Cell cycle restriction of telomere elongation

Telomere elongation by telomerase balances the progressive shortening of ch romosome ends due to the succession of replication cycles [1,2]. Telomerase activity is regulated in vivo at its site of action by the telomere itself . In yeast and human cells, the mean telomere length is maintained at a con stant value through a cis-inhibition of telomerase by factors specifically bound to the telomeric DNA [3-7]. Here, we address an unexplored aspect of telomerase regulation by testing the link between telomere dynamics and cel l cycle progression in the budding yeast Saccharomyces cerevisiae. We follo wed the elongation of an abnormally shortened telomere and observed that, l ike telomere shortening in the absence of telomerase, telomere elongation i s linked to the succession of cell divisions. In cells progressing synchron ously through the cell cycle, telomere elongation coincided with the time o f telomere replication. On a minichromosome, a replication defect partially suppressed telomere elongation, suggesting a coupling between in vivo telo merase activity and conventional DNA replication.