title: An ancient spliceosomal intron in the ribosomal protein L7a gene (Rpl7a) of Giardia lamblia.
abstract: BACKGROUND
  Only 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.
  
  
  RESULTS
  We 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.
  
  
  CONCLUSIONS
  An 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.
authors.affiliation: Canadian Institute for Advanced Research, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada. russella@dal.ca
authors.forename: Anthony G
authors.initials: AG
authors.lastname: Russell
authors.forename: Timothy E
authors.initials: TE
authors.lastname: Shutt
authors.forename: Russell F
authors.initials: RF
authors.lastname: Watkins
authors.forename: Michael W
authors.initials: MW
authors.lastname: Gray
mesh_terms: D000818:Animals; D000953:Antigens, Protozoan; D001483:Base Sequence; D019070:Cell Lineage; D019295:Computational Biology; D017124:Conserved Sequence; D019143:Evolution, Molecular; D016678:Genome; D018503:Genome, Protozoan; D016829:Giardia lamblia; D007438:Introns; D008969:Molecular Sequence Data; D009711:Nucleotides / Q000737:chemistry; D010802:Phylogeny; D022821:RNA Splice Sites; D012342:RNA, Small Nuclear; D012269:Ribosomal Proteins / Q000235:genetics*; D012984:Software; D013045:Species Specificity; D017381:Spliceosomes / Q000235:genetics*
chemical_list: D000953:Antigens, Protozoan; D009711:Nucleotides; D022821:RNA Splice Sites; D012342:RNA, Small Nuclear; D012269:Ribosomal Proteins