Kabaha Majdi M, Zhitomirsky Benny, Schwartz Irit, Tzfati Yehuda
Department of Genetics, The Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel.
Mol Cell Biol. 2008 Mar;28(6):1875-82. doi: 10.1128/MCB.01683-07. Epub 2008 Jan 14.
Telomerase is a ribonucleoprotein reverse transcriptase that copies a short template within its integral telomerase RNA moiety (TER) onto eukaryotic chromosome ends, thus compensating for incomplete replication and degradation. The highly divergent yeast TER is structured in three long arms, with a catalytic core at its center. A binding site for the protein Ku80 is conserved within the 5' arm of TER in Saccharomyces but not in Kluyveromyces budding yeast species. Consistently, KU80 deletion in Kluyveromyces lactis does not affect telomere length, while it causes telomere shortening in Saccharomyces cerevisiae. We found elements in the 5' arm of K. lactis TER that are crucial for telomerase activity and stability. However, we found no indication of the association of Ku80 with this arm. Although the overexpression of Ku80 rescues a particular mutation in K. lactis TER1 that phenocopies a telomerase null mutation, this effect is indirect, caused by the repression of the recombination pathway competing for telomere maintenance. Interestingly, the overexpression of Est3, an essential telomerase protein whose function is still unknown, suppresses the phenotypes of mutations in this arm. These results indicate that the 5' arm of K. lactis TER has critical roles in telomerase function, which may be linked to the function of Est3.
端粒酶是一种核糖核蛋白逆转录酶，它将其完整的端粒酶RNA部分（TER）内的短模板复制到真核染色体末端，从而补偿不完全复制和降解。高度分化的酵母TER由三个长臂构成，其中心有一个催化核心。蛋白质Ku80的结合位点在酿酒酵母TER的5'臂内保守，但在克鲁维酵母属的芽殖酵母物种中不保守。一致地，乳酸克鲁维酵母中的KU80缺失不影响端粒长度，而在酿酒酵母中则导致端粒缩短。我们在乳酸克鲁维酵母TER的5'臂中发现了对端粒酶活性和稳定性至关重要的元件。然而，我们没有发现Ku80与该臂相关联的迹象。尽管Ku80的过表达挽救了乳酸克鲁维酵母TER1中的一个特定突变，该突变模拟了端粒酶缺失突变，但这种效应是间接的，是由竞争端粒维持的重组途径的抑制引起的。有趣的是，Est3（一种功能仍未知的必需端粒酶蛋白）的过表达抑制了该臂中突变的表型。这些结果表明，乳酸克鲁维酵母TER的5'臂在端粒酶功能中具有关键作用，这可能与Est3的功能有关。
