Demonstration of Balbiani ring RNA sequences in polysomes

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.

Characterization of a cloned, moderately repeated sequence from Balbiani ring 2 in Chironomus tentans

pCtBR2-1 is a recombinant plasmid with a 750-bp insert of Chironomus tentans genomic DNA. When pCtBR2-1 was hybridized in situ to salivary gland polytene chromosomes, it hybridized exclusively to Balbiani ring 2 (BR2), a giant chromosomal puff. It was also shown that the insert contained four tandemly repeated sequences that were delineated by HinfI sites which occurred every 190 bp. The purified insert reassociated to C. tentans DNA with a C0t1/2 = 0.48 indicating that the sequence was moderately repeated within the genome. Hybridization of radioactive pCtBR2-1 to nitrocellulose blots containing partial HinfI digests of genomic DNA revealed that the 190-bp repeats were organized into one or more blocks of 11 to 12 copies in tandem. Hybridization of the recombinant plasmid to limit digests of genomic DNA also demonstrated that repeated sequences in BR2 were not homogeneous. As much as 70% of BR2 appeared to be represented by a 26-kb HhaI-resistant core, while the remaining 30% may have HhaI sites at 190-bp intervals, similar to pCtBR2-1.

Selective deletion of large segments of Balbiani ring DNA during molecular cloning

Transcription units in Balbiani ring 1 (BR1) and Balbiani ring 2 (BR2) of Chironomus tenans salivary glands are comprised of about 40 kb of repetitive DNA sequences organized in a satellite-like array. Because of this sequence organization, it was possible to prepare 30 to 40-kb target DNA fragments for cloning by performing limit restriction endonuclease digestion of high-Mr genomic DNA. These fragments were ligated to cohesive termini of the linearized cosmid, pHC79, packaged in vitro, and used to transduce Escherichia coli. Alternatively, target fragments were randomly sheared to a mean length of 8-10 kb, annealed to plasmid pBR322 using homopolymeric tails, and used for bacterial transformation. Recombinant cosmids and plasmids generally contained inserts which were proportional to the length of target fragments used in ligation reactions. However, the subset of recombinants that hybridized to 32P-labeled 75S RNA (highly enriched in BR1 + BR2 sequences) had disproportionately smaller inserts. With the exception of one metastable clone with a 2.1-kb insert, all others had inserts of 0.8 or 0.4 kb. Similar results were obtained in host cells that were recA- or recBC-. The most likely conclusion is that repetitive BR sequences are highly unstable during replication in E. coli and are selectively deleted.

Periodicities and tandem repeats in a Balbiani ring gene

The Balbiani ring (BR) DNAs show prominent periodicities of restriction enzyme sites. Studies using a cloned fragment of the BRc gene strongly suggest that these periodicities reflect the existence of tandemly repetitive sequences within BR DNA. Tandem repeats measuring 54-58 bp have been demonstrated by partial sequence analysis of the BRc clone; the restriction site periodicities suggest the existence of additional 175 (= 3 X 58) and 1050 (= 6 X 175) bp repeat units. The short, medium and long repeats (58, 175 and 1050 bp, respectively) show sequence homology. Constrained unequal crossing over (resulting from misalignment of repeat arrays, usually by one repeat) is proposed as the mechanism for evolution of short, medium and long repeats from each other, in a manner analogous to evolution of satellite DNA sequences. Paradoxically, the dominant restriction site periodicities appear to be more conservative than might be expected on the basis of the overall sequence divergence between the sequenced repeats. This may be a consequence of functionally important, long-range amino acid or oligopeptide periodicities (for example, Asp x Ser or Glu x Ser corresponding to Hinf I sites) in the BRc protein product, in conjunction with preferential use of certain synonymous codons.

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

The main secretory protein fractions from Chironomus tentans have been investigated with particular emphasis on the dominant fraction, component 1, here designated I (Grossbach, 1969). This polypeptide was suggested to be the translatory product of 75SS RNA from Balbiani ring 2 (BR2) because of its size and quantitative prominence. Its molecular weight was estimated by gel filtration in 8 M urea at 850,000 + 101,000 D. During short pulses with radioactive amino acids a large fraction of the label was found in a population of polypeptide chains suggestive of molecules continuously growing to the size of compoenet I. Populations of nascent large protein chains of similar size distribution were dominant in the polysomes and constituted the only population present in the largest polysomes, known to contain 75S RNA from BR2 (and BR1) as predominant or only component (Daneholt et al., 1977; Wieslander and Daneholt, 1977). These data indicate strongly that the large size of component I is not a result of posttranslational modifications. No sequence similarities, using limited proteolysis, were found between component I and component II, both of which have been considered to the BR2 products. There was, furthermore, no detectable immunological identity between component I and smaller secretory protein fractions. The data support Grossbach's and Daneholt's suggestion that component I is closely related to the primary translation product of 75S RNA from the large Balbiani rings.

Sequences translated by Balbiani ring 75S RNA in vitro are present in giant secretory protein from Chironomus tentans

The 75S RNA originating in the large Balbiani rings 1 and 2 (BRI and 2) was isolated and used for in vitro translation in the mRNA dependent reticulocyte lystate. Conditions (K+-concentration, temperature, time etc). were optimized for obtaining translation products of maximal size. Polypeptide chains up to about 500,000 D were obtained but no complete translation products. Trypic fingerprints were performed on the in vitro products as well as on the secretory protein components nos. I and II+III labelled with 35S-methionine. There was a large degree of correspondence between the fingerprint of the in vitro product and that of component I but less to that of component II+III. The results suggest that 75S RNA with an origin in the BR1 and BR2 codes for the giant secretory protein component I.

The size distribution of poly(A) in newly synthesized and old Balbiani ring RNA

The size distribution of newly synthesized and old poly(A) sequences on transcripts of the giant tissue specific puffs, Balbiani rings in salivary glands of Chironomus tentans has been determined. After labeling with [3H]adenosine, poly(A) containing Balbiani ring RNA(75S RNA) was selectively collected by means of a recently developed technique. This combines electrophoretic fractionation and affinity chromatography in one run by insertion of poly(U) immobilized in glass fiber filters in an agarose gel slab. The majority of short-term labeled poly(A) chains released from poly(A) containing 75S RNA molecules is distributed within a narrow size range migrating as one peak with a mean value of 103 +/- 2 nucleotides, which is probably the initial length of poly(A). The labeling pattern of ribonuclease resistant poly(A) stretches after chase with unlabeled adenosine displays a relatively broad and heterogeneous size spectrum from at least 20 to more than 100 nucleotides. The main peak of labeled adenylate core in newly formed poly(A) containing RNA of non-Balbiani ring origin is dispersed within a broader size range than that of Balbiani ring RNA and possesses an average value of 94 +/- 2 nucleotides. During chase conditions, the relative frequency of occurrence of poly(A) chains of 75S RNA in the size range of 100 nucleotides exhibits a significant decrease in parallel with a rather uniform gain in the size classes between 20--50 nucleotides. However, the results are inconsistent with the existence of an age-dependent shortening of poly(A) chains in the balbiani ring RNA. A significant portion of 75S RNA molecules remain associated with poly(A) segments which are essentially of original size even after 21 hr in the presence of unlabeled adenosine. This finding provides support for the possibility that the initiation of the poly(A) shortening in 75S RNA is a stochastic process.

Large-sized polysomes in Chironomus tentans salivary glands and their relation to Balbiani ring 75S RNA

Polysomes from the salivary glands of Chironomus tentans were investigated to determine whether Balbiani ring 75S RNA is incorporated into polysomal structures, and thus probably acts as messenger RNA. A new extraction technique for obtaining ribonucleoproteins was applied that gives a high yield of polysomes with only moderate degradation of the cytoplasmic, high molecular weight RNA. The polysomes sedimented in a broad region (200-2,000S) with a peak value of about 700S, which suggested that they were partly of very large sizes. This was confirmed by visualization of the polysomes in the electron microscope: 400S polysomes contained mainly 11-16 ribosomes, and 1,500S polysomes about 60 ribosomes per polysome. However, polysomes containing 100 or more ribosomes were also observed. It was further established that most of the cytoplasmic 75S RNA was located in polysomes, preferentially in the most rapidly sedimenting ones. From the available information on Balbiani ring RNA in cytoplasm and the present demonstration of 75S RNA molecules in polysomes, it was concluded that at least some Balbiani ring RNA, generated as 75S RNA within the Balbiani rings, eventually enters polysomes without being measurably changed in size. The present information on the potential amino acid coding sequences in 75S RNA is discussed in relation to the large size of the polysomes observed.