Large sized nascent protein as dominating component during protein synthesis in Chironomus salivary glands

Beyond DNA puffs: What can we learn from studying sciarids?

Members of the Sciaridae family attracted the interest of researchers because of the demonstration that the DNA puff regions, which are formed in the salivary gland polytene chromosomes at the end of the fourth larval instar, constitute sites of developmentally regulated gene amplification. Besides contributing to a deeper understanding of the process of gene amplification, the study of sciarids has also provided important insights on other biological processes such as sex determination, programmed cell death, insect immunity, telomere maintenance, and nucleolar organizing regions (NOR) formation. Open questions in sciarids include among others, early development, the role of noncoding RNAs in gene amplification and the relationship between gene amplification and transcription in DNA puff forming regions. These and other questions can now be pursued with next generation sequencing techniques and experiments using RNAi experiments, since this latter technique has been shown to be feasible in sciarids. These new perspectives in the field of sciarid biology open the opportunity to consolidate sciarid species as important emerging models. genesis 54:361-378, 2016. © 2016 Wiley Periodicals, Inc.

The ribosome biogenesis pathway as an early target of benzyl butyl phthalate (BBP) toxicity in Chironomus riparius larvae

Butyl benzyl phthalate (BBP) is a ubiquitous contaminant whose presence in the environment is expected for decades, since it has been extensively used worldwide as a plasticizer in the polyvinyl chloride (PVC) industry and the manufacturing of many other products. In the present study, the interaction of BBP with the ribosome biogenesis pathway and the general transcriptional profile of Chironomus riparius aquatic larvae were investigated by means of changes in the rDNA activity (through the study of the internal transcribed spacer 2, ITS2) and variations in the expression profile of ribosomal protein genes (rpL4, rpL11, and rpL13) after acute 24-h and 48-h exposures to a wide range of BBP doses. Furthermore, cytogenetic assays were conducted to evaluate the transcriptional activity of polytene chromosomes from salivary gland cells, with special attention to the nucleolus and the Balbiani rings (BRs) of chromosome IV. BBP caused a dose and time-dependent toxicity in most of the selected biomarkers, with a general depletion in the gene expression levels and the activity of BR2 after 48-h treatments. At the same time, decondensation and activation of some centromeres took place, while the activity of nucleolus remained unaltered. Withdrawal of the xenobiotic allowed the larvae to reach control levels in the case of rpL4 and rpL13 genes, which were previously slightly downregulated in 24-h tests. These data provide the first evidence on the interaction of BBP with the ribosome synthesis pathways, which results in a significant impairment of the functional activity of ribosomal protein genes. Thus, the depletion of ribosomes would be a long-term effect of BBP-induced cellular damage. These findings may have important implications for understanding the adverse biological effects of BBP in C. riparius, since they provide new sensitive biomarkers of BBP exposure and highlight the suitability of this organism for ecotoxicological risk assessment, especially in aquatic ecosystems.

Amplification and expression of a salivary gland DNA puff gene in the prothoracic gland of Bradysia hygida (Diptera: Sciaridae)

The DNA puff BhC4-1 gene, located in DNA puff C4 of Bradysiahygida, is amplified and expressed in the salivary gland at the end of the fourth larval instar as a late response to the increase in 20-hydroxyecdysone titer that triggers metamorphosis. Functional studies revealed that the mechanisms that regulate BhC4-1 expression in the salivary gland are conserved in transgenic Drosophila. These studies also led to the identification of a cis-regulatory module that drives developmentally regulated expression of BhC4-1-lacZ in the prothoracic gland cells of the ring gland, a compound organ which in Drosophila results from the fusion of the prothoracic glands, the corpus allatum and the corpus cardiacum. Here we have investigated the occurrence of BhC4-1 expression in B. hygida prothoracic glands. We report the identification of the B. hygida prothoracic gland and demonstrate that it releases ecdysone. Using RT-qPCR, western blots and immunolocalization experiments, we demonstrate that the BhC4-1 mRNA and the BhC4-1 protein are both expressed in the B. hygida prothoracic glands at the same time that DNA puff C4 is formed in the salivary gland. We also show that BhC4-1 is concomitantly amplified 4.8-fold in the prothoracic gland and 23-fold in the salivary gland. Our results reveal the occurrence of stage specific expression of a DNA puff gene in the prothoracic glands of B. hygida, and extend previous studies that have shown that DNA puff genes expression is not restricted to the salivary gland. In addition, the description of stage specific gene amplification in the prothoracic glands of B. hygida constitutes the first demonstration that gene amplification in Diptera might occur concomitantly in two different tissues in the same developmental stage.

Differential expression of ribosomal protein gene, gonadotrophin releasing hormone gene and Balbiani ring protein gene in silver nanoparticles exposed Chironomus riparius

The eco- and genotoxicity of silver nanoparticles (AgNPs) was investigated in the fourth instar larvae of the aquatic midge, Chironomus riparius. AgNPs did not have acute toxicity in C. riparius, but did exhibited chronic toxicity on development (pupation and emergence failure) and reproduction. Genotoxicity also occurred in AgNPs exposed C. riparius. Differential Display PCR (DD-PCR), based on the Annealing Control Primer (ACP) technique, was conducted to investigate the underlying toxic mechanism, which identified altered gene expression in C. riparius after treatment with AgNPs. The possible toxicity mechanism of AgNPs in C. riparius involves the down regulation of the ribosomal protein gene (CrL15) affecting the ribosomal assembly and consequently, protein synthesis. Up regulation of the gonadotrophin releasing hormone gene (CrGnRH1) might lead to the activation of gonadotrophin releasing hormone mediated signal transduction pathways and reproductive failure. Up regulation of the Balbiani ring protein gene (CrBR2.2) may be an indication of the organism's protection mechanism against the AgNPs. The overall results suggest that the toxicity of AgNPs towards aquatic organisms should be thoroughly investigated to allow for their safe use, as they seem to exhibit important toxicity towards C. riparius.

Balbiani ring induction in phosphate metabolism

Balbiani rings (BR), giant puffs in Chironomus larval salivary glands, code for giant secretory proteins. As shown earlier, the normally dominant BR2 is turned off with its putative translation product during exposure of larvae to compounds that diminish the stores of P(i). A BR6 develops from a compact chromosome band, and a new giant protein appears in the secretion as the major component. We have determined the sequence of cloned DNA fragments representative for large parts of BR1 and BR2 (normally active) and the inducible BR6. There is an excess of positive charges and high contents of serine/threonine in the coded amino acid composition for the BR1 and BR2 sequences. The coded amino acid sequence for the BR6 clone shares homologies with the others but has an excess of negative charges and lacks serine/threonine. This suggested that the P(i) effects observed earlier could be related to differences in phosphorylation between the normal proteins and the BR6 product. This could be confirmed by measurements of phosphorylation, which occurs in the normal giant proteins mainly at seryl residues. P export with giant secretory protein is normally quantitatively important. Thus, BR6 activation should decrease P loss when P(i) pools are lowered because of inducer action.

Different evolutionary behavior of structurally related, repetitive sequences occurring in the same Balbiani ring gene in Chironomus tentans

The Balbiani ring 2 (BR 2) gene in Chironomus tentans is highly internally repeated. Two types of related repeat units-the alpha and beta types-are tandemly arranged in separate blocks, which together are likely to form the major part of the gene. Every repeat unit has one constant region and one subrepeat region. Here we analyze the length and sequence of a number of repeat units of both types and compare the units within and between the blocks. The approximately 100 alpha repeat units are essentially invariant regarding length and sequence. In contrast, when the approximately 70 beta repeat units are compared, six length variants are found, four of which have been sequenced. The length variations reside in the subrepeat regions and are due to different numbers of whole or half subrepeats. Furthermore, the subrepeat regions differ by several base-pair substitutions, many of which change the amino acid sequence. On the other hand, all beta-type constant regions are of equal length and are virtually homogeneous in sequence. The observed length distributions in combination with analysis of the basepair substitutions in the alpha-and beta-type constant and subrepeat regions suggest that the alpha and beta blocks are of different age, that seemingly homologous repeated regions may play different functional roles at the protein level, and that sequence correction mechanisms are likely to operate to different extents on the constant and subrepeat regions within the beta block.

Transcription of heat shock gene loci versus non-heat shock loci in Chironomus polytene chromosomes: evidence for heat-induced formation of novel putative ribonucleoprotein particles (hsRNPs) in the major heat shock puffs

The heat shock response of Chironomus polytene chromosomes was reexamined. The in vivo effects of heat shock on chromosomal [3H]uridine labeling, RNA polymerase II distribution and ribonucleoprotein (RNP) formation were investigated. One primary result is a clarification of the number and location of chromosomal sites strongly induced by treatment at 37 degrees C for 60 min. In total, seven major heat shock loci were identified by transcription autoradiography in Chironomus tentans: I-20A, II-16B, II-10C, II-4B, II-1C, III-12B, and IV-5C. Secondly, combining immunofluorescence with transcription autoradiography, I find RNA polymerase II occurring after heat shock at multiple chromosomal sites that were also active under normal conditions (20 degrees C). Furthermore, the results demonstrate conclusively that the presence of RNA polymerase II at heat shock and non-heat shock loci is generally correlated with [3H]uridine labeling during heat shock. These latter results extend and corroborate previous findings. Thirdly, the most striking result of this study was revealed in ultrathin sections of puffs by electron microscopy: I discerned a site-specific ultrastructural difference in putative RNP particles between heat shock versus non-heat shock loci. At least three of the seven induced major heat shock puffs (I-20A, III-12B, IV-5C) were observed to contain globular particles that were different, i.e. significantly larger, 250-1,000 A in diameter with a prominent 500-750 A class, than RNP particles of other loci under non-heat shock conditions. These large heat shock puff particles presumably represent nascent or newly synthesized heat shock RNA associated with protein(s) to form heat shock RNPs (hsRNPs). This finding suggests the possible involvement of novel RNPs (hsRNPs) in transcriptional regulation or heat shock RNA turnover and may stimulate further molecular investigations on this subject in both cell physiological and structural terms. I conclude that the locus-specific putative hsRNPs are an intrinsic property of greatly increased heat shock gene transcription.

Demonstration of a 7-nm RNP fiber as the basic structural element in a premessenger RNP particle

Balbiani ring granules in Chironomus salivary glands represent premessenger ribonucleoprotein (RNP) particles, each containing a 35- to 40-kb message for a secretory polypeptide. Their gross structure can be described as an RNP ribbon bent into a toroid. We now demonstrate that an unfolded, thin RNP fiber is observed after low salt treatment of isolated Balbiani ring granules. Moreover, the thin RNP fiber, 7 nm in diameter, can be revealed as the main structural element in Balbiani ring granules studied in situ in 3-D with electron microscope tomography. It is proposed that the thin RNP fiber consists of a premessenger RNA molecule coiled around a filamentous core of polymeric proteins, which has functional implications for processes such as assembly of RNP, intranuclear degradation of RNA, and delivery of RNA through the nuclear pores.

Secretory proteins of Chironomus salivary glands: structural motifs and assembly characteristics of a novel biopolymer

Salivary glands of Chironomus synthesize a family of at least ten secretory proteins that can be grouped into three size classes: the large (about 1000 kDa), intermediate (100- to 200 kDa), and small (less than 100 kDa). After synthesis, secretory proteins undergo a dramatic transformation to form a novel biopolymer. Secretory proteins accumulate in the central lumen of the gland, forming dissociable complexes that appear as a network of smooth fibrils and multistranded beaded fibers. When secretory protein complexes are extruded through the secretory duct, the fibers become oriented in parallel arrays; when these parallel arrays of fibers emerge from the mouth of larvae they are an insoluble, silk-like thread. Regulation of secretory protein-coding gene expression determines which secretory proteins are synthesized, thus, the composition of silk threads. At least two types of threads are produced: larval silk is used to construct tubes for protective housing and assist with feeding; prepupal silk is used to construct tubes for larval/pupal ecdysis (pupation). Variations in composition presumably contribute to different mechanical properties of larval and prepupal silk threads. Since the macroscopic physical properties of polymerized silk most likely reflect the microscopic structure and interaction of secretory proteins, it becomes important to learn the principles which govern secretory protein assembly at the molecular level. Which secretory proteins interact and what are the sites used for intraportein and protein-protein interactions during the assembly of this biopolymer? All eight secretory proteins characterized thus far contain tandemly repeated peptide sequences (ranging from 14-90 amino acids in length) and/or a periodic distribution of Cys residues. These motifs appear to be unique; no other biopolymer has either the repeated peptide sequences or composite structure of chironomid silk threads. The evolutionary conservation of motifs within repeats and among different secretory proteins suggests that the sequences and three-dimensional structures of the motifs may be important for assembly of secretory proteins into complexes, oriented fibers, and silk threads. Further study of secretory protein assembly will bring us closer to understanding how this silk assembles in vivo. By learning principles that nature employs to construct such a novel composite biopolymer, it may become feasible to design and produce new classes of fibers or biomolecular materials with distinctive properties that are currently unavailable.

Heat-shock induction and cytoplasmic localization of transcripts from telomeric-associated sequences in Chironomus thummi

Transcription of telomeric-associated sequences has been detected in the salivary gland cells of the larvae Chironomus thummi. In this species, a heat shock induces puffing at some telomeres, especially at one of the telomeres of chromosome III. We found that this process was concomitant with an increase in the overall telomeric transcript levels. Transcription was also observed in all the telomeres under control conditions, by in situ hybridization, even when these telomeres appeared to be in a nonpuffed state. The telomeric transcripts were found in both, the nuclei and, at higher levels, in the cytoplasmic extracts of salivary gland cells. The heat-shock activation, however, appeared to be restricted to the nuclear level. Telomeric transcription and the peculiar behavior of C. thummi telomeres after a heat shock are discussed.

The complexity of 75S premessenger RNA in balbiani ring granules studied by a new RNA band retardation assay

Under normal growth conditions, Balbiani ring granules constitute premessenger ribonucleoprotein (RNP) particles synthesized in two chromosomal puffs, Balbiani ring (BR) 1 and 2, in the larval salivary glands of Chironomus tentans. At least three genes encoding 75S RNA are present in these two BRs: one in BR1 and two in BR2 (BR2.1 and BR2.2). The complexity of BR granule 75S RNA was studied by agarose gel electrophoresis under non-denaturing conditions. We recorded three main bands, designated I, II and III. Experiments with denaturing gels demonstrated that the differences in migration reflected mainly, but not exclusively, conformational differences. Northern blotting experiments showed that band I contained BR1 sequences, band II contained BR2.1 sequences, and band III contained BR2.2 sequences. To study whether additional genes contributed to the BR granule 75S RNA, an RNA band shift assay was developed. When an oligodeoxyribonucleotide complementary to repetitive BR1 and BR2.2 sequences was hybridized to 75S RNA prior to electrophoresis, bands I and III were retarded but not band II. An oligonucleotide complementary to a repetitive BR2.1 sequence only shifted band II. Since no detectable 75S RNA remained unchanged in these experiments, and all bands were identified by Northern blotting, all the BR granules are likely to originate from the BR1, BR2.1 and BR2.2 genes; no additional genes have to be invoked. Possible applications of the new RNA band shift assay are discussed.

Higher order structure of Balbiani ring premessenger RNP particles depends on certain RNase A sensitive sites

Specific premessenger ribonucleoprotein (RNP) particles, the Balbiani ring (BR) granules from Chironomus tentans salivary glands, were treated with RNase A to study the effect of RNA strand breaks on the higher order structure of the particles. Isolated, radioactively labeled BR granules, known to sediment at 300 S, were digested with RNase A and centrifuged in sucrose gradients. The fractionated particles were subsequently analyzed using electron microscopy and caesium chloride centrifugation. At a low RNase concentration, most of the 300 S particles disintegrated completely, and no metastable degradation products were observed. At intermediate RNase concentrations, no 300 S particles were left, but a minor fraction of the BR granules had unfolded and sedimented at 160 S. These granules could represent particles modified during the RNase treatment or represent a more slowly degrading subfraction of the particles. At a high RNase concentration, no RNP particles at all remained in the gradient. The rapid disintegration of the majority of the BR granules was investigated further by electrophoretic analysis of RNA in the remaining particles. During the RNase treatment BR granules, still sedimenting at 300 S, accumulated strand breaks; in fact, as many as 50 to 100 nicks in the 37 kb RNA could be tolerated. It was concluded from RNA analyses that the disintegration of the BR granules was not dependent on any single nick in the RNA, nor on the accumulation of a certain number of nicks, but rather on one or a few critical strand breaks. We propose that there are organizing sequences essential for particle integrity; once these sequences are nicked, the premessenger RNP particles are rapidly and completely degraded.

A new member of a secretory protein gene family in the dipteran Chironomus tentans has a variant repeat structure

We describe the structure of a gene expressed in the salivary gland cells of the dipteran Chironomus tentans and show that it encodes 1 of the approximately 15 secretory proteins exported by the gland cells. This sp115,140 gene consists of approximately 65 copies of a 42-bp sequence in a central uninterrupted core block, surrounded by short nonrepetitive regions. The repeats within the gene are highly similar to each other, but divergent repeats are present in a pattern which suggests that the repeat structure has been remodeled during evolution. The 42-bp repeat in the gene is a simple variant of the more complex repeat unit present in the Balbiani ring genes, encoding four of the other secretory proteins. The structure of the sp115,140 gene suggests that related repeat structures have evolved from a common origin and resulted in the set of genes whose secretory proteins interact in the assembly of the secreted protein fibers.

Isolation and initial characterization of a specific premessenger ribonucleoprotein particle

A specific type of premessenger ribonucleoprotein (RNP) particle, Balbiani ring (BR) granules, has been isolated from heterogeneous nuclear RNP (hnRNP) in the salivary glands of the dipteran Chironomus tentans. A BR granule contains a single 75S RNA molecule coding for a large secretory protein (Sp1). The isolation procedure is based on the abundance and exceptional size of the BR granules: in EDTA-containing sucrose gradients they sediment as a sharp 300S peak ahead of the remainder of the hnRNP population. The isolated BR granules were identified on the basis of both ultrastructural and biochemical criteria: large spherical particles that contain 75S RNA and BR sequences. A three-dimensional reconstruction of isolated particles by electron microscope tomography further supported the identification of the isolated particles as BR granules. In contrast to the entire hnRNP population, the BR granules exhibited a sharp peak in CsCl gradients with a buoyant density of 1.45 g/cm3. This result indicates that a BR granule consists of 40% RNA and 60% protein by weight, corresponding to a 75S RNA molecule of 12 megadaltons and a total protein content of 18 megadaltons, or about 500 average-sized protein molecules.

Identification of the spI products of Balbiani ring genes in Chironomus thummi

The spI fraction of high molecular weight secretory proteins was analysed in Chironomus thummi. These proteins are encoded by giant Balbiani ring (BR) genes which develop specifically in salivary gland cells. Each component of the spI fraction was studied electrophoretically from early and middle 4th instar larvae and prepupae, as well from galactose-treated larvae where changes in the relative puffing pattern of BR1 and BR2 are known to occur. The spI fraction consists of at least two bands with electrophoretic mobilities slower than those of the spI components of Camptochironomus. The slow migrating component remains throughout the 4th larval instar, while the amount of the faster component changes, being abundant in early 4th instar and prepupae, but not present (or very weak) in middle 4th instar. The correlated shifts in BR puffing pattern during these developmental stages suggest that the slow and fast components are encoded by BR2 and BR1. The spI fraction is modified by galactose treatment, the fast component being induced in parallel with a decrease in the slow component. These changes are correlated with changes in the steady-state levels of RNA: an increase in BR1 RNA and a decrease in BR2 RNA, and of proteins. These proteins could correspond to the spIb and spIa fractions allocated to BR2 and BR1, respectively, in Camptochironomus. After galactose treatment a new faster band sometimes appears, that could correspond to the spIc fraction of Camptochironomus. A possible spId equivalent was also identified. In conclusion the main features of the spI family in C. thummi are similar to those of spI in Camptochironomus.

Secretory protein synthesis in Chironomus salivary gland cells is not coupled with protein translocation across endoplasmic reticulum membranes. Electron microscopic evidence

Fragments of rough endoplasmic reticulum containing polysomes bound to the membranes only at the 5 end were visualized in electron microscopic spreads from Chironomus thummi salivary gland cells. The length of the nascent protein molecules in the polysomes increased from the 5 to the 3 (free) polysome end. The data obtained disagree with the generally accepted model according to which synthesis of secretory proteins is concomitant with the protein transport across the endoplasmic membrane.

Terminal repeats in long repeat arrays are likely to reflect the early evolution of Balbiani ring genes

Balbiani ring (BR) genes in Chironomus tentans are 35 to 40 kb (1 kb = 10(3) bases or basepairs) in length and encode secretory proteins of exceptional size. Each gene contains a large homogeneous core block consisting of approximately 100 tandemly arranged, highly homologous repeat units. The repeat unit has a constant (C) region and a subrepeat (SR) region. The various BR genes exhibit similar C regions, while the SR regions differ as to sequence, length and number of subrepeats. To study early steps in the evolution of the coding repeat arrays of the BR genes we have analyzed the 3' ends of the four BR genes in C. tentans: BR1, BR2.1, BR2.2 and BR6. In each gene the very end of the core block consists of two or three repeat unit variants; in each variant repeat the C region is linked to a Cys region, replacing the SR region. Sequence comparisons between the C regions of the closely related BR1 and BR2 genes show that during evolution the terminal repeat unit variants have to a large extent been isolated from the remainder of the core block and have probably been more conserved than the interior repeat units. Detailed analysis of the structure of the variant repeat units further supports this latter notion and suggests that the BR core blocks have evolved from an array of a simple 36 base-pair long sequence; larger, more complex repeat units containing subrepeats were gradually formed and spread in the block, mainly by homologous unequal recombination events. During this evolution the interior of the core blocks evolved as a homogeneous repetitive structure, while ancestor repeat units remained as sequence relicts in the terminal parts.

Developmentally regulated expression of a Balbiani ring 1 gene for a 180-kD secretory polypeptide in Chironomus tentans salivary glands before larval/pupal ecdysis

The expression of a Balbiani ring 1 gene that codes for a salivary gland-specific 180-kD secretory polypeptide (sp180) is regulated developmentally. Immunoblots of salivary gland protein incubated with an affinity-purified nonapeptide-reactive antibody demonstrated that the salivary gland content of sp180 increases as much as 10-fold between stages 8 and 10 of the fourth larval instar. Hybridization of RNA dot-blots with an oligonucleotide probe indicated that the observed increase in sp180 was preceded by a parallel 20-fold increase in the steady state level of its mRNA beginning between stages 7 and 8. In vitro nuclear transcription experiments demonstrated that there was a 10-fold acceleration in the rate of sp180 gene transcription between stages 6 and 10. The limited period of expression of the sp180 gene contrasted dramatically with the expression of Balbiani ring genes BR1, BR2 alpha, BR2 beta, and BR6, which code for the sp-I family of fibrous secretory polypeptides. The appearance of sp180 in secretion coincided with microscopically visible changes in the bundling of these fibrous polypeptides. At the same time, we noticed changes in the appearance and consistency of feeding tubes that larvae construct with this secretion. These results lead us to propose that sp180 may modify the structure or utilization of fibrous secretory polypeptides specifically for the assembly of pupation tubes necessary for larval/pupal ecdysis.

Possible correlations of polypeptides and Balbiani rings in the salivary glands of Drosophila auraria Peng

Salivary glands of various stocks of Drosophila auraria and some of its close relatives were examined with a variety of electrophoretic techniques both from larval and prepupal stages, and after ecdysterone treatments, in an effort to detect possible translation products of the two Balbiani rings (BR1 and BR2) found in the salivary gland chromosomes of these species. Two polypeptides (P2 and P1), with molecular weights of 12,000 and 53,000, respectively, have been detected, the appearance of which coincides with the presence of BR2. The results do not allow the correlation of BR1 action with any specific polypeptide(s).

A peptide-reactive antibody to a Balbiani ring gene product: immunological evidence that a 6.5-kb RNA in Chironomus tentans salivary glands is mRNA for a 180-kDa nonfibrous component of larval secretion

An immunological approach was utilized to demonstrate that a tissue-specific Balbiani ring (BR) transcript in Chironomus tentans is the mRNA for a secreted 180-kDa polypeptide. Balbiani ring 1 (BR1) on the polytene chromosome IV of larval salivary glands contains a gene comprised of tandemly duplicated nucleotide sequences that are transcribed into a salivary gland-specific, 6.5-kb poly(A)+RNA for which a partial cDNA sequence exists [Dreesen et al., J. Biol. Chem. 260 (1985) 11824-11830]. A nonapeptide was synthesized so that its amino acid sequence corresponded to an open reading frame in the cDNA. This peptide was used to raise rabbit polyclonal antisera and to purify the peptide-reactive antibody by affinity chromatography. The affinity-purified antibody bound specifically to a 180-kDa polypeptide on Western blots containing extracts of total salivary gland protein. Western blot analysis of microdissected cellular vs. lumenal fractions of salivary glands indicated that this 180-kDa polypeptide was primarily localized in the lumen. Consequently, this polypeptide was designated a secretory polypeptide (sp180). Finally, the peptide-reactive antibody was used to localize sp180 in a nonfibrous component of salivary gland secretion by indirect immunofluorescence microscopy.

The 3' ends of two genes in the Balbiani ring c locus of Chironomus thummi

The 3'-end sequences of two nonallelic genes derived from the Balbiani ring c (BRc) locus of Chironomus thummi are described. Only one of the genes appears to be transcribed abundantly in normal late larval salivary glands. The two sequences are highly similar, even in the 3' untranslated regions, but sharply diverge beyond the polyadenylation site. Together with evidence from the 3' ends of BR1 and BR2 genes of C. pallidivittatus and C. tentans, independently characterized by others, this result suggests the existence of a sequence-homogenization mechanism that operates across the 3' ends of all BR genes characterized to date. The 3'-terminal coding region of each BRc gene is divided into two portions by a short intron. The upstream portion is homologous to and continuous with the tandem repeats that make up the internal core of each BR gene; however, that portion is variant in sequence relative to the core, and apparently is not subject to the homogenization process that operates on the core repeats. The portion downstream of the intron encodes a unique, 111-residue polypeptide highly different from the rest of the BRc product. The evolution of the various segments of the BRc genes is discussed.

Correlated changes in steady-state levels of Balbiani ring mRNAs and secretory polypeptides in salivary glands of Chironomus tentans

Balbiani rings (BRs) on polytenized chromosomes in Chironomid salivary glands contain members of a homologous multigene family that encodes a family (the sp-I family) of high Mr secretory polypeptides. Each of these BR genes is comprised largely of tandemly duplicated core repeat sequences consisting of related constant (C) regions and intergenically divergent subrepeat (SR) regions. A set of oligodeoxyribonucleotide probes were synthesized that correspond to the transcribed strand of the SR region of BR1, BR2 alpha, BR2 beta, and BR6 core repeats. Under a defined set of conditions, it was possible to show that each oligonucleotide probe hybridized exclusively to its cognate repeat type without hybridization to other repeat types in cloned DNA templates. These BR probes were then used in dot-blot hybridization experiments to simultaneously follow alterations in the steady-state level of BR mRNAs in response to prolonged exposure of larvae to galactose. The results indicated that the relative amounts of these four BR mRNAs may change in a noncoordinate manner. These BR probes were also used in experiments to compare simultaneously the salivary gland content of sp-I components and specific BR mRNAs in larvae that exhibited naturally occurring or induced alterations in BR gene expression. A correlation was found which suggested that sp-Ia is encoded in a gene comprised of BR1 repeats, sp-Ib is encoded by BR2 beta repeats, sp-Ic is encoded by BR6 repeats and sp-Id is encoded by BR2 alpha repeats.

Individual variations in the content of giant secretory polypeptides in salivary glands of Chironomus

Salivary glands in aquatic larvae of Chironomus are responsible for formation of a fiber that larvae use to construct feeding tubes. Major constituents of this fiber include a family (the sp-I family) of high Mr (1 X 10(6) secretory polypeptides. Because of our interest in the polypeptide composition and polymerization of the salivary fiber we conducted a survey of the electrophoretic pattern of sp-I components found in salivary glands obtained from individual larvae. The survey encompassed ten strains of Chironomus tentans, three strains of Chironomus pallidivittatus and four strains of Chironomus thummi. Salivary glands from C. tentans and C. pallidivittatus contained at least four sp-I components (sp-Ia, sp-Ib, sp-Ic and sp-Id) that behave identically with regard to their electrophoretic mobility and detectability when larvae were exposed to galactose or glycerol. Sp-I components in C. thummi were generally fewer and not directly comparable by electrophoretic mobility to sp-I components in the other two species. During this survey two important alterations were observed in the electrophoretic pattern of sp-I components obtained from C. tentans and C. pallidivittatus. First, all four sp-I components exhibited, with a low frequency, double bands that appeared as slow-versus-fast electrophoretic variants of a particular component. Secondly, the relative steady-state level of each sp-I component fluctuated in comparison to other sp-I components in the same extract. This fluctuation varied such that any one sp-I component might appear as a single prominent component.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel giant secretion polypeptide in Chironomus salivary glands: implications for another Balbiani ring gene

Chironomus salivary glands contain a family of high Mr (approximately 1,000 X 10(3)) secretion polypeptides thought to consist of three components: sp-Ia, sp-Ib, and sp-Ic. The use of a new extraction protocol revealed a novel high Mr component, sp-Id. Results of a survey of individual salivary glands indicated that sp-Id was widespread in more than a dozen strains of C. tentans and C. pallidivittatus. Sp-Id was phosphorylated at Ser residues, and a comparison of cyanogen bromide and tryptic peptide maps of 32P-labeled polypeptides suggested that sp-Ia, sp-Ib, and sp-Id are comprised of similar but nonidentical tandemly repeated amino acid sequences. We concluded that sp-Id is encoded by an mRNA whose size and nucleotide sequence organization are similar to Balbiani ring (BR) mRNAs that code for the other sp-I components. Furthermore, parallel repression of sp-Ib and sp-Id synthesis by galactose led us to hypothesize that both of their genes exist within Balbiani ring 2.

Structural implications of primary sequences from a family of Balbiani ring-encoded proteins in Chironomus

DNA sequencing has revealed an internal, tandemly repetitive structure in the family of giant polypeptides encoded by three types of Balbiani ring (BR) genes, in three different species of Chironomus. Each major BR repeat can be subdivided into two halves: a region consisting of short subrepeats and a more constant region that lacks obvious subrepeats. Comparative predictions of secondary structure indicate that an alpha-helical segment is consistently present in the amino-terminal half of the constant region in all known BR proteins. Comparative predictions, coupled with consideration of the known phosphorylation of serine and threonine residues in BR proteins, suggest that the alpha-helical structure may also extend into the carboxy-terminal half of the constant region, possibly interrupted by beta-turn(s). However, it is also possible that the structure is variable, and that a beta-strand is present in that half in some cases. All of the constant regions conserve one methionine and one phenylalanine residue, as well as all four cysteines; these residues presumably play roles in the packing or cross-linking of aligned constant regions. The structure of the subrepeat region is not clear, but the prevalence of a tripeptide pattern (basic-proline-acidic) suggests some type of structural regularity, possibly an extended helix. The possible significance of these conserved molecular features is discussed in the context of how they may serve the elasticity, insolubility, and hydrophilicity of the fibrils and threads formed by the BR polypeptides.

Conserved and nonconserved structures in the secretory proteins encoded in the Balbiani ring genes of Chironomus tentans

The large, repetitive Balbiani ring (BR) genes, BR 1, 2, and 6, in Chironomus tentans originated from a short ancestral sequence and have all evolved according to analogous amplification schemes. We analyzed the structures of the BR-encoded secretory proteins and defined the parts that have been conserved during the evolutionary process. The BR products show striking similarities, with the BR 1 and BR 2 products being more similar to each other than to the BR 6 product. In the constant (C) region of the repeat units, 7 of the 30 amino acid residues are strictly conserved; 4 of these are the cysteine residues. The subrepeat (SR) regions of all the BR products are dominated by repeated tripeptide elements rich in proline and charged amino acid residues. Most of the amino acid replacements in both regions are conservative. Secondary structure predictions suggested that the C regions of the BR 1 and BR 2 products have several elements of secondary structure: an alpha-helix, a beta-strand, and one or two reverse turns, as in "globular structures." The prediction for the C region of the BR 6 product is similar but lacks a beta-strand. The predictions for the intervening SR regions appear less conclusive, but are clearly different from those for the C regions, and suggest regular structures not differing in their conformational elements. The SR regions evolved from an ancestor sequence similar to the C region; thus, the BR products seem to represent an example of evolution from one structure to two differently folded products.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of the two giant secretory proteins in salivary gland of Chironomus tentans

The two giant secretory proteins, sp-Ia and sp-Ib, in salivary-gland cells of the larva of the fly Chironomus tentans, were isolated by preparative gel electrophoresis and characterized chemically. Their amino acid compositions are dominated by polar amino acids, with about 30% of basic amino acid residues. Crossed immunoelectrophoresis of sp-Ia and sp-Ib provided evidence that they share antigenic determinants. They also have major methionine-containing tryptic peptides in common. CNBr cleavage of sp-Ib gives a small number of low-Mr fragments, indicating that this protein has a repetitive structure.

Self-complementary DNA sequences within the BRc gene of Chironomus thummi

The Balbiani ring c (BRc) DNA of Chironomus thummi consists of tandemly arranged 249-base pair (bp) repeats, which represent the major part of the gene (Bäumlein et al. 1982a) and are transcribed and translated in a periodic polypeptide of unusual size. To obtain further information on the DNA sequence organization of that gene a recombinant phage (lambda CthBRc-1) with a relatively long insert (containing predominantly 249-bp repeats) was studied by electron microscopy (EM). lambda CthBRc-1 was found to undergo specific sequence elimination of BRc DNA resulting in heterogeneous size distribution of insert length within the limits of the cloning capacity of the phage with a maximum around 15 kilobase pairs (kb). The EM analysis of R loops formed between recombinant molecules and poly(A)+RNA (containing the transcripts of BRc and BRb) revealed the existence of self-complementary inverted and direct repeats as further sequence elements of BRc DNA scattered throughout a long portion of the BRc transcription unit. Different intrastrand structures (stems, hairpins, complex loops) originate from the renaturation of several sets of self-complementary repeats. Most double-stranded regions fell into one main-size class with an average length of 0.1 kb. The overall data suggest that self-complementary repeats belong to the same DNA sequence family and are able to cooperate in the formation of loops of different size and complexity. The results are discussed in relation to the functional significance of self-complementary inverted repeats (palindromes) for BRc expression.

Balbiani ring 6 gene in Chironomus tentans: a diverged member of the Balbiani ring gene family

We describe the internal organization of a large part of the Balbiani ring (BR) 6 gene in Chironomus tentans. The BR6 gene is a diverged member of the BR gene family. It displays the characteristic hierarchic organization of repetitive sequences, but in the constant region of the repeat units the overall sequence homology is only 49% when compared to other BR genes. All four cysteines are among the few amino acids conserved in the constant region. In the subrepeat region the central part is built from a repeated tripeptide, Pro-Glu--Arg+. A similar charge distribution adjacent to prolines is found in other BR gene subrepeat regions, most pronouncedly in the BR2-encoded protein. These conserved properties of the BR gene products are relevant to the issue how the various BR gene products interact to form a supramolecular structure, the larval tube, and how functional demands influence the evolution of a eucaryotic gene family.

Inhibition of Balbiani ring transcription following differential arrest of Balbiani ring-coded translation

A differential inhibition of the synthesis of secretory proteins, mainly fractions formed in giant translation units, can be obtained in Chironomus salivary gland cells with low concentrations of the ribosome translocation inhibitor, cycloheximide with or without emetine. Both treatments also lead to puff regression and inhibition of transcription specific to the large Balbiani rings, BR1 and BR2, the loci for the giant secretory proteins. The amount of 75S BR RNA transcribed is also reduced in the cytoplasm and in the poly(A) RNA relative to other transcripts. The half-life of 75S RNA is, however, prolonged so that there is little if any decrease in the cytoplasmic content of 75S RNA. The effect on the Balbiani rings may be due to control emanating from the translational process.

The activity of Balbiani rings 1 and 2 in salivary glands of Chironomus tentans larvae under different modes of development and after pilocarpine treatment

The activity of the Balbiani rings 1 and 2 (BR1 and BR2) in the salivary gland was followed during development of fourth instar larvae of Chironomus tentans under different modes of development, with or without a previous pilocarpine treatment. The activity was determined in parallel by two different methods, by incorporation of [3H]uridine into BR-RNA (75 S) and by morphometry of BR1 and of BR2. In glands of untreated larvae BR2 does not change dramatically except for a depression of activity during oligopause (resting phase). BR1 is completely inactive during this phase but exhibits a pronounced activity maximum in the middle of the prepupal period, in subitaneously developing (i.e., uninterrupted) as well as in postoligopause cultures. After pilocarpine treatment the activity of BR2 (rather than of BR1) is generally increased. The extent of this stimulation, however, is strongly development dependent. A most striking activity difference is observed in postoligopause between animals of stage 5 and of stage 6. The relationship between BR2 activity and degree of emptying of the salivary gland lumen was investigated. A model is proposed in which BR2 activity is conceived as being regulated by two parameters: by the degree of filling of the gland lumen and by the stage and mode of development of the larva.

A variant tandemly repeated nucleotide sequence in Balbiani ring 2 of Chironomus tentans

pCtBR2-2 is a genomic clone from Chironomus tentans that hybridized in situ to Balbiani ring 2 (BR2) on salivary gland polytene chromosome IV. DNA sequencing indicated that the insert contained nearly four copies of a 180 bp tandemly repeated nucleotide sequence that was distinctly different from a previously reported BR2 repeat. Sequence titration experiments detected about 70 copies of the 180 bp repeat per haploid genome, which would correspond to approximately 34% of a 37 kb BR2 gene. Each 180 bp repeat included a conserved 90 bp segment whose sequence was internally nonrepeating (INR), and a variable 90 bp repeated (R) segment comprised of three 30 bp repeats that may have evolved from a 9 bp consensus sequence. Results presented here raise the distinct possibility that other BR genes may contain significantly different repeated sequences that have not been identified.

RNA polymerase B in polytene chromosomes: immunofluorescent and autoradiographic analysis during stimulated and repressed RNA synthesis

The distribution of RNA polymerase B (or II) in native and fixed polytene chromosomes isolated from salivary glands of Chironomus tentans and C. pallidivittatus was investigated by both indirect immunofluorescence and autoradiography. The chromosomes, especially the Balbiani rings (BR2, BR1 and BR3), were examined during periods of stimulated and repressed RNA synthesis. In repressed BR2a and, after the salivary gland chromosomes had been stretched, in various chromosomal segments, it was possible to establish unequivocally that RNA polymerase B is not confined to puffs, but also occurs in interbands. The enzyme was absent from the bands, or at least there was not enough of it to be detected with indirect immunofluorescence. It was shown that the distribution of the indirect immunofluorescence in the chromosomes concurs with that of the 3H-uridine or 3H-UTP labeling. However, RNA polymerase B molecules remain associated with the chromosomal template even after an in vivo alpha-amanitin or actinomycin D treatment to inhibit RNA synthesis. Following heat shocks (37 degrees C to 39 degrees C), transcriptively active RNA polymerase B is still found in interbands, in the BRs and in other puffs that have collapsed as a result of the heat treatment; the greatest enzyme concentrations, however, are in the stimulated heat-shock puffs.

Modulation of 75S RNA synthesis in the Balbiani rings of Chironomus tentans with galactose treatment

Galactose has been used as a tool to modify gene activity in the giant puffs Balbiani ring 2 (BR2) and Balbiani ring 1 (BR1) on chromosome IV in the salivary glands of Chironomus tentans. BR2 decreased gradually and was absent or almost absent after a four day galactose treatment. Concomitant with this morphological change, the labelling of the population of growing 75S RNA molecules in BR2 decreased, and was essentially abolished after four days in galactose. Since the elongation rate at the 75S RNA genes proved to be the same in the galactose treated glands as in the control glands, the decreased labelling in BR2 was likely to correspond to a decreased production of 75S RNA. No changes in the size distribution of the growing 75S RNA molecules were noted during the galactose treatment, suggesting that the modulation of the activity was most likely accomplished at the initiation level, but regulation of a very early premature termination could not be excluded. When galactose was removed from the medium, BR2 attained its normal size and its ordinary RNA labelling. BR1 was studied in parallel with BR2 and it behaved strikingly different: BR1 expanded during the galactose treatment and the amount of growing 75S RNA increased, indicating an enhanced production of this 75S RNA species. Also the modulation of BR1 RNA synthesis was reversible. During the galactose treatment no changes in the labelling of chromosome I-III and of nucleolar RNA were observed suggesting that during the four day treatment, galactose exerted its effect mainly on the synthesis of BR2 and BR1 transcription products. The significance of these observations are considered in relation to the information available on the synthesis of the corresponding secretory polypeptides and the formation of the tube-like burrows. We also discuss the implications of the results for models of the regulation of gene activity and of the puffing process.