Comparison of tsa417-like variant-specific surface protein (VSP) genes in Giardia intestinalis and identification of a novel locus in genetic Group II isolates.

Abstract

<jats:p>A gene (<jats:italic>vsp417</jats:italic>-3<jats:sup>A-II</jats:sup>) encoding a new member of the variant-specific surface protein (VSP) family of <jats:italic>Giardia</jats:italic> is described. <jats:italic>Vsp417</jats:italic>-3<jats:sup>A-II</jats:sup> is detected exclusively in isolates belonging to the genetic Assemblage A/Group II sublineage of <jats:italic>G. intestinalis</jats:italic>, and it is closely related to a previously characterized VSP gene (<jats:italic>tsp11</jats:italic>, herein renamed <jats:italic>vsp417</jats:italic>-2<jats:sup>A-I</jats:sup>) found in Group I isolates of <jats:italic>G. intestinalis</jats:italic>. Analysis of DNA amplified from the genomes of Group II isolates using consensus primers also revealed products corresponding to the <jats:italic>vsp417</jats:italic>-2 locus (<jats:italic>vsp417</jats:italic>-2<jats:sup>A-II</jats:sup> allele), indicating that <jats:italic>vsp417</jats:italic>-2<jats:sup>A-II</jats:sup>

and <jats:italic>vsp417</jats:italic>-3<jats:sup>A-II</jats:sup> represent separate loci in these organisms. Southern hybridizations revealed a single genomic copy of <jats:italic>vsp417</jats:italic>-3<jats:sup>A-II</jats:sup> in Group II isolates, but efforts to detect this locus in Group I organisms were unsuccessful. Together with the <jats:italic>vsp417</jats:italic>-1 (<jats:italic>tsa417</jats:italic>) locus that was first identified in the genetic Group I isolate WB and which has been detected since in all tested genetic Group I isolates (as <jats:italic>vsp417</jats:italic>-1<jats:sup>A-I</jats:sup>) and in genetic Group II isolates (as <jats:italic>vsp417</jats:italic>-1<jats:sup>A-II</jats:sup>), <jats:italic>vsp417</jats:italic>-3<jats:sup>A-II</jats:sup> represents a third member of a stable subset of <jats:italic>tsa417</jats:italic>-like VSP genes in genetic Group II <jats:italic>G. intestinalis</jats:italic>. The encoded polypeptide, <jats:italic>vsp417</jats:italic>-3<jats:sup>A-II</jats:sup> (667 residues, predicted <jats:italic>M</jats:italic><jats:sub>r</jats:sub> 69120) possesses a high cysteine content (12·1 mol%), 28 copies of the metal-binding -CXXC- motif, and a highly conserved C-terminal hydrophobic domain – similar to all other characterized <jats:italic>Giardia</jats:italic> VSP. A revised nomenclature for <jats:italic>Giardia</jats:italic> VSP genes is proposed, prompted by the need to describe the homologues found in the various major genotypes of the species complex, <jats:italic>G. intestinalis</jats:italic>.</jats:p>