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Molecular and Cellular Biology, August 2006, p. 6223-6238, Vol. 26, No. 16
0270-7306/06/$08.00+0     doi:10.1128/MCB.02324-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Expression of rRNA Genes and Nucleolus Formation at Ectopic Chromosomal Sites in the Yeast Saccharomyces cerevisiae

Melanie L. Oakes, Katsuki Johzuka,^, Loan Vu, Kristilyn Eliason,^|| and Masayasu Nomura^*

Department of Biological Chemistry, University of California-Irvine, Irvine, California 92697-1700

Received 5 December 2005/ Returned for modification 31 January 2006/ Accepted 25 May 2006

We constructed yeast strains in which rRNA gene repeats are integrated at ectopic sites in the presence or absence of the native nucleolus. At all three ectopic sites analyzed, near centromere CEN5, near the telomere of chromosome VI-R, and in middle of chromosome V-R (mid-V-R), a functional nucleolus was formed, and no difference in the expression of rRNA genes was observed. When two ribosomal DNA (rDNA) arrays are present, one native and the other ectopic, there is codominance in polymerase I (Pol I) transcription. We also examined the expression of a single rDNA repeat integrated into ectopic loci in strains with or without the native RDN1 locus. In a strain with reduced rRNA gene copies at RDN1 (40 copies), the expression of a single rRNA gene copy near the telomere was significantly reduced relative to the other ectopic sites, suggesting a less-efficient recruitment of the Pol I machinery from the RDN1 locus. In addition, we found a single rRNA gene at mid-V-R was as active as that within the 40-copy RDN1. Combined with the results of activity analysis of a single versus two tandem copies at CEN5, we conclude that tandem repetition is not required for efficient rRNA gene transcription.

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

M.L.O. and K.J. contributed equally to this study.

Present address: National Institute for Basic Biology, 38 Nishigonaka, Myodaijicho, Okazaki 444-8585, Japan.

^|| Present address: Myriad Genetic Laboratories, 320 Wakara Way, Salt Lake City, UT 84108.