E1 and E2 transcription is similarly aeropyle crown region-specific and immediately precedes the time of maximal E1 and E2 RNA accumulation and protein synthesis. Sequences from the 5' flanking regions of E1 and E2 genes have been compared. Several oligonucleotide sequences are present in both genes, and some are duplicated. These are potential cis-acting, regulatory elements.