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We report in this paper that the pentoxyresorufin- O-dealkylase (PROD) protein, encoded by the gene proliferation disrupter ( prod), is associated with the telomeric chromatin in Drosophila melanogaster. It binds to a region just upstream of the promoter of the telomere-specific retrotransposon HeT-A, which is located in the long 3′untranslated region of the element near its oligo(A) tail. Reduction of PROD in prod heterozygote flies results in elevated levels of HeT-A RNA in the ovaries, suggesting that PROD functions as a repressor of HeT-A transcriptional activity at the telomeres. [ABSTRACT FROM AUTHOR]

Chromosome length in Drosophila is maintained by targeted transposition of three non-long terminal repeat retrotransposons, HeT-A, TART, and TAHRE, to the chromosome ends. The length and composition of these retrotransposon arrays can vary significantly between chromosome tips and between fly stocks, but the significance and consequences of these length differences are not understood. A dominant genetic factor, Tel, has been described, which causes a severalfold elongation of the retrotransposon arrays at all telomeres. We used this strain to assess possible affects of extended telomeres on the organism. While we found no effect on life span of the adults, we could demonstrate a correlation between long telomeres and reduced fertility and fecundity in individual females, which is also reflected in abnormal oocyte development. [ABSTRACT FROM AUTHOR]

Drosophila melanogaster extends its telomeres by transposition of two non-LTR retrotransposons, HeT-A and TART, to chromosome ends. We have determined the tissue-specific expression of these two elements by whole-mount in situ hybridization with digoxigenin-labeled RNA sense and antisense probes in the germ line and in a variety of larval tissues during normal development in the wild type and in tissues of mutants that cause overproliferation. Our results indicate that transcript levels, which are a key component in the process of telomere elongation in D. melanogaster, are correlated with cell proliferation in normal tissues and that RNA levels are elevated in growth-stimulated tissues. [ABSTRACT FROM AUTHOR]

The terminal DNA arrays on chromosomes of Drosophila melanogaster are composed of two families of non-LTR retrotransposons, HeT-A and TART. Available evidence suggests that chromosome length in this species and its close relatives is maintained by targeted transposition of these elements, with attachment of the elements to the chromosome end by their 3′ oligo(A) tails. However, the regulation of transposition of these elements and the control of telomere length are poorly understood. Here we present the hypothesis that the forces involved in telomere length regulation in Drosophila are the underlying forces that manifest themselves as telomeric position effect (TPE). Based on recent studies of TPE, which found that expression of a reporter gene is influenced by telomere structure in cis and trans, we propose that the subtelomeric satellite (TAS) in D. melanogaster plays an important role in controlling telomere elongation. Transcription of a HeT-A element is probably initiated at a promoter in the 3′ UTR of an upstream element, and TAS may repress this transcriptional activity in cis and trans. A region of HeT-A not at the extreme 3′ end of the element may act as a transcriptional enhancer that may be modulated by TAS. [ABSTRACT FROM AUTHOR]

Telomere elongation by telomerase is the most widespread mechanism among eukaryotes. However, alternative mechanisms such as homologous recombination between terminal satellite DNAs are probably used in lower dipteran insects and in some plants. Drosophila melanogaster uses the very unusual telomere elongation pathway of transposition of telomere-specific retrotransposable elements. The uniqueness of this telomere elongation mechanism raises the question of its origin. We, therefore, analyzed sequences located at telomeres of fairly distantly related Drosophila species, and in this paper we describe the characterization of complex satellite DNA sequences located at the telomeres of D. virilis and other species in the virilis group. We suggest an involvement of these DNA satellites in telomere elongation by homologous recombination similar to that found in lower dipterans. Our findings raise the possibility that telomere elongation by specific retrotransposons as found in D. melanogaster and its sibling species is a recent event in the evolution of dipteran insects. [ABSTRACT FROM AUTHOR]

Chromosome length in Drosophilais maintained by the targeted transposition of two families of non-LTR retrotransposons, HeT-Aand TART. Although the rate of transposition to telomeres is sufficient to counterbalance loss from the chromosome ends due to incomplete DNA replication, transposition as a mechanism for elongating chromosome ends raises the possibility of damaged or deleted telomeres, because of its stochastic nature. Recent evidence suggests that HeT-Atransposition is controlled at the levels of transcription and reverse transcription. HeT-Atranscription is found primarily in mitotically active cells, and transcription of a w +reporter gene inserted into the 2L telomere increases when the homologous telomere is partially or completely deleted. The terminal HeT-Aarray may be important as a positive regulator of this activity in cis, and the subterminal satellite appears to be an important negative regulator in cis. A third chromosome modifier has been identified that increases the level of reverse transcriptase activity on a HeT-A RNA template and greatly increases the transposition of HeT-A. Thus, the host appears to play a role in transposition of these elements. Taken together, these results suggest that control of HeT-Atransposition is more complex than previously thought. [ABSTRACT FROM AUTHOR]

The telomeric regions in Drosophila cause transcriptional silencing of integrated transgenes. A complex satellite has recently been identified in the subterminal region of the left arm of chromosome 2 that is a good candidate for the source of the observed telomeric silencing, because genetically marked transposable elements that have inserted into this subtelomeric array show repression and variegation of the reporter gene. We asked whether this satellite can also cause transcriptional repression in ectopic chromosomal positions by placing it upstream of a mini- white reporter gene in P element constructs used for germ line transformation. The transgenes are shielded from external influences at the integration site using SU(HW) binding sites at either end. It was found that the satellite represses transcription of the reporter gene in an orientation dependent and an array length dependent manner. The satellite does not, however, induce variegation under the conditions used. The repressed transgenes do not respond to typical modifiers of centromeric position effect variegation, such as Su(var)205 5 , Su(var)2–1 1 , Su(var)3–1 1 , and Su(var)3–6 1 , or to the addition of a Y chromosome. However, as with the original variegating telomeric insertion, suppression in the transgenes is relieved by Su(z)2 5 , suggesting that suppression induced by the subtelomeric satellite retains aspects of telomeric silencing in ectopic positions. [ABSTRACT FROM AUTHOR]

Using recombinant DNA technology we have isolated a 4.4 kb DNA fragment from Drosophila melanogaster which can be localized by in situ hybridization to the region 80C on the left arm of chromosome III. This DNA fragment codes for a 1.4 kb long poly(A)-containing RNA which comprises about 0.6% of the mass of cytoplasmic poly(A) RNA in K cells and Oregon R Embryos. This RNA codes for a 26,000 MW protein of still unknown function. [ABSTRACT FROM AUTHOR]

Telomeres are nucleoprotein structures at the ends of eukaryotic chromosomes that perform a number of vital functions. They allow a cell to distinguish between natural chromosome ends and chromosome breaks in order to delay the cell cycle and repair the broken end. Telomeres also compensate for the inability of DNA polymerase to replicate the chromosome completely. In most eukaryotes a special reverse transcriptase, telomerase, adds telomeric DNA repeats to the chromosome ends using an internal RNA template. However, evidence is accumulating for alternative elongation mechanisms in a variety of eukaryotes. In the yeast Saccharomyces cerevisiae, and possibly in humans, both of which normally use telomerase, a different mechanism can be used for chromosome length maintenance when telomerase is inactive or inactivated. Yeast apparently uses recombination for this purpose; the mechanism in humans is not known. Some insect and plant species, on the other hand, do not use telomerase as their primary mechanism for maintaining chromosome length. Drosophila makes use of specific retrotransposons for this purpose, while other dipterans use recombination. We summarize here the current knowledge of these alternative telomere elongation mechanisms. [ABSTRACT FROM AUTHOR]

Chromosomes not only carry transcribed genes and their regulatory DNA sequences, but also contain regions that are required for the stability and maintenace of the chromosome as a unit. These include centromeres, telomeres and origins of replication. It is clear for replication origins and centromeres that the positions of these chromosomal organelles are determined by sites of the appropriate DNA sequences, but also that functional performance requires one or more contributing proteins. Telomeres are also structurally complex, with one or more DNA components, including simple telomeric repeats and more complex telomere-associated sequences, as well as one or more specific proteins that recognize these sequences. Accumulating evidence suggests that the simple telomeric repeats are required in most, but not all species, although they are not sufficient to determine the chromosomal position of a telomere. [ABSTRACT FROM AUTHOR]

HeT-A elements are Drosophila melanogaster LINE-like retroposons that transpose to broken chromosome ends by attaching themselves with an oligo(A) tail. Since this family of elements is believed to be involved in the vital function of telomere elongation in Drosophila, it is important to understand their transposition mechanism and the molecular aspects of activity. By comparison of several elements we have defined here the unit length of HeT-A elements to be approximately 6 kb. Also, we studied an active HeT-A element that had transposed very recently to the end of a terminally deleted X chromosome. The 12 kb of newly transposed DNA consisted of a tandem array of three different HeT-A elements joined by oligo(A) tails to each other and to the chromosome end broken in the yellow gene. Such an array may have transposed as a single unit or resulted from rapid successive transpositions of individual HeT-A elements. By sequence comparison with another recently transposed HeT-A element, conserved domains in the single open reading frame (ORF), encoding a gag-like polypeptide, of these elements were defined. We conclude that for transposition an intact ORF is required in cis, while the reverse transcriptase is not encoded on the HeT-A element but is provided in trans. This would make HeT-A elements dependent on an external reverse transcriptase for transposition and establish control of the genome over the activity of HeT-A elements. This distinguishes the Drosophila HeT-A element, which has been implicated in Drosophila telomere elongation, from the other, 'selfish' LINE-like elements. [ABSTRACT FROM AUTHOR]

Members of the Drosophila HeT-A family of transposable elements are LINE-like retroposons that are found at telomeres and in centric heterochromatin. We recently characterized an active HeT-A element that had transposed to a broken chromosome end fewer than mine generations before it was isolated. The sequence arerangement of this element, called 9D4, most likely represents the organization of an actively transposing member of the HeT-A family. Here we assess the degree of divergence among members of the HeT-A family and test a model of telomere length maintenance based on HeT-A transposition. The region containing the single open reading frame of this element appears to be more highly conserved than the non-coding regions. The HeT-A element has been implicated in the Drosophila telomere elongation process, because frequent transpositions to chromosome ends are sufficient to counter-balance nucleotide loss due to incomplete DNA replication. The proposed elongation model and the hypothetical mechanism of HeT-A transposition predict a predominant orientation of HeT-A elements with their oligo (A) tails facing proximally at chromosome ends, as well as the existence of irregular tandem arrays of HeT-A elements at chromosome ends resulting from transposition of new HeT-A elements onto chromosome ends with existing elements. Twenty-nine different HeT-A fragments were isolated from directional libraries that were enriched in terminal DNA fragments. Sequence analyses of these fragments and comparisons with the organization of the HeT-A element, 9D4, fit these two predictions and support the model of Drosophila telomere elongation by transposition of HeT-A elements. [ABSTRACT FROM AUTHOR]

Drosophila melanogaster contains a 46 000 MW cytoplasmic protein which is immunologically related to the intermediate filament protein vimentin of vertebrates. A monoclonal antibody raised against this protein was used to study its cross-reactivity with other vertebrate and invertebrate cells. Indirect immunofluorescence showed filamentous meshworks in all species tested. Protein blotting was used to determine the molecular weights of the proteins responsible for the wide range of cross-reactivity of this antibody. We present evidence that vimentin-like proteins are also present in invertebrates and form a cytoplasmic network in Paramecium. Furthermore, we demonstrate in vertebrates and invertebrates the presence of high molecular weight polypeptides which are immunologically related to vimentin. [ABSTRACT FROM AUTHOR]

Using monoclonal antibodies against the major intermediate filament (10 nm) cytoskeletal proteins of Drosophila tissue culture cells, we showed by indirect immunofluorescence and ammuno-electron microscopy that this cytoskeletal material also occurs in a non-filamentous configuration. Patches of fine granular material are detected in the cytoplasm of Kc cells but are absent in another Drosophila cell line (Schneider, line 2). These patches are surrounded by membranes with bound ribosomes, resembling endoplasmic reticulum, and are found throughout the cytoplasm. We suggest that these aggregates are caused by overproduction of intermediate filament material in the Kc cell line. [ABSTRACT FROM AUTHOR]