Est1 and Cdc13 as comediators of telomerase access

Cdc13 and Est1 are single-strand telomeric DNA binding proteins that contri bute to telomere replication in the yeast Saccharomyces cerevisiae. Here it is shown that fusion of Cdc13 to the telomerase-associated Est1 protein re sults in greatly elongated telomeres. Fusion proteins consisting of mutant versions of Cdc13 or Est1 confer similar telomere elongation, indicating th at close physical proximity can bypass telomerase-defective mutations in ei ther protein. Fusing Cdc13 directly to the catalytic core of telomerase all ows stable telomere maintenance in the absence of Est1, consistent with a r ole for Est1 in mediating telomerase access. Telomere length homeostasis th erefore is maintained in part by restricting access of telomerase to chromo some termini, but this limiting situation can be overcome by directly tethe ring telomerase to the telomere.