Larval salivary gland secretion proteins in Drosophila. Identification and characterization of the Sgs-5 structural gene

Gross morphology and adhesion-associated physical properties of Drosophila larval salivary gland glue secretion

One of the major functions of the larval salivary glands (SGs) of many Drosophila species is to produce a massive secretion during puparium formation. This so-called proteinaceous glue is exocytosed into the centrally located lumen, and subsequently expectorated, serving as an adhesive to attach the puparial case to a solid substrate during metamorphosis. Although this was first described almost 70 years ago, a detailed description of the morphology and mechanical properties of the glue is largely missing. Its main known physical property is that it is released as a watery liquid that quickly hardens into a solid cement. Here, we provide a detailed morphological and topological analysis of the solidified glue. We demonstrated that it forms a distinctive enamel-like plaque that is composed of a central fingerprint surrounded by a cascade of laterally layered terraces. The solidifying glue rapidly produces crystals of KCl on these alluvial-like terraces. Since the properties of the glue affect the adhesion of the puparium to its substrate, and so can influence the success of metamorphosis, we evaluated over 80 different materials for their ability to adhere to the glue to determine which properties favor strong adhesion. We found that the alkaline Sgs-glue adheres strongly to wettable and positively charged surfaces but not to neutral or negatively charged and hydrophobic surfaces. Puparia formed on unfavored materials can be removed easily without leaving fingerprints or cascading terraces. For successful adhesion of the Sgs-glue, the material surface must display a specific type of triboelectric charge. Interestingly, the expectorated glue can move upwards against gravity on the surface of freshly formed puparia via specific, unique and novel anatomical structures present in the puparial's lateral abdominal segments that we have named bidentia.

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.

Drosophila ELL is associated with actively elongating RNA polymerase II on transcriptionally active sites in vivo

Several factors have been biochemically characterized based on their ability to increase the overall rate of transcription elongation catalyzed by the multiprotein complex RNA polymerase II (Pol II). Among these, the ELL family of elongation factors has been shown to increase the catalytic rate of transcription elongation in vitro by suppressing transient pausing. Several fundamental biological aspects of this class of elongation factors are not known. We have cloned the Drosophila homolog (dELL) in order to test whether ELL family proteins are actually associated with the elongating Pol II in vivo. Here we report that dELL is a nuclear protein, which, like its mammalian homologs, can increase the catalytic rate of transcription elongation by Pol II in vitro. Interestingly, we find that dELL co-localizes extensively with the phosphorylated, actively elongating form of Pol II at transcriptionally active sites on Drosophila polytene chromosomes. Furthermore, dELL is relocalized from a widespread distribution pattern on polytenes under normal conditions to very few transcriptionally active puff sites upon heat shock. This observation indicates a dynamic pattern of localization of dELL in cells, which is a predicted characteristic of a Pol II general elongation factor. We also demonstrate that dELL physically interacts with Pol II. Our results strongly suggest that dELL functions with elongating RNA polymerase II in vivo.

P-TEFb kinase recruitment and function at heat shock loci

P-TEFb, a heterodimer of the kinase Cdk9 and cyclin T, was isolated as a factor that stimulates formation of productive transcription elongation complexes in vitro. Here, we show that P-TEFb is located at >200 distinct sites on Drosophila polytene chromosomes. Upon heat shock, P-TEFb, like the regulatory factor HSF, is rapidly recruited to heat shock loci, and this recruitment is blocked in an HSF mutant. Yet, HSF binding to DNA is not sufficient to recruit P-TEFb in vivo, and HSF and P-TEFb immunostainings within a heat shock locus are not coincident. Insight to the function of P-TEFb is offered by experiments showing that the direct recruitment of a Gal4-binding domain P-TEFb hybrid to an hsp70 promoter in Drosophilacells is sufficient to activate transcription in the absence of heat shock. Analyses of point mutants show this P-TEFb stimulation is dependent on Cdk9 kinase activity and on Cdk9's interaction with cyclin T. These results, coupled with the frequent colocalization of P-TEFb and the hypophosphorylated form of RNA polymerase II (Pol II) found at promoter-pause sites, support a model in which P-TEFb acts to stimulate promoter-paused Pol II to enter into productive elongation.

Microarray analysis of Drosophila development during metamorphosis

Metamorphosis is an integrated set of developmental processes controlled by a transcriptional hierarchy that coordinates the action of hundreds of genes. In order to identify and analyze the expression of these genes, high-density DNA microarrays containing several thousand Drosophila melanogaster gene sequences were constructed. Many differentially expressed genes can be assigned to developmental pathways known to be active during metamorphosis, whereas others can be assigned to pathways not previously associated with metamorphosis. Additionally, many genes of unknown function were identified that may be involved in the control and execution of metamorphosis. The utility of this genome-based approach is demonstrated for studying a set of complex biological processes in a multicellular organism.

crooked legs encodes a family of zinc finger proteins required for leg morphogenesis and ecdysone-regulated gene expression during Drosophila metamorphosis

Drosophila imaginal discs undergo extensive pattern formation during larval development, resulting in each cell acquiring a specific adult fate. The final manifestation of this pattern into adult structures is dependent on pulses of the steroid hormone ecdysone during metamorphosis, which trigger disc eversion, elongation and differentiation. We have defined genetic criteria that allow us to screen for ecdysone-inducible regulatory genes that are required for this transformation from patterned disc to adult structure. We describe here the first genetic locus isolated using these criteria: crooked legs (crol). crol mutants die during pupal development with defects in adult head eversion and leg morphogenesis. The crol gene is induced by ecdysone during the onset of metamorphosis and encodes at least three protein isoforms that contain 12–18 C2H2 zinc fingers. Consistent with this sequence motif, crol mutations have stage-specific effects on ecdysone-regulated gene expression. The EcR ecdysone receptor, and the BR-C, E74 and E75 early regulatory genes, are submaximally induced in crol mutants in response to the prepupal ecdysone pulse. These changes in gene activity are consistent with the crol lethal phenotypes and provide a basis for understanding the molecular mechanisms of crol action. The genetic criteria described here provide a new direction for identifying regulators of adult tissue development during insect metamorphosis.

Interstrain variability of larval photokinesis in Drosophila melanogaster

Larvae from seven laboratory strains and eight isofemale lines of Drosophila melanogaster differ significantly with regard to their responses to light in a photokinesis assay in which the larvae are tested en masse. Larvae from the CA-2 laboratory stock fail to disperse on assay plates, although observations of individual CA-2 larvae suggest that the larvae are repelled by light. Larvae from all of the other laboratory stocks and all of the isofemale lines (except LI2 and NC5) avoid light in the photokinesis assay. Larvae from some stocks are much more strongly repelled by light than larvae from other stocks. LI2 larvae are unresponsive to light in most replicates of the photokinesis assay, while NC5 larvae are consistently unresponsive to light. Observations of F1 heterozygotes suggest that the allele(s) that affects the vision of LI2 and NC5 larvae has net effects on the animals' behavior that are partially dominant and recessive, respectively.

A new gene nested within the dunce genetic unit of Drosophila melanogaster

The molecular organization of the dunce gene of Drosophila melanogaster has proved to be particularly complex, with two divergently transcribed genes, Sgs-4 and Pig-1, nested within its 79 kb intron (1). Here we report the identification and the molecular characterization of a third gene nested within the transcription unit of dunce. This newly identified gene is located nearly 6 kb downstream Pig-1, within a more upstream dunce intron. The gene is developmentally regulated and transcribed with the same polarity of dunce; several lines of evidence indicate that it might encode for a salivary gland secreted (Sgs) protein.

Noncoordinate expression of Drosophila glue genes: Sgs-4 is expressed at many stages and in two different tissues

The glue genes of Drosophila melanogaster comprise a family of genes expressed at high levels in the salivary glands of late third instar larvae in response to the insect hormone ecdysone. We present evidence that, in contrast to the other glue genes, Sgs-4 is turned on throughout Drosophila development and is not expressed exclusively in the larval salivary glands. Larvae transformed with an Sgs-4/Adh (alcohol dehydrogenase) hybrid gene exhibit Sgs-4-directed Adh expression in the larval proventriculus as well as in the salivary glands as early as the first instar. Sgs-4-specific RNA can be detected at very low levels during all stages of development. During late third instar, levels of Sgs-4 RNA in the salivary glands increase several-thousand-fold, thereby accounting for the large amounts of Sgs-4 protein present in the glue produced by the salivary glands. This pattern of expression is unique to the Sgs-4 gene. While expression of several of the other glue genes can be detected in embryos and early larvae, they appear to be expressed neither throughout development nor in the larval proventriculus. Appearance of the glue gene RNAs in mid third instar salivary glands is noncoordinate, even for the chromosomally clustered genes Sgs-3, Sgs-7, and Sgs-8.

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.

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.

Closely linked DNA elements control the expression of the Sgs-5 glue protein gene in Drosophila

cis-acting sequence regions involved in the regulation of Sgs-5 gene expression were mapped by testing DNA segments containing the Sgs-5 RNA coding region and various amounts of adjacent sequences for the ability to express Sgs-5 RNA. Following injection of the DNA segments into Drosophila embryos, expression of the gene was assayed in the salivary glands of the injected animals after they developed to third instar larvae, these somatically transformed individuals serving as an in vivo transient expression system. The information necessary for the expression of Sgs-5 is contained within 109 bp upstream and 69 bp downstream of the transcribed region. Somatic transformation experiments also show that some feature within the limits of a 1012-bp DNA segment containing the Sgs-5 RNA coding region derived from the Sgs-5 RNA null stock CA-2 must be responsible for the lack of transcription from this allele. The only DNA sequence differences between active and null alleles, within the 1012 bp, are seven single-base-pair substitutions between -84 bp and +175 bp relative to the RNA start site. One or a combination of these sites are likely contributors to the transcriptional inactivity of the Sgs-5CA2 allele.

Genetic localization of a regulatory site necessary for the production of the glue protein P5 in Drosophila melanogaster

The glue proteins are products of a developmentally regulated gene family. These genes are transcriptionally active during the third larval instar and code for the major protein products of salivary glands. The activity of several of the genes can be visualized as intermoult puffs in the polytene salivary gland chromosomes. The amount of one of these proteins, P5, varies widely among wild-type strains. We have used biochemical and genetic methods to investigate the source of this variation. The results ofin vitrotranslation of salivary gland RNA suggest that the variation occurs pretranslationally. Genetic mapping experiments showed that sites on several chromosomes can modulate the amount of P5, but that one site on the third chromosome determines the absence and presence of this protein. We have mapped this glue protein gene, calledGP5, to the interval betweenbx(3–58·8) and sr (3–62·0) which also includes the intermoult puff at 90BC. We discuss the relationship between P5 and the glue protein geneSgs-5which is also located at 90BC.

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

The 68C intermolt puff of Drosophila melanogaster contains a cluster of three glue protein genes, Sgs-3, Sgs-7, and Sgs-8. By analysis of chromosomal rearrangements which break near the glue gene cluster, we have established that a region of no more than 20 kb is required for normal expression of the glue genes and for formation of the 68C puff. Using P element-mediated transformation, we have introduced defined segments of the 68C region into the fly genome and assayed the expression of the Sgs-3 gene. Based on the criteria of correct tissue- and stage-specific expression, transcription of an RNA of appropriate size and abundance, and production of an sgs-3 protein, the correctly regulated expression of the Sgs-3 gene requires less than 3.4 kb of total flanking sequences, approximately 2.3 kb 5' and 1.1 kb 3'. Formation of a new intermolt puff at the site of insertion is not observed for all transformants which produce high levels of Sgs-3 RNA. Only transformants in which the introduced DNA from 68C also contains the Sgs-7 and Sgs-8 genes cause a new intermolt puff at the chromosomal location of the insert.

Larval salivary gland secretion proteins in Drosophila structural analysis of the Sgs-5 gene

The structure of the Drosophila melanogaster salivary gland secretion gene Sgs-5 has been determined by DNA sequence analysis of cloned genomic DNA. This developmentally and tissue-specific gene is a member of the third instar intermolt gene set and is under control of the insect molting hormone ecdysterone. RNA protection experiments show that the RNA coding region of Sgs-5 contains 769 nucleotides and is divided into three exons by two small introns. The protein-coding region appears to begin after a short untranslated RNA leader (33 nucleotides) and to result in a protein of 163 amino acids. The first 18 amino acids give the amino-terminal end the highly hydrophobic nature characteristic of a signal peptide.

An ecdysterone-responsive puff site in Drosophila contains a cluster of seven differentially regulated genes

We have determined the molecular organization of an ecdysterone-responsive puff site in Drosophila melanogaster. The 71E puff site contains a tightly linked cluster of at least seven genes within a neighborhood of 10 X 10(3) base-pairs. All the genes are expressed in a tissue-specific manner in either the larval or the prepupal salivary gland. However, these genes can be divided into two groups on the basis of their temporal pattern of transcription. Six of the genes are expressed only in prepupal salivary glands and are arranged as three divergently transcribed pairs. Nestled within this region is one gene expressed primarily in late third-instar salivary glands. We conclude that this developmentally complex puff site contains six members of the ecdysterone-induced "late"-gene set and one member of the ecdysterone-regulated "intermolt" -gene set. Additional complexity is found at the transcript level: a heterogeneously sized population of RNA molecules arises from each of the seven genes.