The end-to-end association of chromosomes through their telomeres has been observed in normal cells of certain organisms, as well as in senescent and tumor cells. The molecular mechanisms underlying this phenomenon are currently unknown. We show here that five independent mutant alleles in the Drosophila UbcDl gene cause frequent telomere-telomere attachments during both mitosis and male meiosis that are not seen in wild type. These telomeric associations involve all the telomeres of the D. melanogaster chromosome complement, albeit with different frequencies. The pattern of telomeric associations observed in UbcDl mutants suggests strongly that the interphase chromosomes of wild-type larval brain cells maintain a Rabl orientation within the nucleus, with the telomeres and centromeres segregated to opposite sides of the nucleus. The UbcDl gene encodes a class I ubiquitin-conjugating (E2) enzyme. This indicates that ubiquitin-mediated proteolysis is normally needed to ensure proper telomere behavior during Drosophila cell division. We therefore suggest that at least one of the targets of UbcDl ubiquitination is a telomere-associated polypeptide that may help maintain proper chromosomal orientation during interphase.
A 190 bp insertion is associated with the white-eosin mutation in Drosophila melanogaster. This insertion is a member of a family of transposable elements, pogo elements, which is of the same class as the P and hobo elements of D. melanogaster. Strains typically have many copies of a 190 bp element, 10-15 elements 1.1-1.5 kb in size and several copies of a 2.1 kb element. The smaller elements all appear to be derived from the largest by single internal deletions so that all elements share terminal sequences. They either always insert at the dinucleotide TA and have perfect 21 bp terminal inverse repeats, or have 22 bp inverse repeats and produce no duplication upon insertion. Analysis by DNA blotting of their distribution and occupancy of insertion sites in different strains suggests that they may be less mobile than P or hobo. The DNA sequence of the largest element has two long open reading frames on one strand which are joined by splicing as indicated by cDNA analysis. RNAs of this strand are made, whose sizes are similar to the major size classes of elements. A protein predicted by the DNA sequence has significant homology with a human centrosomal-associated protein, CENP-B. Homologous sequences were not detected in other Drosophila species, suggesting that this transposable element family may be restricted to D. melanogaster.
A 1-5 kilobasepair repeated DNA sequence is duplicated in direct orientation so as to flank the suppressor of forked gene in the euchromatin-heterochromatin transition region on the X chromosome of Drosophila melanogaster. These two copies are almost identical, but DNA blotting, analysis of cloned sequences and database searches show that elsewhere in the genome, homologous sequences are poorly conserved. They are often associated with other repeats, suggesting that they may belong to a scrambled and clustered middle repetitive DNA family. The sequences do not appear to be related to transposable elements and their location in different strains is conserved. In situ hybridization to metaphase chromosomes shows that homologous sequences are concentrated in the pericentric regions of the autosomes and the X chromosome. The sequences are not significantly under-represented in DNA from polytene tissue and must lie in the replicated regions of polytene chromosomes. The almost perfect conservation of the two repeats around suppressor of forked in D. melanogaster suggests they arose by duplication or gene conversion. Suppression of recombination in this chromosomal region presumably allows this unusual organization to be stably maintained. In the X-ray induced allele, suppressor offorked-L26, the sequence between the repeats, including the gene, and one copy of the repeat have been deleted.Corresponding
We present an analysis of a chromosomal walk in the region of the euchromatin-heterochromatin transition at the base of the X chromosome of Drosophila melanogaster. This region is difficult to analyse because of the presence of repeated sequences, and we have used cosmids to walk from the last euchromatic gene, suppressor of forked, towards the pericentric heterochromatin. The proximal 30-kb sequence we have isolated consists of repetitive DNA, including four tandem copies of a 5.9-kb sequence. This tandem repeat is itself a mosaic of other, mostly repeated, sequences, including part of a retrotransposon without long terminal repeats, a simple-sequence region of TAA repeats and part of a retrotransposon with long terminal repeats that has not been previously described. Although sequences homologous to these components are found elsewhere in the genome, this arrangement of repeated sequences is only found at the base of the X chromosome. It is conserved in D. melanogaster strains of different geographic origin, but is not conserved in even closely related species.
We have used three cloned DNA sequences consisting of (1) part of the suppressor of forked transcription unit, (2) a cloned 359-bp satellite, and (3), a type I ribosomal insertion, to examine the structure of the base of the X chromosome of Drosophila melanogaster where different chromatin types are found in juxtaposition. A DNA probe from the suppressor of forked locus hybridizes exclusively to the very proximal polytenized part of division 20, which forms part of the beta-heterochromatin of the chromocenter. The cloned 359-bp satellite sequence, which derives from the proximal mitotic heterochromatin between the centromere and the ribosomal genes, hybridizes to the under replicated alpha-heterochromatin of the chromocenter. The type I insertion sequence, which has major locations in the ribosomal genes and in the distal mitotic heterochromatin of the X chromosome, hybridizes as expected to the nucleolus but does not hybridize to the beta-heterochromatic division 20 of the polytene X chromosome. Our molecular data reveal that the suppressor of forked locus, which on cytogenetic grounds is the most proximal ordinary gene on the X chromosome, is very close to the junction of the polytenized and non-polytenized region of the X chromosome. The data have implications for the structure of beta-heterochromatin-alpha-heterochromatin junction zones in both mitotic and polytene chromosomes, and are discussed with reference to models of chromosome structure.
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