Identification of a Giardia krr1 homolog gene and the secondarily anucleolate condition of Giaridia lamblia.

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 krr1 genes actually present in diverse eukaryotes and also seem to present in Archaea. However, 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 search shows 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 case of "lack of mitochondrion," "lack of nucleolus" may not be a primitive feature, but a secondarily evolutionary condition of the parasite.