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Molecular and Cellular Biology, March 2006, p. 2226-2236, Vol. 26, No. 6
0270-7306/06/$08.00+0     doi:10.1128/MCB.26.6.2226-2236.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Condensin Loaded onto the Replication Fork Barrier Site in the rRNA Gene Repeats during S Phase in a FOB1-Dependent Fashion To Prevent Contraction of a Long Repetitive Array in Saccharomyces cerevisiae

Laboratory of Genome Dynamics, National Institute for Basic Biology,¹ School of Life Science, Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan,² Iwate College of Nursing, Iwate 020-0151, Japan,³ School of Advanced Science, Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan⁴

Received 22 September 2005/ Returned for modification 13 October 2005/ Accepted 28 December 2005

An average of 200 copies of the rRNA gene (rDNA) is clustered in a long tandem array in Saccharomyces cerevisiae. FOB1 is known to be required for expansion/contraction of the repeats by stimulating recombination, thereby contributing to the maintenance of the average copy number. In fob1 cells, the repeats are still maintained without any fluctuation in the copy number, suggesting that another, unknown system acts to prevent repeat contraction. Here, we show that condensin acts together with FOB1 in a functionally complemented fashion to maintain the long tandem repeats. Six condensin mutants possessing severely contracted rDNA repeats were isolated in fob1 cells but not in FOB1⁺ cells. We also found that the condensin complex associated with the nontranscribed spacer region of rDNA with a major peak coincided with the replication fork barrier (RFB) site in a FOB1-dependent fashion. Surprisingly, condensin association with the RFB site was established during S phase and was maintained until anaphase. These results indicate that FOB1 plays a novel role in preventing repeat contraction by regulating condensin association and suggest a link between replication termination and chromosome condensation and segregation.

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