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.

Chromatin Elimination in Ascaris lumbricoides Takes Place Within a Short, Precisely Defined Chromosomal DNA Segment

Fritz Mller, Chantal Wicky and Heinz Tobler

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.