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In the yeast Saccharomyces cerevisiae, telomere elongation is negatively regulated by the telomere repeat-binding protein Rap1p, such that a narrow length distribution of telomere repeat tracts is observed. This length regulation was shown to function independently of the orientation of the telomere repeats. The number of repeats at an individual telomere was reduced when hybrid proteins containing the Rap1 p carboxyl terminus were targeted there by a heterologous DNA-binding domain. The extent of this telomere tract shortening was proportional to the number of targeted molecules, consistent with a feedback mechanism of telomere length regulation that can discriminate the precise number of Rap1 p molecules bound to the chromosome end.
Stephane Marcand,* Eric Gilson, David Shore
Telomeres, the ends of linear eukaryotic chromosomes, are essential structures formed by specific protein-DNA complexes that protect chromosomal termini from degradation and fusion (I ). One of the essential functions of telomeres is to allow the complete replication of chromosome ends, which cannot be accomplished by known DNA polymerases (2). The progressive loss of DNA that would occur after each round of replication is balanced by a ribonucleoprotein terminal transferase enzyme called telomerase, which specifically extends the 3' G-rich telomeric strand in an RNAtemplated reaction (3). In most organisms, telomeric DNA consists of a tandem array of short repeats. In yeast, the telomeric DNA is organized in a nonnucleosomal structure based on an array of the telomere repeat-binding protein Rap1p (4, 5).
In the human germline, cells express telomerase and maintain a constant average telomere length. This initial size appears to determine the replicative life-span of somatic cells, in which telomerase activity is usually undetectable and telomere repeats are progressively lost at each cell division (6). In unicellular organisms like S. cerevisiae, telomere length is kept within a narrow size distribution, specific for a given strain, and does not appear to vary with growth conditions or culturing time (7). Telomere length regulation can be viewed as the resuit of a balance between elongation and shortening. It has been proposed that this equilibrium is determined by negative regulation of telomerase activity by the telomere itself when a specific threshold length is reached (8, 9), or by a progressive increase in the frequency of intrachromatid terminal excision as a function of...