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

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 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.

Interaction of β-Giardin with the Bop1 Protein in Giardia lamblia

“…Recent finding of krr homolog gene and purification of fibrillarin in G. lamblia suggest a common mechanism of pre-RNA processing operative in this anucleolate eukaryote (Ghosh et al 2001;Xin et al 2005). This study shows a novel interaction between Bop1 and β-giardin in G. lamblia, but does not provide any concrete information on the function of Bop1.…”

An inventory of yeast proteins associated with nucleolar and ribosomal components

“…Recently, Xin et al . [46] described a typical nucleolar protein in Giardia lamblia and concluded that Giardia once had nucleoli. We conclude that, so far, 'genome fusion' hypotheses are compatible with current data on nucleolar evolution.…”

RNase MRP and the RNA processing cascade in the eukaryotic ancestor

“…Short ITS1 regions are also found in other species with reduced genomes such as Trichomonas vaginalis and the microsporidian E. cuniculi both of which have had MRP-rRNAs [31]. G. lamblia does not contain a nucleolus, but it is expected to contain a eukaryotic-like rRNA processing system due to the presence of many pre-rRNA processing proteins [64]. Some proteins that are usually shared between P and MRP have also been found in G. lamblia [65].…”