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Nucleic Acids Research 2006 34(2):620-634; doi:10.1093/nar/gkj466
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Published online 30 January 2006

© The Author 2006. Published by Oxford University Press. All rights reserved
The online version of this article has been published under an open access model. Users are entitled to use, reproduce, disseminate, or display the open access version of this article for non-commercial purposes provided that: the original authorship is properly and fully attributed; the Journal and Oxford University Press are attributed as the original place of publication with the correct citation details given; if an article is subsequently reproduced or disseminated not in its entirety but only in part or as a derivative work this must be clearly indicated. For commercial re-use, please contact journals.permissions{at}oxfordjournals.org

Article

Deletion of the Tetrahymena thermophila rDNA replication fork barrier region disrupts macronuclear rDNA excision and creates a fragile site in the micronuclear genome

J. S. Yakisich and G. M. Kapler*

Department of Molecular and Cellular Medicine, Texas A&M University System Health Science Center College Station, TX 77843-1114, USA

*To whom correspondence should be addressed. Tel: +1 979 847 8690; Fax: +1 979 847 9481; Email: gkapler{at}tamu.edu

Received November 15, 2005. Revised January 9, 2006. Accepted January 9, 2006.

During macronuclear development the Tetrahymena thermophila ribosomal RNA gene is excised from micronuclear chromosome 1 by site-specific cleavage at chromosome breakage sequence (Cbs) elements, rearranged into a ‘palindromic’ 21 kb minichromosome and extensively amplified. Gene amplification initiates from origins in the 5' non-transcribed spacer, and forks moving toward the center of the palindrome arrest at a developmentally regulated replication fork barrier (RFB). The RFB is inactive during vegetative cell divisions, suggesting a role in the formation or amplification of macronuclear rDNA. Using micronuclear (germline) transformation, we show that the RFB region facilitates Cbs-mediated excision. Deletion of the RFB inhibits chromosome breakage in a sub-population of developing macronuclei and promotes alternative processing by a Cbs-independent mechanism. Remarkably, the RFB region prevents spontaneous breakage of chromosome 1 in the diploid micronucleus. Strains heterozygous for {Delta}RFB and wild-type rDNA lose the {Delta}RFB allele and distal left arm of chromosome 1 during vegetative propagation. The wild-type chromosome is subsequently fragmented near the rDNA locus, and both homologs are progressively eroded, suggesting that broken micronuclear chromosomes are not ‘healed’ by telomerase. Deletion of this 363 bp segment effectively creates a fragile site in the micronuclear genome, providing the first evidence for a non-telomere cis-acting determinant that functions to maintain the structural integrity of a mitotic eukaryotic chromosome.


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 Mol. Biol. CellHome page
J. S. Yakisich, P. Y. Sandoval, T. L. Morrison, and G. M. Kapler
TIF1 Activates the Intra-S-Phase Checkpoint Response in the Diploid Micronucleus and Amitotic Polyploid Macronucleus of Tetrahymena
Mol. Biol. Cell, December 1, 2006; 17(12): 5185 - 5197.
[Abstract] [Full Text] [PDF]


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