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Molecular and Cellular Biology, January 2006, p. 438-447, Vol. 26, No. 2
0270-7306/06/$08.00+0     doi:10.1128/MCB.26.2.438-447.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Distinct Paths To Stop Codon Reassignment by the Variant-Code Organisms Tetrahymena and Euplotes

Joe Salas-Marco,^1, Hua Fan-Minogue,¹ Adam K. Kallmeyer,¹ Lawrence A. Klobutcher,² Philip J. Farabaugh,³ and David M. Bedwell^1*

Department of Microbiology, University of Alabama at Birmingham, Birmingham, Alabama 35294,¹ Department of Molecular, Microbial and Structural Biology, University of Connecticut Health Center, Farmington, Connecticut 06030,² Department of Biological Sciences, University of Maryland—Baltimore County, Baltimore, Maryland 21250³

Received 8 August 2005/ Returned for modification 31 August 2005/ Accepted 25 October 2005

The reassignment of stop codons is common among many ciliate species. For example, Tetrahymena species recognize only UGA as a stop codon, while Euplotes species recognize only UAA and UAG as stop codons. Recent studies have shown that domain 1 of the translation termination factor eRF1 mediates stop codon recognition. While it is commonly assumed that changes in domain 1 of ciliate eRF1s are responsible for altered stop codon recognition, this has never been demonstrated in vivo. To carry out such an analysis, we made hybrid proteins that contained eRF1 domain 1 from either Tetrahymena thermophila or Euplotes octocarinatus fused to eRF1 domains 2 and 3 from Saccharomyces cerevisiae. We found that the Tetrahymena hybrid eRF1 efficiently terminated at all three stop codons when expressed in yeast cells, indicating that domain 1 is not the sole determinant of stop codon recognition in Tetrahymena species. In contrast, the Euplotes hybrid facilitated efficient translation termination at UAA and UAG codons but not at the UGA codon. Together, these results indicate that while domain 1 facilitates stop codon recognition, other factors can influence this process. Our findings also indicate that these two ciliate species used distinct approaches to diverge from the universal genetic code.

Present address: Department of Systems Biology, Harvard Medical School, Boston, Mass.

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