Positive and negative regulation of telomerase access to the telomere.

The protective caps on chromosome ends - known as telomeres - consist of DNA and associated proteins that are essential for chromosome integrity. A fundamental part of ensuring proper telomere function is maintaining adequate length of the telomeric DNA tract. Telomeric repeat sequences are synthesized by the telomerase reverse transcriptase, and, as such, telomerase is a central player in the maintenance of steady-state telomere length. Evidence from both yeast and mammals suggests that telomere-associated proteins positively or negatively control access of telomerase to the chromosome terminus. In yeast, positive regulation of telomerase access appears to be achieved through recruitment of the enzyme by the end-binding protein Cdc13p. In contrast, duplex-DNA-binding proteins assembled along the telomeric tract exert a feedback system that negatively modulates telomere length by limiting the action of telomerase. In mammalian cells, and perhaps also in yeast, binding of these proteins probably promotes a higher-order structure that renders the telomere inaccessible to the telomerase enzyme.