Abstract
Background5S rRNA is a highly conserved ribosomal component. Eukaryotic 5S rRNA and its associated proteins (5S rRNA system) have become very well understood. Giardia lamblia was thought by some researchers to be the most primitive extant eukaryote while others considered it a highly evolved parasite. Previous reports have indicated that some aspects of its 5S rRNA system are simpler than that of common eukaryotes. We here explore whether this is true to its entire system, and whether this simplicity is a primitive or parasitic feature.Methodology/Principal FindingsBy collecting and confirming pre-existing data and identifying new data, we obtained almost complete datasets of the system of three isolates of G. lamblia, two other parasitic excavates (Trichomonas vaginalis, Trypanosoma cruzi), and one free-living one (Naegleria gruberi). After comprehensively comparing each aspect of the system among these excavates and also with those of archaea and common eukaryotes, we found all the three Giardia isolates to harbor a same simplified 5S rRNA system, which is not only much simpler than that of common eukaryotes but also the simplest one among those of these excavates, and is surprisingly very similar to that of archaea; we also found among these excavates the system in parasitic species is not necessarily simpler than that in free-living species, conversely, the system of free-living species is even simpler in some respects than those of parasitic ones.Conclusion/SignificanceThe simplicity of Giardia 5S rRNA system should be considered a primitive rather than parasitically-degenerated feature. Therefore, Giardia 5S rRNA system might be a primitive system that is intermediate between that of archaea and the common eukaryotic model system, and it may reflect the evolutionary history of the eukaryotic 5S rRNA system from the archaeal form. Our results also imply G. lamblia might be a primitive eukaryote with secondary parasitically-degenerated features.
Highlights
Giardia lamblia is one of the most widespread intestinal protozoan parasites in the world
5S rRNA genes, though there were only eight copies, far fewer than the number seen in most common eukaryotes, were annotated in G. lamblia genome database [16]
The genome databases of two other G. lamblia isolates, assemblage B isolate GS and assemblage E isolate P15, are available [21,22]. We identified their 5S rRNA system to confirm whether the simplicity of the 5S rRNA system is a common trait among different G. lamblia isolates
Summary
Giardia lamblia is one of the most widespread intestinal protozoan parasites in the world. Phylogenomic analyses based on large phylogenomic datasets have placed G. lamblia within excavate, one of the three primary divisions of eukaryotes [13] No matter it is primitive or not, it is amazing that it was reported that in this organism no 5S rRNA was detected in either experiments or BLAST searches against its genome database [14,15], since 5S rRNA is so ubiquitous and highly conserved in both prokaryotic and eukaryotic cells. There were only two components of TFIIIB (TBP and BRF) identified in the G. lamblia genome database [17]. It is unknown whether the entire G. lamblia 5S rRNA system is simpler than that of common eukaryotes yet. We here explore whether this is true to its entire system, and whether this simplicity is a primitive or parasitic feature
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