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Nucleic Acids Research Advance Access published online on October 28, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn706
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Published by Oxford University Press 2008
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nucleic Acid Enzymes

The divergent eukaryote Trichomonas vaginalis has an m7G cap methyltransferase capable of a single N2 methylation

Augusto Simoes-Barbosa1,2, Camila Louly1, Octávio L. Franco2, Mary A. Rubio3, Juan D. Alfonzo3 and Patricia J. Johnson1,*

1Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, 609 Charles E. Young Drive East, Los Angeles, CA 90095-1489, USA, 2Centro de Analises Proteomicas e Bioquimicas, Pos-Graduacao em Ciencias Genomicas e Biotecnologia, Universidade Catolica de Brasilia SGAN 916, Brasilia DF 70790-160, Brazil and 3Department of Microbiology and RNA Group, The Ohio State University, 484 West 12th Ave, Columbus, OH 43210, USA

*To whom correspondence should be addressed. Tel: +1 310 825 4870; Fax: +1 310 206 5231; Email: johnsonp{at}ucla.edu

Received August 12, 2008. Revised September 25, 2008. Accepted September 29, 2008.

Eukaryotic RNAs typically contain 5' cap structures that have been primarily studied in yeast and metazoa. The only known RNA cap structure in unicellular protists is the unusual Cap4 on Trypanosoma brucei mRNAs. We have found that T. vaginalis mRNAs are protected by a 5' cap structure, however, contrary to that typical for eukaryotes, T. vaginalis spliceosomal snRNAs lack a cap and may contain 5' monophophates. The distinctive 2,2,7-trimethylguanosine (TMG) cap structure usually found on snRNAs and snoRNAs is produced by hypermethylation of an m7G cap catalyzed by the enzyme trimethylguanosine synthase (Tgs). Here, we biochemically characterize the single T. vaginalis Tgs (TvTgs) encoded in its genome and demonstrate that TvTgs exhibits substrate specificity and amino acid requirements typical of an RNA cap-specific, m7G-dependent N2 methyltransferase. However, recombinant TvTgs is capable of catalysing only a single round of N2 methylation forming a 2,7-dimethylguanosine cap (DMG) as observed previously for Giardia lamblia. In contrast, recombinant Entamoeba histolytica and Trypanosoma brucei Tgs are capable of catalysing the formation of a TMG cap. These data suggest the presence of RNAs with a distinctive 5' DMG cap in Trichomonas and Giardia lineages that are absent in other protist lineages.


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