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J. Biol. Chem., Vol. 280, Issue 21, 20740-20751, May 27, 2005

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Pyrrolysine and Selenocysteine Use Dissimilar Decoding Strategies^*

Yan Zhang, Pavel V. Baranov, John F. Atkins¶||, and Vadim N. Gladyshev**

From the Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664, the Department of Human Genetics, University of Utah, Salt Lake City, Utah 84112, and the ^¶Biosciences Institute, University College Cork, Cork, Ireland

Selenocysteine (Sec) and pyrrolysine (Pyl) are known as the 21st and 22nd amino acids in protein. Both are encoded by codons that normally function as stop signals. Sec specification by UGA codons requires the presence of a cis-acting selenocysteine insertion sequence (SECIS) element. Similarly, it is thought that Pyl is inserted by UAG codons with the help of a putative pyrrolysine insertion sequence (PYLIS) element. Herein, we analyzed the occurrence of Pyl-utilizing organisms, Pyl-associated genes, and Pyl-containing proteins. The Pyl trait is restricted to several microbes, and only one organism has both Pyl and Sec. We found that methanogenic archaea that utilize Pyl have few genes that contain in-frame UAG codons, and many of these are followed with nearby UAA or UGA codons. In addition, unambiguous UAG stop signals could not be identified. This bias was not observed in Sec-utilizing organisms and non-Pyl-utilizing archaea, as well as with other stop codons. These observations as well as analyses of the coding potential of UAG codons, overlapping genes, and release factor sequences suggest that UAG is not a typical stop signal in Pyl-utilizing archaea. On the other hand, searches for conserved Pyl-containing proteins revealed only four protein families, including methylamine methyltransferases and transposases. Only methylamine methyltransferases matched the Pyl trait and had conserved Pyl, suggesting that this amino acid is used primarily by these enzymes. These findings are best explained by a model wherein UAG codons may have ambiguous meaning and Pyl insertion can effectively compete with translation termination for UAG codons obviating the need for a specific PYLIS structure. Thus, Sec and Pyl follow dissimilar decoding and evolutionary strategies.

Received for publication, February 8, 2005 , and in revised form, March 22, 2005.

^* This work was supported in part by National Institutes of Health (NIH) Grant GM061603 (to V. N. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| Supported by NIH Grant GM48152 and an award from Science Foundation Ireland.

^** To whom correspondence should be addressed: Dept. of Biochemistry, University of Nebraska, Lincoln, NE 68588-0664. Tel.: 402-472-4948; Fax: 402-472-7842; E-mail: vgladyshev1{at}unl.edu.

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