Abstract
BackgroundOnly one spliceosomal-type intron has previously been identified in the unicellular eukaryotic parasite, Giardia lamblia (a diplomonad). This intron is only 35 nucleotides in length and is unusual in possessing a non-canonical 5' intron boundary sequence, CT, instead of GT.ResultsWe have identified a second spliceosomal-type intron in G. lamblia, in the ribosomal protein L7a gene (Rpl7a), that possesses a canonical GT 5' intron boundary sequence. A comparison of the two known Giardia intron sequences revealed extensive nucleotide identity at both the 5' and 3' intron boundaries, similar to the conserved sequence motifs recently identified at the boundaries of spliceosomal-type introns in Trichomonas vaginalis (a parabasalid). Based on these observations, we searched the partial G. lamblia genome sequence for these conserved features and identified a third spliceosomal intron, in an unassigned open reading frame. Our comprehensive analysis of the Rpl7a intron in other eukaryotic taxa demonstrates that it is evolutionarily conserved and is an ancient eukaryotic intron.ConclusionAn analysis of the phylogenetic distribution and properties of the Rpl7a intron suggests its utility as a phylogenetic marker to evaluate particular eukaryotic groupings. Additionally, analysis of the G. lamblia introns has provided further insight into some of the conserved and unique features possessed by the recently identified spliceosomal introns in related organisms such as T. vaginalis and Carpediemonas membranifera.
Highlights
One spliceosomal-type intron has previously been identified in the unicellular eukaryotic parasite, Giardia lamblia
We identify an intron in ribosomal protein L7a gene (Rpl7a), the gene encoding the G. lamblia ribosomal protein L7a and an additional spliceosomal intron in an unassigned open reading frames (ORFs) that encodes a non-conserved protein
A conventional spliceosomal intron in the Giardia lamblia Rpl7a gene In comparing ribosomal protein L7a homologs from various organisms, we noted that the predicted sequence for the G. lamblia protein [GenBank:EAA41652] appeared abnormally truncated at the C-terminal end relative to other eukaryotic and archaeal sequences, terminating at position 171 of S. cerevisiae L7a (Fig. 1)
Summary
One spliceosomal-type intron has previously been identified in the unicellular eukaryotic parasite, Giardia lamblia (a diplomonad). The introns from these two organisms display significant sequence similarity at their 5' ends This suggests that similar splicing mechanisms may be employed to remove introns in organisms from these two eukaryotic groups, members of the proposed eukaryotic supergroup Excavata [6]. These sequence motifs have been shown to be important for efficient in vivo splicing of introns in T. vaginalis [3]; since only one intron has been identified in G. lamblia so far, the extent of conservation of these sequence motifs in other G. lamblia introns is not known. These features may be derived and be unique among spliceosomal introns found in these long-branching eukaryotic taxa
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