Telomere formation on macronuclear chromosomes of Oxytricha trifallax and O. fallax: alternatively processed regions have multiple telomere addition sites

The Common-Region-flanking positions also differ from the arm-distal positions in that they are "multi-TAS" regions: anchored PCR shows heterogeneous patterns of telomere addition sites, but arm-distal sites do not. The multi-TAS patterns are reproducible, but are sensitive to the sequence of the allele being processed. Thus, random degradation following chromatid cutting does not create this heterogeneity; these telomere addition sites also must be dictated by cis-acting sequences.Most ciliates show such micro-heterogeneity in the precise positions of telomere addition sites. Telomerase is believed to be tightly associated with, and act in concert with, the chromatid-cutting nuclease: heterogeneity must be the result of intervening erosion activity. Our "weak-sites" hypothesis explains the correlation between alternative chromatid cutting at the Common Region boundaries and their multi-TAS character: when the chromatid-breakage machine encounters either a weak binding site or a weak cut site at these regions, then telomerase dissociates prematurely, leaving the new end subject to erosion by an exonuclease, which pauses at cis-acting sequences; telomerase eventually heals these resected termini. Finally, we observe TAS positioning influenced by trans-allelic interactions, reminiscent of transvection.Telomeres cap the tips of eukaryotic chromosomes and protect them from involvement in the double-strand break repair mechanism and in classic breakage-fusion-bridge catastrophes described by McClintock [1]. Telomeres are maintained at full length, against their shortening at replication, by restoration of 3' terminal repeats by telomerase. However, telomerase is not expressed in most mammalian somatic cells, and once somatic lineages expose their chromosome ends, crisis ensues and normally results in lineage senescence. Oncogenic transformation requires the re-expression of telomerase [2]. We study telomeres in ciliated protozoa, because telomeres comprise an unprecedent