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Eukaryotic Cell, February 2005, p. 421-431, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.421-431.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Elimination of Foreign DNA during Somatic Differentiation in Tetrahymena thermophila Shows Position Effect and Is Dosage Dependent{dagger}

Yifan Liu,1,{ddagger} Xiaoyuan Song,1 Martin A. Gorovsky,1 and Kathleen M. Karrer1,2*

Department of Biology, University of Rochester, Rochester, New York,1 Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin2

Received 14 October 2004/ Accepted 23 November 2004

In the ciliate Tetrahymena thermophila, approximately 15% of the germ line micronuclear DNA sequences are eliminated during formation of the somatic macronucleus. The vast majority of the internal eliminated sequences (IESs) are repeated in the micronuclear genome, and several of them resemble transposable elements. Thus, it has been suggested that DNA elimination evolved as a means for removing invading DNAs. In the present study, bacterial neo genes introduced into the germ line micronuclei were eliminated from the somatic genome. The efficiency of elimination from two different loci increased dramatically with the copy number of the neo genes in the micronuclei. The timing of neo elimination is similar to that of endogenous IESs, and they both produce bidirectional transcripts of the eliminated element, suggesting that the deletion of neo occurred by the same mechanism as elimination of endogenous IESs. These results indicate that repetition of an element in the micronucleus enhances the efficiency of its elimination from the newly formed somatic genome of Tetrahymena thermophila. The implications of these data in relation to the function and mechanism of IES elimination are discussed.

* Corresponding author. Mailing address: Department of Biological Sciences, Marquette University, Milwaukee, WI 53201-1881. Phone: (414) 288-1474. Fax: (414) 288-7357. E-mail: kathleen.karrer{at}marquette.edu.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

{ddagger} Present address: Laboratory of Chromatin Biology, Rockefeller University, New York, N.Y.

Eukaryotic Cell, February 2005, p. 421-431, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.421-431.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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