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MBE Advance Access published online on May 4, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msi161
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org
Accepted April 22, 2005

Research Article

Unique tRNA introns of an enslaved algal cell

Oliver Kawach 1, Christine Voß 1, Jonci Wolff 1, Katalin Hadfi 1, Uwe-G. Maier 1*, and Stefan Zauner 1

1 Philipps-University Marburg, Cellbiology, Karl-von-Frisch-Strasse, D-35032 Marburg

* To whom correspondence should be addressed.
Uwe-G. Maier, E-mail: maier{at}staff.uni-marburg.de


  Abstract

Nucleomorphs are remnant nuclei of eukaryotic, secondary endosymbionts exclusively found in cryptophytes and chlorarachniophytes. The nucleomorph of the cryptophyte Guillardia theta codes for 36 tRNA genes, 15 of them predicted to contain introns and one pseudo-tRNA. Some of the predicted intervening sequences are manifested at positions not known in Eukarya, even tRNAs with more than one intron were suggested. By isolating RT-PCR products of the spliced tRNAs we verify processing of all predicted intron-harbouring tRNAs and demonstrate splicing of the smallest introns (3nt) investigated so far. However, the spliced intervening sequences are in some cases shifted in respect to the predicted ones. Moreover, we show that introns, if inserted into the B-box of tRNA genes in the nucleomorph of cryptophytes, mimic promoter regions and do not abolish transcription by RNA polymerase III. Consequently, internal nucleomorph-encoded tRNA promoter regions are in some case dissected from the sequence of the mature tRNAs. By reanalysing tRNA introns of a recently sequenced red algae we furthermore show that splicing of introns at unusual positions may be introduced in cryptophytes by its secondary endosymbiont. However, in contrast to the rest of the symbiont genome, introns are not minimized in quantity instead scattered along the tRNA genes.


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