Worm Breeder's Gazette 11(5): 57
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
The chromosomes of Ascaris lumbricoides undergo chromatin diminution in those cells destined to form the somatic lineages of the animal. During this process, the heterochromatic ends of the chromosomes break off and eventually degrade in the cytoplasm. The shortened chromosomes are stable, suggesting that elimination is accompanied by a de novo formation of telomeres. In order to test this hypothesis, we decided to analyze the structure of the somatic telomeres of A. lumbricoides. Bal 31 experiments revealed that they carry multiple copies of the tandemly repeated hexamer TTAGGC, a sequence that is similar to those found at the chromosomal ends of vertebrates and some lower eukaryotes. Since we expected the regions flanking the somatic telomeres to be involved in the elimination process, we designed a cloning strategy for their isolation from the genome of A. lumbricoides. One of the positive clones obtained (pTel 1) was analyzed in detail. By sequencing we found pTel 1 to carry 27 perfect repeats of the hexamer TTAGGC, oriented 5' to 3' towards the end of the fragment. Thus, the orientation of the G-rich strand corresponds exactly to the defined orientation of eukaryotic telomeric repeats. The subtelomeric DNA sequences of pTel 1 are unique and therefore specific for a single somatic telomere. The corresponding region within the germ line genome is located at an internal chromosomal site rather than on a telomere. Elimination, however, does not take place merely at the locus cloned in pTel 1, but also at many more sites, all of them being scattered throughout a 3 kb long specific DNA segment within this region. The nucleotide sequence of this DNA segment is very AT-rich, but so far no specific elimination signals could be identified. Comparison of pTel 1 with the corresponding germ line sequences shows that chromosomal fragmentation is followed by the addition of tandem arrays of the telomeric repeating units TTAGGC, which are not present on the germ line chromosome at this site. The interesting question, whether this de novo formation of telomeres is carried out by the action of a telomere terminal transferase activity, is currently investigated in our laboratory.