Determination of the multiplicity of the silk fibroin gene and detection of fibroin gene-related DNA in the genome of Bombyx mori

Evidence from flagelliform silk cDNA for the structural basis of elasticity and modular nature of spider silks

Orb-web weaving spiders rely on their aerial nets to entrap flying prey. A key mechanical feature of orb-web design is the high elasticity of the capture spiral. We report the cloning of substantial cDNA for flagelliform gland silk protein, which forms the core fiber of the catching spiral. Like all silks, the flagelliform protein is composed largely of iterated sequences. The dominant repeat of this protein is Gly-Pro-Gly-Gly-X, which can appear up to 63 times in tandem arrays. This motif likely forms Pro2-Gly3 type II beta-turns and the resulting series of concatenated beta-turns are thought to form a beta-spiral. We propose that this spring-like helix is the basis for the elasticity of silk. The variable fifth position of the motif (X) is occupied by a small subset of residues (Ala, Ser, Tyr, Val). Moreover, these X amino acids occur in specific patterns throughout the repeats. This ordered variation strongly suggests that with hydration, the beta-spirals form hydrogen-bonded networks that increase the elasticity of flagelliform silk. The self-assembly of flagelliform protein monomers into silk fibers may be promoted by beta-spiral/beta-spiral interactions. Additionally, the other two motifs in the flagelliform protein, Gly-Gly-X and a spacer that disrupts the glycine-rich regions, may contribute to the alignment of monomers into fibers. The flagelliform protein cDNA was compared to the other members of the spider silk gene family. We show that all spider silk proteins can be characterized as sets of shared structural modules. The occurrence of these modules among the proteins is inconsistent with the phylogenetic relationships inferred from the C-terminal regions. This observation, along with the high level of variation among individual flagelliform protein repeats, but striking lack of such variation in the other silk proteins, suggests that unusual homogenization processes are involved in silk protein evolution.

Structural features of mag, a gypsy-like retrotransposon of Bombyx mori, with unusual short terminal repeats

Mag is a retrotransposon found as an insert in the Sericin 2 gene. It is present in a few copies--4 to 15--dispersed in the genome of different strains of Bombyx mori as well as in Bombyx mandarina. Flanked by a 5 bp target sequence with no sequence specificity, it is bordered by direct repeats of 77 nucleotides. Despite their unusual short size, these terminal repeats and their immediately adjacent sequences present all the signals necessary for transcription into genomic RNA and for reverse transcription. Mag contains two overlapping open reading frames which are organized as the gag and pol genes of retroviruses and encode putative nucleic acid binding peptide, protease, reverse transcriptase, RNase H and endonuclease in this order. Sequence comparison of these proteins places mag within the gypsy group of LTR retrotransposons next to the echinoderm element SURL.

Cloning and characterization of the highly polymorphic Ser2 gene of Bombyx mori

Three alleles of the sericin (Ser) 2-encoding gene (Ser2), called L, C and mC, were isolated from a Bombyx mori genomic library, and two related ones, called mCL and Cv, were also characterized in B. mori European strains. The Ser2 gene gives rise to two middle silk gland mRNAs by differential splicing. The size of a short mRNA (3.1 kb) is constant, but the length of a longer one ranges from 5 to 6.4 kb depending on the Ser2 allele. These length variations probably result from unequal recombinations in a region which contains about 30 well conserved 45-bp repeats coding for a Ser-like peptide. Furthermore, the L allele (and probably the mCL one) contains a 4.4-kb retrotransposon, resembling the copia-like ones of Drosophila.

A single gene produces multiple sericin messenger RNAs in the silk gland of Bombyx mori

The sericins are a family of major cocoon proteins specifically synthesized in the middle silk gland of the silkworm Bombyx mori. The 5' part of one sericin gene had been cloned and described by Okamoto et al. (1982, J. Biol. Chem. 257, 15192-15199). Using a differential screening procedure of Bombyx genomic libraries, we obtained the 3' part of this gene. We demonstrate that it consists of a single gene extending over 24 kb, present in two allelic forms in hybrid strains. This gene encodes for four mRNAs which result from a unique transcript by an alternative splicing mechanism. This explains, at least partially, the diversity of the sericins found in the cocoon.

Transposable genetic element found in the 5'-flanking region of the fibroin H-chain gene in a genomic clone from the silkworm Bombyx mori

A transposable genetic element was found in the 5'-flanking region of the fibroin H-chain gene in one of the genomic clones from the silkworm Bombyx mori. This element, named K-1.4, is about 1 X 4 X 10(3) base-pairs long, contains an open reading frame of only 225 base-pairs and has inverted repeats of 12 base-pairs at both ends. Duplication of three base-pairs seems to have occurred when this element was integrated into the silkworm genome. About 15 copies of K-1.4 are present per haploid genome of various silkworm strains. Genomic loci of some of these elements are different among different strains or even among individual offspring of the same parents. K-1.4 is present also in the genome of Bombyx mandarina. The K-1.4-related sequences are present in some species belonging to the family Saturniidae.

Structural organization of the P25 gene of Bombyx mori and comparative analysis of its 5' flanking DNA with that of the fibroin gene

We have cloned a large portion of the P25 gene of Bombyx mori encoding the 25,000 dalton polypeptide which associates with fibroin to constitute the major silk protein. Its structure has been investigated by restriction mapping R-loop analysis, S1 nuclease protection experiments and nucleotide sequencing of the region spanning the 5' end of the gene and its flanking DNA. This has permitted a comparative sequence analysis of the DNA from the P25 and fibroin genes. The genes demonstrate no relatedness in their coding regions but they exhibit large blocks of sequence homology in their 5' flanking regions. In particular, the DNA upstream of the P25 gene possesses a sequence very similar to a region of fibroin 5' flanking DNA that is known to possess transcription modulation signals. The functional significance of these homologous regions is discussed with regard to the highly coordinated expression of these two genes.

Increased level of prolactin gene sequences in bromodeoxyuridine treated GH cells

The 5-bromodeoxyuridine-resistant (BrdUrdr) derivative (F1BGH12C1) of prolactin nonproducing (PRL-) rat pituitary tumor cell-subclone GH12C1, synthesize prolactin (PRL) in the presence of the drug. Analysis of nuclear RNA isolated from BrdUrd treated F1BHG12C1 cells demonstrated several high molecular weight RNA PRL sequences, similar to those observed in the nuclear RNA fraction of PRL producing (PRL+) GH3 cells. No such RNAPRL sequences could be detected in nuclear RNA fraction of untreated F1 BGH12C1 cells. PRL sequences in the genome of GH3 (PRL+), GH12C1 (PRL-) and F1BGH12C1 (PRL-, BrdUrdr) GH cells could be identified by blot analysis in 4.8-5.2kb fragment of restriction endonuclease, Hind III digested DNA. Both PRL+ and PRL- cells seem to have approximately the same level of PRL gene sequences in total cell DNA. However Hind III digested DNA of BrdUrd treated F1BGH12C cells revealed the presence of significantly higher levels of PRL gene sequences, in comparison, to that observed in total DNA of untreated cells. The increased level of PRL gene sequences was dependent on the period of drug treatment and a parallel increase in the cytoplasmic RNAPRL sequences was also observed.

Reiteration frequency of procollagen genes in the guinea pig genome. Collagen genes are not amplified during granuloma fibroblasts differentiation

Procollagen mRNA was purified from collagen synthesizing polysomes obtained from an experimental guinea pig granuloma, and iodinated in vitro. The procollagen 125I-labelled mRNA was hibridized with granuloma and liver guinea pig DNA in vast DNA excess conditions. A Cot 1/2 800-900 mol . s . 1-1 for both tissues was obtained from the hybridization curves. With these results, we could suggest the existence of 11-13 procollagen genes per haploid genome. By the analysis of the hybridization data it was possible to infer that there is no genomic amplification in tissues highly specialized in the synthesis of collagen such as granuloma.

A structural analysis of Balbiani ring dna sequences in Chironomus tentans

The Balbiani rings in the salivary gland polytene chromosomes of Chironomus tentans include the most active structural genes in this organ. Two of them (BR1 and BR2) contain repetitive sequences and are transcribed into giant RNA molecules. On Southern blots of restriction digests, we have identified fragments of genomic DNA which contain BR sequences. One of these fragments with a length of about 150 bp has been cloned and shown to hybridize preferentially to the BR1 transcription unit. Determination of its nucleotide sequence revealed several recognition sites for restriction enzymes which cleave the giant BR gene(s) into small pieces of approximately 240 bp. It is concluded that the cloned fragment represents part of the basic 240 bp repeat unit of a BR1 gene. Data obtained from partial restriction digests using the cloned DNA segment as a probe indicate that probably the entire BR1 gene comprises tandem repeats of 240 bp. Evidence is presented that the cloned BR1 sequence significantly cross-hybridizes to BR2 and to a lesser extent to BR6. BR2 sequences are present on a MboI fragment of 40 kb and seem to be organized in a very similar way as found for the BR1 gene.

Amplification of genes for chorion proteins during oogenesis in Drosophila melanogaster

The endochorion and exochorion of Drosophila eggs are synthesized by the ovarian follicle cells during a brief period of about 5 hr. In this terminal phase of egg chamber development, the structural genes for several abundant chorion proteins are expressed at high levels according to a temporally regulated program. The female-sterile mutation ocelliless maps at the site of the genes for two of these proteins, the 36,000- and 38,000-dalton chorion proteins (c36 and c38), which are closely linked. The mutation results in a cis-acting reduction in the amounts of c36 and c38 that accumulate in late-stage egg chambers. We have investigated the mechanism that underlies this decreased production by using cDNA clones complementary to these gene sequences. Unexpectedly, it was found that, in normal females, the genes for c36, c38, and at least one other chorion protein are specifically amplified more than 10-fold in the DNA of late-stage egg chambers. The extra replication involves at least some adjacent chromosomal sequences and begins prior to the onset of mRNA and protein synthesis. The additional DNA remains stable after gene expression has ceased. The behavior of these genes is thus reminiscent of the properties of the DNA puffs that have been described in several groups of Diptera. The extent of amplification of c36 and c38, but not of the 18,000-dalton chorion protein c18 (which is unlinked), was decreased in the egg chambers of flies homozygous for ocelliless, suggesting that altered gene dosage may be responsible for the decreased synthesis of chorion proteins in the mutant.

Genes and mRNAs coding for zein polypeptides in Zea mays

Zein messenger RNAs from maize endosperm were purified by successive oligo(dT)-cellulose chromatography and sucrose gradient centrifugation. Polyacrylamide gel electrophoresis under denaturing conditions revealed the presence of two size classes of zein messenger RNAs of Mr 3.5 x 10(5) and 4.10 x 10(5). The mRNA was shown to synthesize the major zein polypeptides, to have a base composition characteristic of a poly(A)-containing RNA and to be transcribed by reverse transcriptase into complementary DNA. The r0t1/2 of the hybridization curve of cDNA hybridized to an excess of mRNA was shown to be 7 x 10(-2) M . s indicating that about 15 non-cross-hybridizing sequences are present in the zein mRNA preparations. The kinetics of cDNA annealing with an excess of maize DNA from 2 n cells suggest a ten-times reiteration of each mRNA sequence. This result is confirmed from saturation experiments, where in cDNA excess to DNA, the number of zein genes per haploid maize genome was estimated as about 120 copies. Similar experiments carried out on DNA from normal and mutant endosperms (3n cells) indicate the absence of large amplifications or deletions of zein genes in the tissue devoted to zein synthesis.

The adaptation of the silkgland cell to the production of fibroin in Bombyx mori L

At the end of the larval life, the posterior silk gland of Bombyx mori is highly specialized in the biosynthesis of a specific protein : silk fibroin. The successive steps of fibroin production : amino supply, synthesis and secretion are described. Their analysis shows that fibroin synthesis is important enough to orient the overall cellular activities. Thus, the terminal differentiation of the posterior silk gland cell corresponds to the cells adaptation to the production of fibroin. Cytological and biochemical studies of the silk gland development show that specialization occurs discontinuously; the fourth molt, when fibroin is no more produced, is a phase of regression of the cellular adaptation whereas cell differentiation proceeds during the growth phase of the following fifth intermolt. After the spinning of the cocoon, the cells are lysed and disappear entirely at the nymphal stage. Biometrical analysis of silk production of different Bombyx strains in relation with the development of the proteosynthesis apparatus leads to the conclusion that the specific messenger RNA content determines the amount of synthetized fibroin. At maximum secretion, the mRNAF recruits almost all the cell ribosomes. The variations of the size of the proteosynthesis machinery are sufficient to explain the differences of productivity of the various silkworm strains. Different experimental factors affect silk production. Topical applications of juvenile hormone induce an increase of the RNA content and a consequent rise of the amount of secreted protein. In contrast, starvation reduces the silk production by acting at both transcriptonal and translational levels. Current researches on this system are devoted to the study of the differential gene expression, with particular interest to the regulation of the transcription of the specific fibroin messenger RNA.

Chicken ribosomal DNA: gene frequency and purification by R-loop hybridization

The genes for ribosomal RNA of chicken are present in about 200--240 copies per haploid genome. The ribosomal genes were separated from the bulk of DNA by different methods. In CsCl-actinomycin D gradients the GC-rich ribosomal DNA is shifted to the lower density side of the bulk of DNA. After hybridization with iodinated ribosomal RNA under conditions where R-loops are formed ribosomal DNA has an increased density. The R-loops can be separated from the bulk of DNA in Cs2-SO4 gradients.

The synthesis and processing of the messenger RNAs specifying heavy and light chain immunoglobulins in MPC-11 cells

The nuclear precursors of the immunoglobulin messenger RNAs of MPC-11 cells were characterized with respect to size, amount per cell and extent of polyadenylation. These cells produce three Ig mRNAs: a 1.8 kb component coding for a gamma2b heavy chain (H mRNA), a 1.2 kb mRNA coding for a k light chain (L mRNA) and a 0.8 kb mRNA coding for the constant region portion of the k light chain (Lf mRNA). To identify the pre-mRNAs without ambiguity, we constructed recombinant DNA plasmids containing H and L cDNA sequences, and used the cloned cDNAs as hybridization probes for analysis of steady state nuclear RNA and in DNA excess hybridization experiments with pulse-labeled nuclear RNA. The nuclear molecules containing Ig sequences consist of an 11 kb component (H1), which we believe to be the primary transcript of the H gene, 5.3 kb (L1), and 3.3 kb (L2) components, which seem to be primary transcripts of the L and L1 genes, components corresponding to mature size H, L and Lf mRNAs, and several intermediate-sized components which include the processing derivatives. The precursor role of these nuclear molecules was established by studies of their labeling kinetics and by appropriate pulse-chase experiments. All the pre-mRNA species including H1, L1 and L2 contain poly(A), thus suggesting that polyadenylation is an early event in the processing of these mRNAs. The MPC-11 cell contains about 30,000 and 40,000 cytoplasmic H and L mRNA molecules, respectively, which must be produced within one cell generation (approximately 24 hr). In comparison, the nucleus contains about 100-150 molecules of total pre-mRNA and only about 10-15 molecules of presumptive primary transcripts for each of these Ig species. These values indicate very rapid transcription rates (greater than 20 transcripts per min) and exceptionally fast processing rates (approximately 0.5 min for the primary transcripts and approximately 5 min for overall nuclear processing) for the Ig mRNAs. Thus rapid transcription and processing, together with high cytoplasmic stability, account for the high abundance of Ig mRNAs in the myeloma cell.

The genes for 18S, 5.8S and 28S ribosomal RNA of Bombyx mori are organized into tandem repeats of uniform length

The organization of the multiple genes for 18S, 5.8S and 28S rRNA in the genome of the silkworm, Bombyx mori was determined by restriction endonuclease digestion and Southern blot hybridization. The ribosomal genes (rDNA) are tandemly reiterated, with a uniform repeat length of 6.9 . 10(6) daltons. Each rDNA repeat has a single site for EcoRI, HindIII, HpaI and SmaI and each of these sites has been mapped with respect to the others and to the rRNA genes; each repeat consists of a transcribed region (6 . 10(6)daltons) containing the 18S, 5.8S and 28S rRNA genes (5' leads to 3') and also a small non-transcribed spacer (approximately 10(6) daltons). Complete rDNA repeats were cloned using the vector RSF2124 and grown in Escherichia coli. Characterization of the rDNA plasmids confirmed the conclusions from studies of the total rDNA. The organization of B. mori rDNA is similar to that of other eukaryotes, except for the absence of heterogeneity in the rDNA repeat length; thus, there is neither variation in the length of the non-transcribed spacer nor the presence of inserts in a detectable portion of the rDNA. The utility of this map, and particularly of the rDNA plasmids, for detailed studies of rRNA transcription and processing is discussed.

Synthesis of histone messenger RNAs by RNA polymerase II in nuclei from S phase HeLa S3 cells

Nuclei were isolated from synchronized HeLa S3 cells and transcribed utilizing their endogenous RNA polymerases. Our data suggest that S phase nuclei are capable of synthesizing histone mRNA sequences while nuclei from G1 phase cells are not. Transcription of histone mRNA sequences by S phase nuclei can be abolished completely by low levels of alpha-amanitin (1.0 microgram/ml, a concentration which completely inhibits RNA polymerase II). From these results it appears that transcription of the histone mRNA sequences occurs during the S phase but not during the G1 phase of the cell cycle and that RNA polymerase II is responsible for histone gene readout.

The repetition frequency of DNA in Balbiani ring 2 of Chironomus thummi

The RNA of Balbiani ring BR2 of polytene chromosomes from Chironomus thummi salivary glands was microisolated and reassociated in the presence of an excess of total larval DNA. BR2 RNA reacts as a single component with a C0t 1/2 of 8.6. Ribosomal precursor RNA from microisolated nucleoli reassociates under identical conditions with a C0t 1/2 of 12.3. These C0t 1/2-values suggest repetition frequencies in the range of 35 and 50 for ribosomal DNA and Balbiani ring 2 DNA, respectively. The data presented here favour the view that the gene for BR2 RNA of C. thummi is internally repeated and contains only one type of DNA sequence.

The nucleotide sequence of two silk gland alanine tRNAs: implications for fibroin synthesis and for initiator tRNA structure

The nucleotide sequences of two major alanine tRNAs from the Bombyx mori posterior silk gland have been determined. One of these tRNAs appears to be specific to the silk gland, where its accumulation is associated with the rapid production of fibroin. Both sequences are identical, with the exception of a single nucleotide in the anticodon stem. A striking feature of both alanine tRNAs is that loop IV contains sequences previously believed to be restricted to initiator tRNA.

Characterization and in vitro translation of polyadenylated messenger ribonucleic acid from Neurospora crassa

Ribonucleic acid (RNA) extracted from Neurospora crassa has been fractionated by oligodeoxythymidylic acid [oligo(dT)]-cellulose chromatography into polyadenylated messenger RNA [poly(A) mRNA] and unbound RNA. The poly(A) mRNA, which comprises approximately 1.7% of the total cellular RNA, was further characterized by Sepharose 4B chromatography and polyacrylamide gel electrophoresis. Both techniques showed that the poly(A) mRNA was heterodisperse in size, with an average molecular weight similar to that of 17S ribosomal RNA (rRNA). The poly(A) segments isolated from the poly(A) mRNA were relatively short, with three major size classes of 30, 55, and 70 nucleotides. Gel electrophoresis of the non-poly(A) RNA indicated that it contained primarily rRNA and 4S RNA. The optimal conditions were determined for the translation of Neurospora mRNA in a cell-free wheat germ protein-synthesizing system. Poly(A) mRNA stimulated the incorporation of [14C]leucine into polypeptides ranging in size from 10,000 to 100,000 daltons. The RNA that did not bind to oligo(dT)-cellulose also stimulated the incorporation of [14C]leucine, indicating that this fraction contains a significant concentration of mRNA which has either no poly(A) or very short poly(A) segments. In addition, the translation of both poly(A) mRNA and unbound mRNA was inhibited by 7-methylguanosine-5'-monophosphate (m7G5'p). This is preliminary evidence for the existence of a 5'-RNA "cap" on Neurospora mRNA.