Xin De-Dong, Wen Jian-Fan, He De, Lu Si-Qi
Mol Biol Evol. 2005 Mar;22(3):391-4. doi: 10.1093/molbev/msi052. Epub 2004 Nov 17.
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.
长期以来，蓝氏贾第鞭毛虫一直被认为是最原始的真核生物之一，且与原核生物和真核生物之间的过渡阶段接近，但最近一些支持性特征受到了挑战，比如缺乏线粒体和高尔基体。此前也有报道称蓝氏贾第鞭毛虫缺乏核仁，而在其他真核细胞中核仁是前体核糖体RNA加工和核糖体组装的场所。在此，我们报道了在无核仁真核生物蓝氏贾第鞭毛虫中鉴定出酵母同源基因krr1。krr1基因在酵母中编码前体核糖体RNA加工蛋白之一，在蓝氏贾第鞭毛虫中也有活跃转录。蓝氏贾第鞭毛虫krr1推导的蛋白质序列与含有单个KH结构域的酵母KRR1p高度相似。我们的数据库搜索表明，krr1基因实际上存在于多种真核生物中，似乎在古细菌中也存在。然而，只有包括蓝氏贾第鞭毛虫在内的真核生物同源物具有单个KH结构域，该结构域包含保守基序KR(K)R。另一种重要的前体核糖体RNA加工蛋白纤维蛋白原此前也在蓝氏贾第鞭毛虫中被鉴定出来。此外，我们的数据库搜索显示，真核细胞中近一半的其他核仁定位蛋白基因在贾第虫中也有同源物。因此，我们认为前体RNA加工的共同机制可能在无核仁真核生物蓝氏贾第鞭毛虫和其他真核生物中起作用，并且与“缺乏线粒体”的情况类似，“缺乏核仁”可能不是一个原始特征，而是该寄生虫的次生进化条件。
