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

Abstract

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 Δfob1 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 Δfob1 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.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank Kunihiro Matsumoto, Andrew W. Murray, Masahiro Ajimura, Takehiko Kobayashi, Akio Toh-e, and Yoshikazu Ohya for providing the yeast strains, plasmids, and genomic library. We are grateful to Austen Ganley for critically reading the manuscript.

This work was supported in part by Grants-in-Aid for Scientific Research 13480234 and 13141205 from the Ministry of Education, Culture, Sports, Science and Technology.