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K Beckingham

Publications and source records attributed to K Beckingham.

49 records · Page 3Linked to original sources
PubMed1988

Genes with specific functions in the ovarian follicles of Calliphora erythrocephala (Diptera).

Working with the large dipteran Calliphora erythrocephala, we have performed differential screening to identify genes actively expressed in the previtellogenic and early vitellogenic stages of oogenesis but silent during the early stages of embryogenesis. Clones containing sequences homologous to four such genes have been characterized. Two clones are homologous to the yolk protein 1 gene of Drosophila melanogaster. These two clones are expressed not only in the columnar follicle cells surrounding the oocyte but also in the border cells--a highly specialized subgroup of the follicle cells. This indicates a new function for these cells though previously to contribute mainly in the formation of the micropyle. A third clone, which is related to the D. melanogaster vitelline membrane protein genes of the cluster at chromosomal locus 26A, is expressed only in the perioocyte follicle cells and not the border cell population. The fourth clone encodes a sequence of unknown function which is abundantly expressed in the germ line cells of the follicle. Transcripts homologous to this clone persist into the mature follicle and initially appear concentrated at the anterior pole of the oocyte. The distribution of repetitious DNA within these four clones indicates that the C. erythrocephala genome has a short interspersion arrangement of repetitive DNA.

Animals↗
PubMed1987

Structure and sequence of the Drosophila melanogaster calmodulin gene.

A series of phage clones overlapping the single calmodulin gene locus of Drosophila melanogaster has been isolated and the exons of the gene positioned and sequenced within these clones. A calmodulin cDNA clone of the electric eel was used to identify these clones and to position the two major protein-coding exons of the gene. cDNA clones for D. melanogaster calmodulin were then isolated, characterized and used to identify the remaining exons. The gene consists of four exons separated by three introns of 3400 to 4300 bases in length. Exon 1 consists of the 5' untranslated region and the initiator ATG; exon 2 encodes amino acid residues 1 to 58.3; exon 3 encodes residues 58.3 to 139.3; and exon 4 encodes residues 139.3 to 148 and the 3' untranslated region. From the sequence of the 3' untranslated region and the lengths of the cDNA clones, two or three polyadenylation sites are indicated. Sequences potentially involved in the control of transcription of the gene and splicing of the mRNA product have been identified. Comparison of the intron-exon structures of the D. melanogaster calmodulin gene, the chick calmodulin gene, and other genes of the troponin C superfamily reinforces previous hypotheses that these genes arose from a common progenitor and permits identification of four introns that were probably present in the progenitor gene structure. The D. melanogaster calmodulin gene contains three of these introns, and the chick gene contains all four. These gene comparisons also indicate that the region of these genes encoding Ca2+-binding loop 3 is highly variable in structure. The chick and D. melanogaster calmodulin genes differ in this region, the chick gene containing a fifth intron here that is absent from the D. melanogaster gene.

Animals↗
PubMed1987

Isolation and characterization of abl gene sequences in Calliphora erythrocephala.

Screening of genomic DNA libraries with hybridization probes derived from a Drosophila melanogaster c-abl proto-oncogene homologue resulted in the isolation of a set of related sequences from the dipteran Calliphora erythrocephala. Although the region encompassing the c-abl protein kinase domain encodes a polypeptide extremely similar to the Drosophila gene, considerable inter- and intraspecific divergence is found adjacent to this region. Restriction-site heterogeneity and cross-hybridization studies between individual cloned isolates suggest that abl homologues represent a small gene family in the Calliphora genome. As is the case in Drosophila, abl-related transcripts appear to be low in abundance, are synthesized during oogenesis and stored as a maternal mRNA.

Animals↗
PubMed1986

3B55: a repetitious sequence family which is transcribed and proportionately replicated in germ-line polyploid nuclei of Calliphora erythrocephala.

The chromosomes of dipteran polyploid nurse cell nuclei are functionally analogous to the oocyte lampbrush chromosomes of the Amphibia. In investigating the transcriptional and replicative activity of these nuclei in Calliphora erythrocephala we have identified a cloned highly repetitious DNA fragment which shows enhanced transcriptional activity in these cells and different replicative behavior in these germ-line polyploid nuclei as opposed to somatic polytene nuclei. The clone, 3B55, contains a 6.8-kb insert which consists primarily of tandemly repeated 200-bp sequences defined by RsaI sites. From Southern hybridizations to diploid (embryonic) genomic DNA, 3B55-related DNA was calculated to represent a significant fraction of the haploid genome (0.8% or 5000 kb). In situ hybridizations established that these sequences are present in the pericentric regions of four of the six chromosomes. Thus the 3B55 sequences have the properties of a satellite-type DNA family. Quantitation of the 3B55 200-bp monomer (which represents approximately 60% of the genomic 3B55 DNA sequences in all tissues examined) revealed that in somatic polytene salivary gland nuclei, 3B55 DNA is highly under-replicated to give a monomer representation of only 48 +/- 11 kb per haploid genome. However, in germ-line nurse cell nuclei, 3B55 DNA is proportionately replicated to give a monomer genomic representation (3560 +/- 344 kb) equivalent to that of diploid DNA (3011 +/- 202 kb). Transcripts complementary to 3B55 sequences are at least 25 times more abundant in total nurse cell nuclear RNA than in total embryonic nuclear RNA. These findings suggest an association of the 3B55 sequence family with some germ-line specific function.

Animals↗
PubMed1985

A stochastic mechanism controls the relative replication of equally competent ribosomal RNA gene sets in individual dipteran polyploid nuclei.

The endoreplication of the two nucleolar organizers (NOs) of the diploid genome has been examined in individual polyploid nuclei of the dipteran Calliphora erythrocephala. Crosses between two strains with diagnostic nontranscribed spacer polymorphisms in their rRNA genes were used to provide progeny with distinguishable NOs, and single nuclei of two highly polyploid cell types--salivary gland and nurse cells--were examined from individual F1 animals. Initially the representation of the two NOs in total polyploid tissue DNA was determined. This revealed that, although the NO regions present in one of the strains (Tom) were very similar in spacer composition, they displayed two types of behavior in the hybrids containing the single NO region typical of the second strain (Karla). In TW phenotype F1 progeny, very little replication of the Tom NO relative to the Karla NO occurred, whereas in TS phenotype progeny replication of the Tom and Karla NOs was approximately equivalent. When individual polyploid nuclei of the TS phenotype animals were examined, however, the relative replication of the Tom and Karla NOs was found not to be a fixed genetic property but to vary dramatically from cell to cell. This was true even for the nurse cell nuclei within a single ovarian follicle, which are the products of only four mitotic divisions of a single germ-line cell. These findings indicate that for NOs of similar replicative competence, a stochastic mechanism governs the relative usage of each NO for endoreplication and that the relative activity of the two NOs is not stably determined through the mitotic divisions preceding polyploidization. Stochastic selection after mitotic DNA replication could be a general phenomenon governing the relative usage (transcription) of different, but equally competent, alleles of any gene in individual cells, if the required factors are in short supply.

Animals↗
PubMed1985

Both nucleolar organizers are replicated in Dipteran polyploid tissues: a study at the level of individual nuclei.

Working with the Dipteran Calliphora erythrocephala, we have tested the hypothesis that only one nucleolar organizer region (NO) is replicated during polyploidization. NO replication was examined in two very different highly polyploid nuclear types: salivary gland nuclei and nurse cell nuclei. Two strains of the organism containing NO regions with highly diagnostic nontranscribed spacer (NTS) polymorphisms were prepared and reciprocal single pair-matings between members of the strains were performed. The representation of the two distinguishable NOs in diploid and polyploid DNAs of individual F1 progeny from each cross was then examined. DNA from a total polyploid nuclear DNA preparation and from individual polyploid nuclei of both tissue types was analyzed. Our results show conclusively that both genomic NOs are replicated in individual polyploid nuclei of both types. Further, evidence for variation in the relative replication of cistrons from the two NOs by individual nuclei was obtained. The cistron types present in the NOs of both strains showed differential replication upon polyploidization. In general, the patterns of differential cistron replication seen in salivary gland and nurse cell nuclei were similar.

Animals↗
PubMed1984

Conserved 5' flank homologies in dipteran 5S RNA genes that would function on 'A' form DNA.

We have sequenced the 480 base pair (bp) repeating unit of the 5S RNA genes of the Dipteran fly Calliphora erythrocephala and compared this sequence to the three known 5S RNA gene sequences from the Dipteran Genus Drosophila (1,2). A striking series of five perfectly conserved homologies identically positioned within the 5' flanks of all four Dipteran 5S RNA coding regions has thus been identified. The spacing (12-13 bp) between all of these homologies is typical of A form rather than B form DNA. Given that the eukaryotic 5S RNA gene specific initiation factor TFIIIA (3) is a DNA unwinding protein (4), a role for these Dipteran 5' flank homologies in initiation site selection on 5S RNA genes transiently unwound for transcription is suggested. One of the Dipteran homology blocks is highly conserved in sequence and position in all but one of the eukaryotic 5S RNA gene sequences known to date (17/18 genes). Its sequence (consensus: TATAAG) and position (average center: -26 bp) are highly reminiscent of the polymerase II gene 'TATA' box (5).

Animals↗
PubMed1984

The intron boundaries and flanking rRNA coding sequences of Calliphora erythrocephala rDNA.

We have sequenced the available cloned examples of the intron-coding sequence junctions for the rDNA of the higher Dipteran, Calliphora erythrocephala. The introns interrupt the rDNA at the same position as the type 1 intron family detected in Drosophila melanogaster and D. virilis (10,11). A duplication of 14 base pairs of the 28S rRNA coding sequence surrounds a short version of the major genomic length class of introns. This same duplication is associated with boundaries of the type 1 introns in D. virilis and D. melanogaster (10, 13,14). We have detected considerable homology between the 3' intron sequences of C. erythrocephala and D. virilis. The rRNA coding sequences flanking the introns are extremely homologous in C. erythrocephala, D. melanogaster and D. virilis, with only one small region of significant divergence. This corresponds to a variable stem region previously identified in eukaryotic 28S rRNA at a site analogous to the L1 ribosomal protein binding site of prokaryotic 23S rRNA (27).

Animals↗
PubMed1984

Different chromatin states of the intron- and type 1 intron+ rRNA genes of Calliphora erythrocephala.

In most species of dipteran fly examined, a fraction of the rDNA cistrons are interrupted by introns. These dipteran intron+ rRNA genes are unique in that they are transcriptionally inactive. Previous studies have investigated the mechanism underlying this transcriptional repression for rRNA genes carrying the best characterized sequence family of such introns, the so-called type 1 introns first identified in Drosophila melanogaster. These studies have established that cloned examples of both intron-free and type 1 intron+ rRNA genes will support transcription in a cell-free system and suggest therefore that a difference in the chromatin state of the two gene types must underlie their very different potential for in vivo transcription. We have examined this possibility for the type 1 intron+ rDNA cistrons of Calliphora erythrocephala by in situ hybridization studies using the polytene chromosome complement of the pupal bristle-forming (trichogen) cells. These studies show that the chromatin configuration of the two gene types is strikingly different. The intron-free genes are preferentially localized in the actively transcribed fibrillar center of the nucleolus. The intron+ genes are preferentially condensed in the blocks of heterochromatin attached to the nucleolus.

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