SUMMARY
DNA looping is one of the mechanisms involved in telomere maintenance. It probably provides a
solution not only to 'the end-replication problem', but also for the protection of chromosomal ends
against degradation enzymes and, as typical double-strand breaks, from DNA repair machinery.
Telomeric loops (t-loops) formed by an invasion of protruding 3' overhangs into the doublestranded
telomeric regions were observed in a variety of organisms ranging from ciliates to
mammals. Genetic data indicate that looping also occurs at the telomeres of Saccharomyces
cerevisiae, suggesting its importance for telomere function in yeast. However, several observations
argue against the presence of 'true' t-loops in the budding yeast telomeres (e.g. the lack of TRF-like
protein, heterogeneous telomeric sequences). Instead, telomeres in S. cerevisiae appear to form
fold-back structures mediated by protein-protein interactions. To directly visualize the telomeric
structure in budding yeast, we developed a system based on a mini-chromosome carrying an array
of lac operator sequences allowing its purification by the lac repressor affinity column. In contrast
to budding yeast, the fission yeast Schizosaccharomyces pombe contains a homologue of the
human telomeric protein TRF2, designated Taz1p. As the TRF2 protein has been implicated in
remodelling telomeres into t-loops, the ability of Taz1p to promote t-loop formation is examined by
electron microscopy using purified protein and synthetic templates containing a double-stranded
fission yeast telomeric tract. Our studies should shed some light not only on telomeric architecture
in yeast, but should also be instrumental in deciphering detailed telomeric structure in higher
eukaryotes.
KEY WORDS
telomere; t-loop; Taz1; fission yeast; budding yeast; chromatin; silencing
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