Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, the Chinese Academy of Sciences, Kunming, Yunnan 650223, China 2. Graduate School of the Chinese Academy of Science, Beijing 100039, China
The nucleolus of the eukaryotic cell is the site of ribosome biogenesis and contains many protein factors required for ribosome biogenesis. As it has been previously shown that Giardia did not have a nucleolar structure, we want to know how ribosome biogenesis occurs in Giardia. Are there any differences between Giardia and the other eukaryotes with nucleolar structures? We first identified 129 conserved common proteins which are involved in ribosome biogenesis in typical eukaryotes. Then, we used these proteins as query sequences to identify the orthologs in Giardia genomic database. Our results indicate that there are 89 orthologs of the 129 ribosomal biogenesis proteins in Giardia, some of which are involved in rRNA methylation or pseudouridine and present in 90S, pre-40S and pre-60S particles. These data suggest the ribosome biogenesis system of Giardia is similar to that of typical eukaryotes. However, there are 40 ribosomal proteins that do not have obvious orthologs in Giardia. This may imply that the ribosome biogenesis system of Giardia is simpler than that of other eukaryotes. According to these results, we argue that although Giardia does not possess nucleolus, its mechanism and pathway of ribosome biogenesis are similar to that of typical eukaryotes but simpler, and are different from that of prokaryotes. The present investigation provides significant insights into the origin and evolution of the nucleolus.