SummaryTelomerase is often re-activated in human cancers and is widely used to immortalize cells in culture. In addition to the maintenance of telomeres, telomerase has been implicated in cell proliferation, genomic instability and apoptosis. Here we show that human telomerase reverse transcriptase (hTERT) is targeted to the mitochondria by an N-terminal leader sequence, and that mitochondrial extracts contain telomerase activity. In seven different human cell lines, mitochondrial telomerase increases hydrogen-peroxide-mediated mitochondrial DNA damage. hTERT expression did not alter the rate of hydrogen peroxide breakdown or endogenous cellular levels . Because the damaging effects of hydrogen peroxide are mediated by divalent metal ions (Fenton chemistry), we examined the levels of bioavailable metals. In all cases, higher levels of chelatable metals were found in hTERTexpressing cells. These results suggest that mitochondrial telomerase sensitizes cells to oxidative stress, which can lead to apoptotic cell death, and imply a novel function of telomerase in mitochondrial DNA transactions.

Nucleotide excision repair is a highly conserved DNA repair mechanism present in all kingdoms of life. The incision reaction is a critical step for damage removal and is accomplished by the UvrC protein in eubacteria. No structural information is so far available for the 3 0 incision reaction. Here we report the crystal structure of the N-terminal catalytic domain of UvrC at 1.5 Å resolution, which catalyzes the 3 0 incision reaction and shares homology with the catalytic domain of the GIY-YIG family of intron-encoded homing endonucleases. The structure reveals a patch of highly conserved residues surrounding a catalytic magnesium-water cluster, suggesting that the metal binding site is an essential feature of UvrC and all GIY-YIG endonuclease domains. Structural and biochemical data strongly suggest that the N-terminal endonuclease domain of UvrC utilizes a novel one-metal mechanism to cleave the phosphodiester bond.