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MBE Advance Access published online on November 17, 2004
Molecular Biology and Evolution, doi:10.1093/molbev/msi052
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved
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Accepted November 8, 2004

Letter

Identification of a Giardia krr1 Homolog Gene and the Secondarily Anucleolate Condition of Giaridia lamblia

De-Dong Xin 1, Jian-Fan Wen 2*, De He 1, and Si-Qi Lu 3

1 Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming 650223, China; Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
2 Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming 650223, China
3 Capital University of Medical Sciences, Beijing 100054, China

* To whom correspondence should be addressed.
Jian-Fan Wen, E-mail: wenjf{at}mail.kiz.ac.cn


  Abstract

Giaridia lamblia was long considered to be one of the most primitive eukaryotes and to lie close to the transition between prokaryotes and eukaryotes, but several supporting features such as lack of mitochondrion and Golgi have been challenged recently. It was also reported previously that G. lamblia lacked nucleolus, which is the site of pre-rRNA processing and ribosomal assembling in the other eukaryotic cells. Here we report the identification of the yeast homolog gene, krr1, in the anucleolate eukaryote, G. lamblia. The krr1 gene encoding one of the pre-rRNA processing proteins in yeast, is actively transcribed in G. lamblia. The deduced protein sequence of G. lamblia krr1 is highly similar to yeast KRR1p that contains a single-KH domain. Our database searches indicated that actually krr1 genes present in diverse eukaryotes and also seem to present in Archaea. But only the eukaryotic homologs, including that of G. lamblia, have the single-KH domain which contains the conserved motif KR(K)R. Fibrillarin, another important pre-rRNA processing protein has also been identified previously in G. lamblia. Moreover, our database searching showes that nearly half of the other nucleolus-localized protein genes of eukaryotic cells also have their homologs in Giardia. Therefore, we suggest that a common mechanism of pre-RNA processing may operate in the anucleolate eukaryote G. lamblia and in the other eukaryotes, and that like the ‘lack of mitochondrion’, ‘lack of nucleolus’ may not be a primitive feature yet, but a secondarily evolutionary condition of the parasite.

Keywords: Giaridia lamblia; KRR1p; nucleolus; KH domain; Giardia lamblia; krr1 gene; nucleolus; rRNA-processing; evolution.
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