Eukaryotic Cell
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Eukaryotic Cell, February 2003, p. 95-102, Vol. 2, No. 1
1535-9778/03/$08.00+0     DOI: 10.1128/EC.2.1.95-102.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Selection on the Genes of Euplotes crassus Tec1 and Tec2 Transposons: Evolutionary Appearance of a Programmed Frameshift in a Tec2 Gene Encoding a Tyrosine Family Site-Specific Recombinase

Thomas G. Doak,1 David J. Witherspoon,2 Carolyn L. Jahn,3 and Glenn Herrick1*

Department of Pathology, School of Medicine, University of Utah, Salt Lake City, Utah 84132,1 Idaho Technology, Inc., Salt Lake City, Utah 84108,2 Department of Cell and Molecular Biology, Medical School, Northwestern University, Chicago, Illinois 60611-30083

Received 24 June 2002/ Accepted 5 September 2002

The Tec1 and Tec2 transposons of the ciliate Euplotes crassus carry a gene for a tyrosine-type site-specific recombinase. The expression of the Tec2 gene apparently uses a programmed +1 frameshift. To test this hypothesis, we first examined whether this gene has evolved under purifying selection in Tec1 and Tec2. Each element carries three genes, and each has evolved under purifying selection for the function of its encoded protein, as evidenced by a dearth of nonsynonymous changes. This distortion of divergence is apparent in codons both 5' and 3' of the frameshift site. Thus, Tec2 transposons have diverged from each other while using a programmed +1 frameshift to produce recombinase, the function of which is under purifying selection. What might this function be? Tyrosine-type site-specific recombinases are extremely rare in eukaryotes, and Tec elements are the first known eukaryotic type II transposons to encode a site-specific recombinase. Tec elements also encode a widespread transposase. The Tec recombinase might function in transposition, resolve products of transposition (bacterial replicative transposons use recombinase or resolvase to separate joined replicons), or provide a function that benefits the ciliate host. Transposons in ciliated protozoa are removed from the macronucleus, and it has been proposed that the transposons provide this "excisase" activity.

* Corresponding author. Mailing address: Department of Pathology, University of Utah, 30 North 1900 East, 5C124 SOM, Salt Lake City, UT 84132-2501. Phone: (801) 581-6001. Fax: (801) 581-4517. E-mail: glenn.herrick{at}path.utah.edu.

Eukaryotic Cell, February 2003, p. 95-102, Vol. 2, No. 1
1535-9778/03/$08.00+0     DOI: 10.1128/EC.2.1.95-102.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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