Biomedical subjects

H Tobler

Publications and source records attributed to H Tobler.

At least 19 recordsLinked to original sources

PubMed2000

Chromatin diminution in the parasitic nematodes ascaris suum and parascaris univalens.

Chromatin diminution in Parascaris univalens and Ascaris suum undoubtedly represents an interesting case of developmentally programmed DNA rearrangement in higher eukaryotes. It is a complex mechanism involving chromosomal breakage, new telomere addition and DNA degradation, and occurs in all presomatic cells. The process is rather specific with respect to its developmental timing and the chromosomal regions that are eliminated. The functional significance of chromatin diminution still remains an enigma. The fact, however, that single-copy, protein-coding genes are contained in the eliminated DNA demonstrates that in P. univalens and A. suum, there is a qualitative difference between germ-line and somatic genomes, and suggests that chromatin diminution may be used as a "throw-away" approach to gene regulation. We present a hypothesis as to how, during evolution, a partial genome duplication might have been linked to the process of chromatin diminution, in order to provide a selective advantage to parasitic DNA-eliminating nematodes.

Animals↗

PubMed2000

The C. elegans Mi-2 chromatin-remodelling proteins function in vulval cell fate determination.

The Mi-2 protein is the central component of the recently isolated NuRD nucleosome remodelling and histone deacetylase complex. Although the NuRD complex has been the subject of extensive biochemical analyses, little is known about its biological function. Here we show that the two C. elegans Mi-2 homologues, LET-418 and CHD-3, play essential roles during development. The two proteins possess both shared and unique functions during vulval cell fate determination, including antagonism of the Ras signalling pathway required for vulval cell fate induction and the proper execution of the 2 degrees cell fate of vulval precursor cells, a process under the control of LIN-12 Notch signalling.

Adenosine Triphosphatases↗

PubMed1999

Developmentally regulated telomerase activity is correlated with chromosomal healing during chromatin diminution in Ascaris suum.

Telomerase is the ribonucleoprotein complex responsible for the maintenance of the physical ends, or telomeres, of most eukaryotic chromosomes. In this study, telomerase activity has been identified in cell extracts from the nematode Ascaris suum. This parasitic nematode is particularly suited as a model system for the study of telomerase, because it shows the phenomenon of chromatin diminution, consisting of developmentally programmed chromosomal breakage, DNA elimination, and new telomere formation. In vitro, the A. suum telomerase is capable of efficiently recognizing and elongating nontelomeric primers with nematode-specific telomere repeats by using limited homology at the 3' end of the DNA to anneal with the putative telomerase RNA template. The activity of this enzyme is developmentally regulated, and it correlates temporally with the phenomenon of chromatin diminution. It is up-regulated during the first two rounds of embryonic cell divisions, to reach a peak in 4-cell-stage embryos, when three presomatic blastomeres prepare for chromatin diminution. The activity remains high until the beginning of gastrulation, when the last of the presomatic cells undergoes chromatin diminution, and then constantly decreases during further development. In summary, our data strongly argue for a role of this enzyme in chromosome healing during the process of chromatin diminution.

Animals↗

PubMed1999

Anterior organization of the Caenorhabditis elegans embryo by the labial-like Hox gene ceh-13.

The Caenorhabditis elegans lin-39, mab-5 and egl-5 Hox genes specify cell fates along the anterior-posterior body axis of the nematode during postembryonic development, but little is known about Hox gene functions during embryogenesis. Here, we show that the C. elegans labial-like gene ceh-13 is expressed in cells of many different tissues and lineages and that the rostral boundary of its expression domain is anterior to those of the other Hox genes. By transposon-mediated mutagenesis, we isolated a zygotic recessive ceh-13 loss-of-function allele, sw1, that exhibits an embryonic sublethal phenotype. Lineage analyses and immunostainings revealed defects in the organization of the anterior lateral epidermis and anterior body wall muscle cells. The epidermal and mesodermal identity of these cells, however, is correctly specified. ceh-13(sw1) mutant embryos also show fusion and adhesion defects in ectodermal cells. This suggests that ceh-13 plays a role in the anterior organization of the C. elegans embryo and is involved in the regulation of cell affinities.

Amino Acid Sequence↗

PubMed1997

The expression of the C. elegans labial-like Hox gene ceh-13 during early embryogenesis relies on cell fate and on anteroposterior cell polarity.

Clusters of homeobox-containing HOM-C/hox genes determine the morphology of animal body plans and body parts and are thought to mediate positional information. Here, we describe the onset of embryonic expression of ceh-13, the Caenorhabditis elegans orthologue of the Drosophila labial gene, which is the earliest gene of the C. elegans Hox gene cluster to be activated in C. elegans development. At the beginning of gastrulation, ceh-13 is asymmetrically expressed in posterior daughters of anteroposterior divisions, first in the posterior daughter of the intestinal precursor cell E and then in all posterior daughters of the AB descendants ABxxx. In this paper, we present evidence that supports position-independent activation of ceh-13 during early C. elegans embryogenesis, which integrates cell fate determinants and cell polarity cues. Our findings imply that mechanisms other than cell-extrinsic anteroposterior positional signals play an important role in the activation and regulation of the C. elegans Hox gene ceh-13.

Animals↗

PubMed1996

cec-1, a soma-specific chromobox-containing gene in C. elegans.

The chromo domain is a phylogenetically conserved sequence motif which was identified as a region of homology between the repressor protein Pc and the heterochromatin constitutive protein HP1 of Drosophila. The specific function of the chromo domain is not yet understood, but it seems to be required for protein-protein interactions in chromatin-associated complexes. Here, we present a new chromobox-containing gene from Caenorhabditis elegans (cec-1). It encodes a nuclear protein that is present in all somatic cells from the 50- to 80-cell stage on throughout development and in adult animals. No cec-1 protein was detected in the cells of early embryos, in germ cells, and in their precursor cells Z2 and Z3. cec-1 mRNA, however, is already present in all the blastomeres of early embryos. Immunolocalization experiments revealed a homogeneous distribution of CEC-1 within interphase nuclei, while during mitosis CEC-1 seems to dissociate from the condensing chromosomes. The expression pattern of the cec-1 gene suggests that it may represent a new regulatory gene in C. elegans.

Amino Acid Sequence↗

PubMed1996

Telomeric repeats (TTAGGC)n are sufficient for chromosome capping function in Caenorhabditis elegans.

Telomeres are specialized structures located at the ends of linear eukaryotic chromosomes that ensure their complete replication and protect them from fusion and degradation. We report here the characterization of the telomeres of the nematode Caenorhabditis elegans. We show that the chromosomes terminate in 4-9 kb of tandem repeats of the sequence TTAGGC. Furthermore, we have isolated clones corresponding to 11 of the 12 C. elegans telomeres. Their subtelomeric sequences are all different from each other, demonstrating that the terminal TTAGGC repeats are sufficient for general chromosomal capping functions. Finally, we demonstrate that the me8 meiotic mutant, which is defective in X chromosome crossing over and segregation, bears a terminal deficiency, that was healed by the addition of telomeric repeats, presumably by the activity of a telomerase enzyme. The 11 cloned telomeres represent an important advance for the completion of the physical map and for the determination of the entire sequence of the C. elegans genome.

Animals↗

PubMed1996

Chromatin diminution in nematodes.

The process of chromatin diminution in Parascaris and Ascaris is a developmentally controlled genome rearrangement, which results in quantitative and qualitative differences in DNA content between germ line and somatic cells. Chromatin diminution involves chromosomal breakage, new telomere formation and DNA degradation. The programmed elimination of chromatin in presomatic cells might serve as an alternative way of gene regulation. We put forward a new hypothesis of how an ancient partial genome duplication and chromatin diminution may have served to maintain the genetic balance in somatic cells and simultaneously endowed the germ line cells with a selective advantage.

Animals↗

PubMed1996

A newly formed telomere in Ascaris suum does not exert a telomere position effect on a nearby gene.

During the process of chromatin diminution in Ascaris suum (formerly named Ascaris lumbricoides var. suum), developmentally regulated chromosomal fragmentation and new telomere addition occur within specific chromosomal breakage regions (CBRs). The DNA sequences flanking one of these CBRs (CBR-1) were analyzed, and two protein-encoding genes were found on either side. The noneliminated gene, agp-1, whose AUG start codon is located within approximately 2 kb of the boundary of CBR-1, encodes a putative GTP-binding protein which is structurally related to eukaryotic and prokaryotic elongation factors. Northern (RNA) blot analyses revealed that transcripts of this gene are present at all developmental stages, suggesting that the massive chromosomal rearrangements associated with the process of chromatin diminution have no influence on agp-1 expression. This demonstrates that addition of new telomeres in CBR-1 does not result in a telomeric position effect, a phenomenon previously described in Saccharomyces cerevisiae.

Amino Acid Sequence↗

PubMed1994

Tas, a retrotransposon from the parasitic nematode Ascaris lumbricoides.

The cloned retrotransposon Tas OE3 from the genome of the parasitic nematode Ascaris lumbricoides was completely sequenced. The element is flanked by long terminal repeats (LTR) and contains three distinct regions encoding putative proteins typical for retroid elements. The first region, ORF1, encodes a putative Gag protein including a 'Leu zipper', a nucleic acid binding motif, as well as an aspartic protease domain. The second region contains an incomplete ORF (ORF2) with sequence similarities to known retroviral reverse transcriptases (RT), ribonucleases H and integrases. A third ORF, which is located adjacent to the 3' LTR, might encode an env-like protein. Based on amino-acid sequence analysis of the RT domain, Tas falls into a new subgroup of LTR-containing retrotransposons.

Amino Acid Sequence↗

PubMed1994

Caenorhabditis elegans unc-51 gene required for axonal elongation encodes a novel serine/threonine kinase.

Mutations in the unc-51 gene of the nematode Caenorhabditis elegans result in various abnormalities in axonal elongation and axonal structures. We cloned the unc-51 gene by tagging with the transposon Tc1. The wild-type unc-51 gene, which rescued the mutant phenotypes, encodes a novel serine/threonine kinase of 856 amino acids. Mutation sites were identified in the unc-51 gene of six mutants. A Lys-->Met mutation created in vitro in the kinase domain led to the loss of rescuing activity and was dominant negative, indicating that the kinase domain of Unc-51 is essential for the function. Expression of an unc-51/lacZ fusion gene was observed in many neurons at all stages. We propose that protein phosphorylation by the unc-51 product is important for axonal elongation and possibly for axonal guidance.

Alleles↗

PubMed1994

Ribosomal heterogeneity from chromatin diminution in Ascaris lumbricoides.

The genome of Ascaris lumbricoides encodes both germline- and soma-specific proteins homologous to the eukaryotic small ribosomal protein (Rp) S19. The two Ascaris homologs differ by 24 amino acid substitutions and are both components of the small ribosomal subunits. In oocytes, the germline RpS19 homolog (RpS19G) predominates. During chromatin diminution, however, the gene is eliminated from all presomatic cells, and RpS19G is replaced by the product of the somatic gene (RpS19S). Chromatin diminution in A. lumbricoides causes a change in the protein composition of ribosomes during development and represents an alternative means of gene regulation.

Amino Acid Sequence↗

PubMed1994

Extremely stable transcripts may compensate for the elimination of the gene fert-1 from all Ascaris lumbricoides somatic cells.

The single-copy gene fert-1 becomes eliminated from all somatic cells during the process of chromatin diminution in Ascaris lumbricoides var. suum. By using Northern blot and in situ hybridization techniques, we have analyzed its rather unusual expression pattern. Different splicing and 3' end formation events generate in a developmentally regulated manner various poly(A)+ and poly(A)- fert-1 RNA species. The lack of any significant open reading frame in most of its RNA products indicates that fert-1 may function as structural RNA rather than encoding a protein. Fert-1 transcripts are produced in the precursors of the gametes, but degraded at the time of meiosis and not passed on to the zygote. Embryonic transcription of fert-1 sets in as soon as the female nucleus has completed its meiosis. Our data thus demonstrate that the Ascaris transcription apparatus is active prior to the general onset of zygotic transcription, which we think takes place in the four- to six-cell-stage embryos. Upon elimination of fert-1 gene from the somatic cells, most of its transcripts disappear. Two short fert-1 RNA products, however, are stably maintained throughout development until the second larval stage, which is more than 1 month after the elimination of their coding sequences. Possible functions of fert-1 are discussed.

Amino Acid Sequence↗

PubMed1992

The two main rDNA size classes of Ascaris lumbricoides: comparison of transcription termination and spacer organization.

Structural and functional analyses were carried out to compare transcription termination and intergenic spacer organization between the two heterogeneous ribosomal DNA size classes of Ascaris lumbricoides. By performing mung bean nuclease mapping in vivo, we localized the 3' end of the mature 26S rRNA to the same position in both forms. This site coincides with the in vivo and in vitro transcription termination site of the 40S-precursor rRNA from both rDNA size classes. We demonstrate that the 3' boundary of sequences necessary for faithful transcription termination extends 35 bp beyond the end of the 26S rRNA gene and that these sequences are perfectly conserved within the two size classes. The intergenic spacers of both rDNA size classes have been completely sequenced; they are 2410 bp and 1937 bp long and show a sequence homology of 92.8%. There is no evidence for the presence of reiterated termination sites or promoters within the relatively short intergenic spacers of the two rRNA gene classes.

Animals↗

PubMed1992

Unusual features of the retroid element PAT from the nematode Panagrellus redivivus.

The PAT retroid transposable elements differ from other retroids in that they have a 'split direct repeat' structure, i.e., and internal 300bp sequence is found repeated, about one half at each element extremity. A very abundant transcript of about 900 nt, the start of which maps to the preferentially deleted portion of PAT elements, is detected on total Panagrellus redivius RNA bearing Northern blots. A potentially corresponding ORF encodes a protein of 265 residues having a carboxy terminal Cystein motif, believed to be exclusively characteristic of the GAG protein in retoid elements. A much fainter, 1800nt long transcript, is also detected on Northern blots and maps slightly downstream of the first ORF. The predicted protein sequence of this region bears motifs typical of reverse transcriptase and RNaseH, as found in the Pol genes of retroid elements. Peptide motif similarities are greatest with the DIRS-1 element derived from Dictyostelium discoideum. The possibility of using PAT elements as transposon tagging system for Caenorhabditis elegans is discussed.

Amino Acid Sequence↗

PubMed1992

Chromatin diminution in nematode development.

Chromatin diminution in Parascaris and Ascaris represents the classical case of a developmentally programmed genome rearrangement. The process is very specific with respect to ontogenetic timing and chromosomal localization, and involves chromosomal breakage, new telomere formation and DNA degradation. Recent evidence from Ascaris lumbricoides var. suum suggests that chromatin diminution might have a function in gene regulation.

Animals↗

PubMed1992

Nucleotide sequence of PAT, a retroid element with unusual DR organization, isolated from Panagrellus redivivus.

We have isolated several copies of the transposable element PAT of Panagrellus redivivus and sequenced one full length, presumably autonomous, 5514 bp entity. The terminal sequences are found repeated inside the element, probably representing the homologous of the long terminal repeats in common retroid elements. Two major open reading frames are present with features typical of GAG and Pol. Both the structural features and open reading frame characteristics assign PAT to the retroid family of transposable elements, and more precisely to the gypsy class of retroids when putative functional domains of Pol are compared to published sequences.

Amino Acid Sequence↗

PubMed1991

New telomere formation after developmentally regulated chromosomal breakage during the process of chromatin diminution in Ascaris lumbricoides.

During the process of chromatin diminution, which takes place in all presomatic cells of the early Ascaris embryo, the heterochromatic termini of the chromosomes are lost. Here we show that the newly formed ends of the reduced somatic chromosomes carry tandem repeats of the telomeric sequence TTAGGC. Comparison of a cloned somatic telomere with the corresponding germline chromosomal region revealed that these telomeric repeats are not present at or near the chromosomal breakage site. They are most likely added by a telomerase-mediated event. Chromosomal breakage, which precedes the telomere addition process, takes place within a short, specific chromosomal region (CBR); however, it does not occur at a single locus, but rather at many different sites. Altogether, our data show that chromatin diminution in Ascaris is a complex molecular process that includes site-specific chromosomal breakage, new telomere formation, and DNA degradation.

Animals↗