Human chromosomes end in telomeres, repetitive DNA sequences that serve as a buffer to protect the coding material of the genome. During DNA replication telomeres shrink due to the end replication problem, leading to cell-cycle arrest when telomeres reach a critical length. This arrest provides an important barrier against cancer formation by limiting the number of division human cells can undergo. In stem cells and 90% of cancer cells the ribonucleoprotein (RNP) telomerase counteracts telomere shrinkage, allowing these cell types to divide indefinitely. Telomerase is a reverse transcriptase that adds telomeric repeats to the chromosome end by copying them from a template contained in its RNA subunit. Recruitment of telomerase to telomeres occurs in S-phase of the cell cycle but the molecular mechanism of the process is only partially understood. Using biochemical, biophysical, and cell biological approaches in combination with genome editing I have defined the protein-protein interface required for telomerase recruitment to telomeres. Furthermore, live cell single-molecule imaging of telomerase revealed that telomerase monitors telomeres by forming frequent, short protein-protein interactions with telomeres. Only rarely are these brief interactions converted into stable, long lasting interactions consistent with telomere elongation. These observations explain how a small pool of telomerase RNPs can maintain all telomeres. Finally, using zero mode waveguides, I have developed a single molecule assay to study the telomerase catalytic cycle. Together, these approaches will allow me to provide a comprehensive picture of telomerase mediated telomere maintenance, hopefully leading to new strategies to prevent telomerase action in cancer cells.

Jens Schmidt (University of Colorado Boulder)