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Molecular and Cellular Biology, March 2008, p. 1443-1455, Vol. 28, No. 5
0270-7306/08/$08.00+0     doi:10.1128/MCB.01614-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Recombination-Based Telomere Maintenance Is Dependent on Tel1-MRN and Rap1 and Inhibited by Telomerase, Taz1, and Ku in Fission Yeast ^,

Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois 60607

Received 31 August 2007/ Returned for modification 9 October 2007/ Accepted 12 December 2007

Fission yeast cells survive loss of the telomerase catalytic subunit Trt1 (TERT) through recombination-based telomere maintenance or through chromosome circularization. Although trt1 survivors with linear chromosomes can be obtained, they often spontaneously circularize their chromosomes. Therefore, it was difficult to establish genetic requirements for telomerase-independent telomere maintenance. In contrast, when the telomere-binding protein Taz1 is also deleted, taz1 trt1 cells are able to stably maintain telomeres. Thus, taz1 trt1 cells can serve as a valuable tool in understanding the regulation of telomerase-independent telomere maintenance. In this study, we show that the checkpoint kinase Tel1 (ATM) and the DNA repair complex Rad32-Rad50-Nbs1 (MRN) are required for telomere maintenance in taz1 trt1 cells. Surprisingly, Rap1 is also essential for telomere maintenance in taz1 trt1 cells, even though recruitment of Rap1 to telomeres depends on Taz1. Expression of catalytically inactive Trt1 can efficiently inhibit recombination-based telomere maintenance, but the inhibition requires both Est1 and Ku70. While Est1 is essential for recruitment of Trt1 to telomeres, Ku70 is dispensable. Thus, we conclude that Taz1, TERT-Est1, and Ku70-Ku80 prevent telomere recombination, whereas MRN-Tel1 and Rap1 promote recombination-based telomere maintenance. Evolutionarily conserved proteins in higher eukaryotic cells might similarly contribute to telomere recombination.

Published ahead of print on 26 December 2007.

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