Drosophila glue gene Sgs-3: sequences required for puffing and transcriptional regulation

Drosophila Glue: A Promising Model for Bioadhesion

The glue produced by Drosophila larvae to attach themselves to a substrate for several days and resist predation until the end of metamorphosis represents an attractive model to develop new adhesives for dry environments. The adhesive properties of this interesting material have been investigated recently, and it was found that it binds as well as strongly adhesive commercial tapes to various types of substrates. This glue hardens rapidly after excretion and is made of several proteins. In D. melanogaster, eight glue proteins have been identified: four are long glycosylated mucoproteins containing repeats rich in prolines, serines and threonines, and four others are shorter proteins rich in cysteines. This protein mix is produced by the salivary glands through a complex packaging process that is starting to be elucidated. Drosophila species have adapted to stick to various substrates in diverse environmental conditions and glue genes appear to evolve rapidly in terms of gene number, number of repeats and sequence of the repeat motifs. Interestingly, besides its adhesive properties, the glue may also have antimicrobial activities. We discuss future perspectives and avenues of research for the development of new bioadhesives mimicking Drosophila fly glue.

Fine infrastructure of released and solidified Drosophila larval salivary secretory glue using SEM

The Golgi-derived large secretory granules of Drosophila salivary glands (SGs) constitute the components of the salivary glue secretion (Sgs). The Sgs represents a highly special and unique extracellular composite glue matrix that has not yet been identified outside of Cyclorrhaphous Dipterans. For over half a century, the only major and unambiguously documented function of the larval salivary glands was to produce a large amount of mucinous glue-containing secretory granules that, when released during pupariation, serves to affix the freshly formed puparia to a substrate. Besides initial biochemical characterization of the Sgs proteins and cloning of their corresponding Sgs genes, very little is known about other properties and functions of the Sgs glue. We report here observations on the fine SEM-ultrastructure of the Sgs glue released into to the lumen of SGs, and after it has been expectorated and solidified into the external environment. Surprisingly, in contrast to long held expectations, it appears to be a highly structured bioadhesive mass with an internal spongious to trabecular infrastructure, reflecting the state of its hydratation. We also found that in addition to its cementing properties, it is highly efficient at glueing and trapping microorganisms, and thus may serve a potentially very important immune and defense role. High hydration capacity, the speed by which this glue can dry, uniqueness of its protein composition and spongious infrastructure can provide inspiration for development of potential biomimetics that can attach completely different or incompatible surfaces with high efficiency and strength.

Evolution of salivary glue genes in Drosophila species

Background

At the very end of the larval stage Drosophila expectorate a glue secreted by their salivary glands to attach themselves to a substrate while pupariating. The glue is a mixture of apparently unrelated proteins, some of which are highly glycosylated and possess internal repeats. Because species adhere to distinct substrates (i.e. leaves, wood, rotten fruits), glue genes are expected to evolve rapidly.

Results

We used available genome sequences and PCR-sequencing of regions of interest to investigate the glue genes in 20 Drosophila species. We discovered a new gene in addition to the seven glue genes annotated in D. melanogaster. We also identified a phase 1 intron at a conserved position present in five of the eight glue genes of D. melanogaster, suggesting a common origin for those glue genes. A slightly significant rate of gene turnover was inferred. Both the number of repeats and the repeat sequence were found to diverge rapidly, even between closely related species. We also detected high repeat number variation at the intrapopulation level in D. melanogaster.

Conclusion

Most conspicuous signs of accelerated evolution are found in the repeat regions of several glue genes.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1364-9) contains supplementary material, which is available to authorized users.

Steroid regulation of glue protein genes in Drosophila melanogaster

Transcript accumulation of the 3C glue protein gene Sgs-4 was induced in cultured salivary glands of Drosophila third instar larvae by supplementing the culture medium with 20-OH-ecdysone. The salivary glands were isolated from hormone-deficient larvae of the temperature-sensitive mutant l(1)su(f)ts67g, which were shifted from permissive (25 degrees C) to restrictive temperature (30 degrees C) at 60 h after oviposition. At the permissive temperature the glue protein are expressed during the latter half of the third instar. At the restrictive temperature there is no detectable or an extremely reduced accumulation of the 3C glue protein gene transcripts in these larvae. Induction of transcript accumulation was demonstrated by increased amounts of glue gene RNAs in the 20-OH-ecdysone supplemented salivary glands. Maximum accumulation was reached within 1 h after supplementation. The induction of accumulation was inhibited by a concentration of cycloheximide that repressed total protein synthesis, suggesting that 20-OH-ecdysone acts indirectly on the 3C glue gene by inducing synthesis of a protein(s) required transcript accumulation. We also show that there is a more rapid disappearance of 3C transcripts from salivary glands cultured in the presence of 20-OH-ecdysone than from glands cultured in its absence. This hormone-induced disappearance is, in contrast to the 68C transcripts, not inhibited by cycloheximide.

Glue secretion in the Drosophila salivary gland: a model for steroid-regulated exocytosis

Small hydrophobic hormones like steroids control many tissue-specific physiological responses in higher organisms. Hormone response is characterized by changes in gene expression, but the molecular details connecting target-gene transcription to the physiology of responding cells remain elusive. The salivary glands of Drosophila provide an ideal model system to investigate gaps in our knowledge, because exposure to the steroid 20-hydroxyecdysone (20E) leads to a robust regulated secretion of glue granules after a stereotypical pattern of puffs (activated 20E-regulated genes) forms on the polytene chromosomes. Here, we describe a convenient bioassay for glue secretion and use it to analyze mutants in components of the puffing hierarchy. We show that 20E mediates secretion through the EcR/USP receptor, and two early-gene products, the rbp(+) function of BR-C and the Ca2+ binding protein E63-1, are involved. Furthermore, we demonstrate that 20E treatment of salivary glands leads to Ca2+ elevations by a genomic mechanism and that elevated Ca2+ levels are required for ectopically produced E63-1 to drive secretion. The results presented establish a connection between 20E exposure and changes in Ca2+ levels that are mediated by Ca2+ effector proteins, and thus establish a mechanistic framework for future studies.

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.

The trithorax group gene moira encodes a brahma-associated putative chromatin-remodeling factor in Drosophila melanogaster

The genes of the trithorax group (trxG) in Drosophila melanogaster are required to maintain the pattern of homeotic gene expression that is established early in embryogenesis by the transient expression of the segmentation genes. The precise role of each of the diverse trxG members and the functional relationships among them are not well understood. Here, we report on the isolation of the trxG gene moira (mor) and its molecular characterization. mor encodes a fruit fly homolog of the human and yeast chromatin-remodeling factors BAF170, BAF155, and SWI3. mor is widely expressed throughout development, and its 170-kDa protein product is present in many embryonic tissues. In vitro, MOR can bind to itself and it interacts with Brahma (BRM), an SWI2-SNF2 homolog, with which it is associated in embryonic nuclear extracts. The leucine zipper motif of MOR is likely to participate in self-oligomerization; the equally conserved SANT domain, for which no function is known, may be required for optimal binding to BRM. MOR thus joins BRM and Snf5-related 1 (SNR1), two known Drosophila SWI-SNF subunits that act as positive regulators of the homeotic genes. These observations provide a molecular explanation for the phenotypic and genetic relationships among several of the trxG genes by suggesting that they encode evolutionarily conserved components of a chromatin-remodeling complex.

Sodium salicylate decreases intracellular ATP, induces both heat shock factor binding and chromosomal puffing, but does not induce hsp 70 gene transcription in Drosophila

Sodium salicylate has long been known to be an inducer of the heat shock puffs and presumably heat shock gene transcription in the polytene chromosomes of Drosophila salivary gland cells. Stress-induced transcription of the heat shock genes is mediated by the transcription factor known as Heat Shock Factor (HSF). In yeast, sodium salicylate has been reported to induce the DNA binding of HSF but not heat shock gene transcription itself, and similar findings have been reported in human cells. This apparent discrepancy in the induction of certain aspects of the heat shock response between these organisms prompted us to carefully reexamine the induction of the heat shock response in Drosophila salivary gland cells of third instar larvae and Drosophila tissue culture (SL2) cells. Sodium salicylate (3-30 mM) decreases intracellular ATP levels in SL2 cells and induces HSF binding activity in SL2 and salivary gland cells in a dose-dependent manner. Despite the induction of HSF binding and heat shock puffs in polytene chromosomes, we found no evidence for increased hsp 70 gene transcription suggesting that chromosomal puffing and gene transcription may be separable events. Salicylate did not induce the HSF hyperphosphorylation that is normally associated with HSF activation. Furthermore, salicylate (30 mM) prevented heat-induced hyperphosphorylation of HSF and hsp 70 gene transcription indicating that salicylate's inhibitory effect on hsp 70 transcription may be independent of its effect on HSF binding activity. We propose that the reduction in intracellular ATP caused by the addition of salicylate likely plays a role in the activation of HSF binding and the inhibition of both HSF hyperphosphorylation and hsp 70 gene transcription.

DNA puff C4 of Bradysia hygida (Diptera: Sciaridae) contains genes unequally amplified and differentially expressed during development

We report here the isolation and characterization of a 2.3 kb genomic EcoRI fragment that co-localizes in the DNA puff C4 of Bradysia hygida with a 4 kb EcoRI fragment previously characterized as containing part of a gene amplified and expressed in the salivary gland at the time when puff C4 expands. Verification of the relative amount of DNA complementary to these two genomic fragments shows that they are unequally amplified in the salivary gland. The fragment containing part of the gene expressed when puff C4 expands amplifies about eight times more than the 2.3 kb fragment. This 2.3 kb fragment also carries sequences complementary to RNA species present in the gland in a period when puff C4 has already receded. Based on these data we discuss the nature of the DNA puff and the possible way in which amplification is occurring at these sites.

Hormonal control of gene expression: interactions between two trans-acting regulators in Drosophila

The steroid hormone 20-hydroxyecdysone (20HE) and the Broad-Complex locus (BRC) are involved in regulating developmental changes in gene expression around the time of metamorphosis in Drosophila. We have investigated the regulatory interactions between 20HE, BRC, and a set of genes expressed in the fat body of third-instar Drosophila larvae. RNA levels for two hormone-inducible genes, Larval Serum Protein-2 and P1, accumulate to normal levels in BRC-mutant larvae. In contrast, RNA levels for the P6 gene were affected by mutations at BRC. On the basis of the results of experiments in which hormone concentrations were varied in BRC-mutant or wild-type larvae, we conclude that 20HE can both increase and decrease P6 RNA levels in the absence of BRC product(s). BRC appears to be a trans-acting modulator of the response of P6 to the hormone. We propose that BRC attenuates the repressive effect of the hormone, expanding the range of hormone concentrations that induce the gene, thus allowing P6 RNA to reach high levels during the third larval instar. The results are discussed in relation to other genes that are regulated by the same two trans-acting factors. A model is presented that refines the model of Ashburner et al. (1974, Cold Spring Harbor Symp. Quant. Biol. 38: 655-662) for the hormonal regulation of gene activity.

Cooperative enhancement at the Drosophila Sgs-3 locus

The Drosophila glue gene Sgs-3 is specifically expressed in the secretory cells of the salivary glands of third instar larvae. We have assayed the expression of gene fusions to determine the role of cis-acting Sgs-3 sequences in conferring this pattern of expression. These experiments define two regulatory regions required for expression of reporter genes from the Sgs-3 promoter. One region, between 106 and 56 bp upstream of the Sgs-3 mRNA 5' end is sufficient for low but correct tissue- and stage-specific expression. A second region, lying between 629 and 130 bp 5' of the RNA start site is functionally equivalent; that is, it alone will also direct low level, specific expression. These two regions act synergistically to give high level expression. More distant upstream regions function to further increase levels of expression. These two regulatory elements can confer a salivary gland-specific pattern of expression on a heterologous promoter and are also sufficient to drive gene expression in other Drosophila species, implying conservation of regulators.

Promoter is an important determinant of developmentally regulated puffing at the Sgs-4 locus of Drosophila melanogaster

Sgs-4 is one of the eight known genes coding for larval secretion proteins in Drosophila melanogaster. High-level transcription of the endogenous Sgs genes in salivary glands is accompanied by chromosome puffing at the Sgs gene loci. Naturally occurring mutations of the Sgs-4 promoter region diminish both the level of Sgs-4 expression and the puff size; in null-producers no puff is formed. P element-mediated transformation experiments were performed to clarify this apparent causal relation between transcription and puffing. Sgs-4 upstream sequences, unchanged or recombined with sequences from differently expressed alleles, were fused with Sgs-4 coding and downstream sequences or with the coding sequence of the viral oncogene v-mil. Analyses of the expression of these fragments at the RNA and protein levels and of their capacity for puff formation demonstrate uncoupling of transcription and puffing. That is, high-level transcription is independent of chromosome puffing and does not necessarily induce puffing, and developmentally regulated chromosome puffing is independent of significant transcriptional activity within the puff. Our results show that the strength of the Sgs-4 promoter located within the upstream region from -1 to -840 determines the formation of a puff. No specific effects could be detected on either transcription or puffing by decondensed versus compact chromatin adjoining the transposed DNA at the sites of insertion in transformants. A model in which trans-acting factors binding to the promoter region initiate puffing is proposed.

Regulatory elements and interactions in the Drosophila 68C glue gene cluster

We reviewed studies on the developmental regulation of the 68C glue gene cluster of Drosophila melanogaster. Extensive transformation analyses of Sgs-3 have shown that four regions necessary for normal expression can be distinguished. The first(+10 to -50) contains the transcription start site and TATA motif. This region can be replaced functionally by corresponding sequences from the hsp70 gene, but it is sensitive to point mutations in the TATA sequence. The second region (-50 to -98) contains more than one upstream sequence that, in combination with the other elements, leads to stage and tissue-specific expression. The third region (centered at -600) contains an element that enhances transcript levels some 20-fold. The final region (between -1.65 and -2.35 kb) contains elements having modest (twofold to threefold) effects on expression, one of which is contained in the coding sequences of Sgs-7, a second member of the cluster.

Vectors for Drosophila P-element-mediated transformation and tissue culture transfection

We describe nine P-element vectors that can be used to study gene regulation and function in Drosophila. These vectors were designed for use in germline transformation and cell culture transfection assays. One set consists of five P elements that can be used to study transcriptional regulatory sequences. These vectors contain several unique restriction sites for insertion of a foreign promoter upstream from either a cat or lacZ reporter gene. Two of the beta-galactosidase-coding vectors also require the insertion of a start codon for translation of the reporter enzyme and thus can be used to study translational regulatory sequences. The second set of P elements consists of four vectors that contain the Drosophila cytoplasmic actin 5C promoter and polyadenylation signals. Upon insertion of a foreign DNA segment, these vectors direct constitutive expression of the encoded RNA and protein.

Molecular cloning, genomic organization, expression and evolution of 12S seed storage protein genes of Arabidopsis thaliana

We have identified a number of genes of the flowering plant Arabidopsis thaliana that are abundantly expressed during embryogenesis. In this paper we discuss four of these genes, which comprise a gene family: complete genomic nucleotide sequence of two of the genes and partial sequence of the other two shows that they are all homologous to the 12S globulin seed storage protein genes of other angiosperms. The four genes fall into three subfamilies, as defined by cross-hybridization. One subfamily contains two genes in the Landsberg erecta strain, but only a single gene in the Columbia strain of Arabidopsis. The other two of these 12S gene subfamilies contain only single genes in both strains. Thus, the seed storage protein gene family in Arabidopsis appears much simpler than that in other higher plants.These genes are expressed during the latter half of embryogenesis, a period in which abscisic acid (ABA) is thought to play a role in gene regulation, and known to play a role in seed physiology. We observed no significant difference in the expression profiles of these four genes in ABA-deficient and ABA-insensitive mutants of Arabidopsis, except that the onset of detectable expression of all of the transcripts is slightly delayed in both types of mutants.

Evolution and expression of the Sgs-3 glue gene of Drosophila

A cluster of three glue genes is present at chromosomal site 68C in the Drosophila melanogaster genome. In this study, we have used a comparative approach to investigate both the regulation and the evolution of the largest of these three genes, Sgs-3. The homologous genes from two related Drosophila species (D. erecta and D. yakuba) have been introduced into the D. melanogaster genome by P-factor-mediated transformation. When the resulting transformant lines were assayed for expression of the introduced genes, near-normal patterns of expression were seen. This demonstrates that the cis-acting regulatory sequences of the introduced Sgs-3-homologous glue genes are capable of interacting effectively with the transcriptional machinery of D. melanogaster. We have also determined the sequences of the Sgs-3-homologous glue genes from D. simulans, D. erecta and D. yakuba. These sequences were compared and used in two ways. The first was to locate conserved sequence elements in regions known to be involved in regulation of the gene. Several such elements were found; they represent potential sites of cis-acting regulatory sequences. Second, we looked at the evolution of the glue gene protein-coding regions. A very rapidly evolving central region of the protein-coding sequences was found; this region contains a striking series of tandem repeats of a five amino acid residue sequence in all four species. Also a number of conserved aspects of the Sgs-3-homologous proteins were found; these features may be essential to their function as a glue.

26-[125I]iodoponasterone A is a potent ecdysone and a sensitive radioligand for ecdysone receptors

The effects of ecdysone, the steroid molting hormone of arthropods, are of considerable interest both to insect physiologists and to those studying steroid-regulated gene expression. Yet progress in understanding ecdysone receptors has been inhibited by the lack of a suitable highly radioactive hormone analog with high affinity for the receptor. Here we report that the synthetic ecdysteroid 26-iodoponasterone A is one of the most active ecdysones known, inducing half-maximal morphological transformation in Drosophila Kc167 cells when present at 0.5 nM. 26-[125I]Iodoponasterone A can be prepared at a specific activity of 2175 Ci/mmol (1 Ci = 37 GBq) by reaction of the precursor 26-mesylinokosterone with carrier-free Na125I. The radiolabeled material binds to Kc167 cell ecdysone receptors specifically and with affinity (Kd ca. 3.8 X 10(-10) M). Thus, 26-[125I]iodoponasterone A appears to be a superior radioligand for ecdysone receptors on grounds both of affinity and of specific activity. Its ready availability should greatly facilitate studies of these receptors.

Transcription from each of the Drosophila act5C leader exons is driven by a separate functional promoter

The Drosophila act5C gene has two leader exons at which transcription initiation occurs. In this way two classes of transcripts that are different with respect to the 5'-untranslated sequences are synthesized. Both are present in Drosophila Kc cell mRNA. To define the sequences necessary for transcription from each start point and to determine if each is driven by a separate promoter, 5'-flanking regions from the act5C gene were inserted upstream from the bacterial chloramphenicol acetyltransferase gene and tested for promoter activity by transient assays in Drosophila Kc cells. We show that both leader exons are preceded by separate, functional, promoters. The exon 1 proximal promoter contains at least two regions important for optimal expression. One is at more than 1.9 kb upstream from the exon 1 cap site while the other lies between 1.2 and 0.09 kb of the cap site. The promoter elements necessary for transcription from exon 2 are within 450 bp upstream from its cap site. The data suggest that, in some constructions, transcription initiation at exon 1 inhibits transcription initiation at exon 2. There is a sequence of dyad symmetry which is present upstream from both exon 1 and exon 2 of the form CC(A-rich)6GG. The same sequences are found upstream from many mammalian and chicken actin genes and of the human and mouse c-fos genes, where they are believed to be transcription regulatory sequences.

Regulatory sequences of the Sgs-4 gene of Drosophila melanogaster analysed by P element-mediated transformation

The X chromosomally located allele Sgs-4c for a larval secretion protein of Drosophila melanogaster is normally expressed in female larvae of the strain Oregon R and is hyperexpressed in male larvae exhibiting dosage compensation; the allele Sgs-4d in the strain Samarkand is weakly expressed and is not hyperexpressed in male larvae showing a dosage effect. P element-mediated transformation of upstream DNA sequences from both alleles combined with Sgs-4d coding and downstream sequences was performed to localize sequences which are responsible for the level of gene expression and for hyperexpression of Sgs-4c in male larvae. Our results demonstrate that weak expression and dosage effect are inherited with the upstream region from -1 to -838. This Samarkand fragment differs from the homologous Oregon R region only by a C to T transition at -344 which lies within an assumed binding sequence for the ecdysone receptor complex of dyad base symmetry. Replacing the Samarkand upstream region from -1 to -838 by the Oregon R region restores normal Sgs-4 expression and dosage compensation. Hyperexpression in male larvae displays high sensitivity to position effect and is nearly completely inhibited in one transformed line under heterozygous conditions. The integration of an Sgs-4d transposon into a weak spot of polytene chromosome 2L results in a decrease in gene expression. The GTT- and GT-rich regions at -1.2 and -2.0 kb do not obviously influence Sgs-4 expression but possibly play a role in induction of stage-specific chromosome puffing.