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

High molecular mass complexes of aquatic silk proteins

Little is known about specific protein protein associations that take place during formation of Chironomus tentans silk. The aim of this study was to learn if C. tentans salivary glands contain biochemically discrete silk protein complexes. Examination of native extracts by non-denaturing agarose gel electrophoresis and immunoblotting revealed two SDS-resistant complexes: C1a, nominally containing silk proteins spIa, sp185 and sp140, and C1b, containing spIb, sp185 and sp140. The data also implied that C1a and C1b can further associate into SDS-sensitive homo- or hetero-oligomers. Sedimentation of extracts in preparative glycerol gradients resulted in a heterogeneous distribution of C1a and C1b centered near 30S. Examination of gradient fractions by denaturing polyacrylamide gel electrophoresis and immunoblotting indicated that C1a and C1b co-sediment with spIs, sp185, and sp140; however, these fractions also contained sp40, sp17 and sp12. In contrast, two other silk proteins sedimented throughout the gradient. Electron micrographs of a complex-containing fraction showed discrete, sometimes oligomeric lattice-like structures that, over time, assembled in vitro into multistranded beaded fibers. It is proposed that C1a and C1b are quaternary structures that are intermediates in the assembly pathway of C. tentans silk.

Disulfide bonds in a recombinant protein modeled after a core repeat in an aquatic insect's silk protein

We constructed a gene encoding rCAS, recombinant constant and subrepeat protein, modeled after tandem repeats found in the major silk proteins synthesized by aquatic larvae of the midge, Chironomus tentans. Bacterially synthesized rCAS was purified to near homogeneity and characterized by several biochemical and biophysical methods including amino-terminal sequencing, amino acid compositional analysis, sedimentation equilibrium ultracentrifugation, and mass spectrometry. Complementing these techniques with quantitative sulfhydryl assays, we discovered that the four cysteines present in rCAS form two intramolecular disulfide bonds. Mapping studies revealed that the disulfide bonds are heterogeneous. When reduced and denatured rCAS was allowed to refold and its disulfide bonding state monitored, it again adopted a conformation with two intramolecular disulfide bonds. The inherent ability of rCAS to quantitatively form two intramolecular disulfide bonds may reflect a previously unknown feature of the in vivo silk proteins from which it is derived.

A new member of the balbiani ring multigene family in the dipteran Chironomus tentans consists of a single-copy version of a unit repeated in other gene family members

The known Balbiani ring (BR) multigene family members in the dipteran Chironomus tentans encode salivary gland secretory proteins in the size range between 38 and 1,000 kDa. The proteins interact to form protein fibers used by the aquatic larvae to spin feeding and protective larval tubes or pupation tubes. Here, we describe a new BR multigene family member, the sp17 gene, which codes for an 89-amino-acid-long protein with a relative mobility of 17k. The gene has a high content of charged amino acid residues and consists of two structurally different halves. Five regularly spaced cysteine codons are present in the 5' half while the 3' half contains five proline codons. These two different halves exhibit similarities to the C and SR regions, respectively, which form the tandemly repeated units in the about 40-kb-long BR genes and which also, in different versions, are the building blocks of all genes in the BR multigene family. In this multigene family, encoding interacting structural proteins, the long BR genes with their 125-150 tandemly arranged repeat units as well as the short sp17 gene with its single-copy version of such a repeat unit, have therefore evolved from a common ancestor.

Secondary structure of synthetic peptides derived from the repeating unit of a giant secretory protein from Chironomus tentans

The secretory proteins of Chironomus tentans larvae, which are used to construct underwater feeding and pupation tubes, assemble into complexes in vitro. Members of a family of 1000 kDa proteins, the spIs, appear to form the fibrous backbone of the assembled complexes. The spIs consist of a core of tandemly repeating units of 60 to 90 amino acids that can be subdivided into two regions: the subrepeat region, made up of short internal repeats, and the constant region, which lacks simple subrepeats. We have synthesized peptides representative of the constant and subrepeat regions of one of the spIs, and have examined their secondary structure using Fourier transform IR and CD spectroscopy. The IR spectrum of the constant peptide indicates that this peptide has alpha-helical regions and beta-turns. The CD spectrum confirms this. The IR spectrum of the subrepeat peptide is similar to that of the poly(Gly)II helix, and also may indicate the presence of beta-turns. The CD spectrum is consistent with this helical structure. Extrapolation of these results to intact spIs is in agreement with secondary structure prediction and modeling studies. Our results indicate that the alpha-helices and poly(Gly)II-like helices are not arranged as coiled-coils, which are often found in fibrous proteins. We suggest that these structural elements may be in an unusual arrangement in the spIs, organized as alternating alpha-helices and poly(Gly)II or collagen-like helices, interspersed with beta-turns.

Balbiani ring 1 gene in Chironomus tentans. Sequence organization and dynamics of a coding minisatellite

Balbiani ring (BR) genes in diptera encode large secretory proteins and are classical model systems for studies of gene expression. In Chironomus tentans, four closely related BR genes, BR 1, BR 2.1, BR 2.2 and BR 6 form a gene family. The BR genes have been partially characterized and are known to contain long arrays of tandemly arranged repeat units with an hierarchical repeat organization. Here, we report the sequence organization of the complete transcribed part of the BR 1 gene in C. tentans. The gene contains five exons and four introns. Three of the introns are located at the 5' end and the fourth at the 3' end of the gene. Exon 4 is approximately 35,000 bases long and is built completely from tandemly organized repeats. We show that this long repeat block contains two types of related repeat units, beta and gamma. Each type forms a large uninterrupted array, a 5' beta array and a 3' gamma array with a sharp border between them. In the hierarchical repeat structure in each repeat array, all repeats are virtually identical at one level of repetition, but shown differences at the next level. The whole repeat block in the BR 1 gene fluctuates in size between different alleles, but not by more than 10%. In contrast, within the block, the beta and gamma arrays vary in length between 8000 and 29,000 bases in an inverse fashion, together keeping the overall length requirement. We propose that the length of exon 4 is conserved by selection of cross-over products of a given length, and that the internal hierarchical sequence organization in the BR 1 gene is a consequence of the combined action of several different sequence turnover mechanisms, all dependent on the unequal pairing of homologous sequences at different, competing levels of repetition.

Translocation of a specific premessenger ribonucleoprotein particle through the nuclear pore studied with electron microscope tomography

A specific premessenger ribonucleoprotein (RNP) particle in the salivary glands of the dipteran Chironomus tentans was studied with electron microscope tomography during translocation from the cell nucleus to the cytoplasm. The RNP particle consists of a thin RNP fiber tightly folded into a ribbon, which is bent into a ring-like structure. Upon translocation through the pore, the particle is first orientated in a specific manner at the pore entrance, and subsequently the bent ribbon is gradually straightened and transported through the pore with the 5' end of the RNA in the lead. Concomitantly, the elementary RNP fiber constituting the ribbon is gradually unpacked and will appear more or less extended on the cytoplasmic side of the pore complex. The ordered nature of the process suggests a specific recognition of the RNP particle at the nuclear pore.

Sequence organization of the Balbiani ring 2.1 gene in Chironomus tentans

Balbiani rings are giant chromosomal puffs, containing related genes that provide unique possibilities for in vivo analysis of gene expression at the chromatin and ribonucleoprotein levels. Here, the 5' end of the Balbiani ring 2.1 gene in the dipteran Chironomus tentans has been isolated and the sequence organization of the entire Balbiani ring gene is presented. The gene contains five exons, one being extremely small, only 6 base pairs, and one being extremely large, approximately 30 kilobase pairs. Three introns are located at the 5' end and a fourth one is located at the 3' end. The central 30-kilobase-pair exon is entirely built from tandemly organized repeats. All repeats are virtually identical except for a few variant repeats at both ends of the repeat array. The number of repeats may vary between alleles and the length of the gene therefore changes between 30 and 35 kilobase pairs.

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.

Amino acid sequence of PR-39. Isolation from pig intestine of a new member of the family of proline-arginine-rich antibacterial peptides

We recently isolated from pig intestine and characterized a 31-residue antibacterial peptide named cecropin-P1 with activity against Escherichia coli and several other Gram-negative bacteria. The isolation involved a number of batch-wise steps followed by several chromatography steps. The continued investigation of these antibacterial peptides has now yielded another antibacterial peptide with high activity against both E. coli and Bacillus megaterium. Amino acid analysis showed a very high content of proline (49 mol%) and arginine (26 mol%), an intermediate level of phenylalanine and low levels of leucine, tyrosine, isoleucine, and glycine. The primary structure was determined by a combination of Edman degradation, plasma desorption mass spectrometry and C-terminal sequence analysis by carboxypeptidase Y degradation using capillary zone electrophoresis for detection of liberated residues. The calculated molecular mass was 4719.7 Da, which is in excellent agreement with 4719 Da obtained by plasma desorption mass spectrometry. The peptide was named PR-39 (proline-arginine-rich with a size of 39 residues). The lethal concentration of the peptide was determined against six Gram-negative and four Gram-positive strains of bacteria.

A tissue-specific puff (Balbiani ring a) in Chironomus thummi may contain a gene encoding a 67-kDa protein which exhibits non-tissue-specific expression

A 2.3-kb genomic clone has been isolated from the region where the tissue-specific puff, Balbiani ring a (BRa), is found on chromosome IV of the special lobe of Chironomus thummi salivary gland cells. The clone was characterized by nucleotide sequence analysis. Two clusters of direct tandem repeats were identified, as well as large and small open reading frames (ORFs). The large ORF was fused to an Escherichia coli lacZ gene. Antibodies against the beta-galactosidase/ORF fusion protein reacted selectively on Western blots with a 67-kDa protein. Western-blot analysis and immunoelectron microscopy showed that this protein was distributed in the cells of all larval tissues examined. We concluded that BRa, a tissue-specific puff, whose activity correlates with the synthesis of 160-kDa secretory protein [Kolesnikov et al., Chromosoma 83 (1981) 661-677], may also contain a gene which is not expressed in a tissue-specific manner.

Functional analysis of the 3'-terminal part of the Balbiani ring 2.2 gene by interspecies sequence comparison

An interspecies comparison was made between the 3' ends of Balbiani ring genes from Chironomus. The comparison was focused on the BR2.2 gene, and a part at the 3' end from Chironomus pallidivittatus (which included also a segment of the gene core) was cloned. Its sequence, and other previously published BR sequences from this species and from Chironomus tentans were used in the analysis. The 3' parts of these repetitive genes can be divided into a region belonging to the core of the genes followed by a terminal region. In the core region the repeats (each of which consists of a constant part and a subrepeated part) are highly similar and the constant parts show little interspecies differentiation. Furthermore, the two parts of the repeats are units in an evolutionary and probably also functional sense. The terminal region contains modified constant units, usually isolated between acidic so-called cys regions, the whole arrangement lying upstream of an intron toward a 3'-terminal exon. Most of the modified constant units are mosaics in rates of evolution with stable outer quarters bordering to equally stable cys regions and a central half with a very high rate of evolution. One of the terminal units, present only in the BR2.2 gene and second from the end, differs distinctly not only from corresponding core units but also from other terminal units in the three normally active BR genes. It lies upstream of all cys regions and is evolutionarily conserved over most of its length. Furthermore, two-dimensional protein structure prediction does not exclude an endoproteolytic cleavage site in this unit.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Balbiani ring 3 in Chironomus tentans encodes a 185-kDa secretory protein which is synthesized throughout the fourth larval instar

We have continued to map and identify genes encoding a family of secretory proteins. These proteins are synthesized in larval salivary glands of the midge, Chironomus tentans, and assemble in vivo into insoluble silk-like threads. The genes for several secretory proteins exist in Balbiani rings (BRs) on salivary-gland polytene chromosomes. A randomly primed cDNA clone, designated pCt185, hybridized in situ to BR3 and was shown on Northern blots to originate from a salivary gland-specific 6-kb poly(A) + RNA. The partial cDNA sequence contained 483 nucleotides including one open reading frame (ORF) encoding 160 amino acids (aa). A striking feature of the ORF was the periodic distribution of cysteine residues (Cys-X-Cys-X-Cys-X6-Cys) which occurred approximately every 22 aa. A cDNA-encoded 18-aa sequence was selected for chemical peptide synthesis. When affinity-purified antipeptide antibodies were incubated with a Western blot containing salivary-gland proteins they reacted specifically with a 185-kDa secretory protein (sp185). Developmental studies showed that sp185 and its mRNA were present in salivary glands throughout the fourth larval instar. Thus sp185 and a family of 1000-kDa secretory proteins are encoded by a class of genes that are expressed throughout the fourth instar. This contrasts with the developmentally regulated expression of the sp140 and sp195 genes whose expression is maximal during the prepupal stages of larval development.

The Balbiani ring 3 gene in Chironomus tentans has a diverged repetitive structure split by many introns

A set of approximately 15 secretory proteins is synthesized by the salivary gland cells in the midge Chironomus tentans. These proteins are secreted but do not form insoluble fibers until they are transported out of the gland lumen. A Balbiani ring (BR) gene family consisting of four genes (BR1, BR2.1, BR2.2 and BR6) have previously been shown to encode four of these proteins, sp-I a to d, with relative molecular weights of 1 x 10(6). Each BR gene contains an uninterrupted block in which about 100 repeats are tandemly arranged. The repeats are virtually identical and efficient homogenization mechanisms must operate within each block. Here we describe a new BR gene, the BR3 gene, which according to structural similarities may belong to the BR gene family, but at the same time exhibits a strikingly different structure. The gene encodes a 10.9 kb transcript that contains 38 introns and is spliced into a 5.5 kb mRNA. The mRNA is translated into a cysteine-rich 185 kDa major component of the gland secretion. The coding sequence in the gene is built from diverged repeats in which mainly the cysteine codons are preserved and the sequence is split by the introns into 17 to 678-bp long exons. The introns are located at defined positions in relation to the repeat structure. In sharp contrast to the uninterrupted array of identical repeats in the BR1-BR6 genes, the repeats in the BR3 gene are not efficiently homogenized and have diverged extensively from each other. We propose that the splitting of the repeat structure into variable sized exons prevents homogenizations dependent on unequal aligning of homologous sequences.

Subrepeats within the BR1 beta repeat unit in Chironomus pallidivittatus can be classified into different types depending on codon usage

A new type of repeat unit was isolated from Balbiani ring 1 of Chironomus pallidivittatus and designated BR1 beta repeat. It consists of a constant and a subrepeated part, like previously described units belonging to the core blocks of the BR genes. The subrepeated part contains 10-codon subrepeats with an arrangement similar to the subrepeats of the previously described BR2 beta gene. The present unit differs from earlier reported core units firstly in a much lower number of copies (about 15) per genome, which are tandemly arranged. Secondly, the number of subrepeats per BR1 beta repeat unit can show great variations. On the basis of the pattern of codon usage, three types of subrepeats can be distinguished. One type lies 5'-proximal in the subrepeat array and consists of variable numbers of subrepeats almost identical at the nucleotide level. The last complete subrepeat represents another type, with consistent differences in codon usage as compared to subrepeats of the proximal type. Finally, there is an intermediate type represented by the subrepeat preceding the distal one. Here, codon characteristics from proximal and distal subrepeats are mixed in a patchy and irregular way. The evolution of the arrays can be understood either as being the result of subrepeat formation in two steps (occurring before and after amplification of whole repeat units) or as the result of a continuous process in which there is evidence for participation of gene conversion.

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.

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.

Three-dimensional structure of a specific pre-messenger RNP particle established by electron microscope tomography

Electron microscope tomography has been used to refine the structural analysis of individual RNP particles synthesized on Balbiani ring genes in Chironomus tentans. The spherical particles have a diameter of 500 A and are composed of a thick RNP ribbon bent into an asymmetrical, four-domain, ring-like configuration. The first domain, containing the 5' end of the transcript, and the fourth domain, containing the 3' end, are close to each other.

A BR 1 gene in Chironomus tentans has a composite structure: a large repetitive core block is separated from a short unrelated 3'-terminal domain by a small intron

The large Balbiani ring (BR) genes in the dipteran genus Chironomus have been considered to be homogeneous repetitive structures. Analysis of a genomic DNA segment now reveals that a BR 1 gene in C. tentans is a composite gene, consisting of two different types of sequences. A 15-20 kb core block of tandemly arranged repeat units extends close to the 3' end of the BR 1 gene and ends in repetitive structures partly different from the repeat units in the core block. A 55 bp long intron separates the core block, which probably constitutes a single exon, from a non-related 3'-exon, comprising the final 332 bp of the translated part of the gene. According to hydrophobicity and secondary structure predictions, the 3'-exon encoded peptide is distinctly different from the repetitive core block domain and attains a globular structure. The carboxyl-terminal peptide domain is likely to be a general feature of BR encoded proteins and may have important functions in the excretion and polymerisation of the secretory proteins.

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)