COMPARISON OF TSA417-LIKE VARIANT-SPECIFIC SURFACE PROTEIN (VSP) GENES IN GIARDIA-INTESTINALIS AND IDENTIFICATION OF A NOVEL LOCUS IN GENETIC GROUP-II ISOLATES

A gene (vsp417-3(A-II)) encoding a new member of the variant-specific surface protein (VSP) family of Giardia is described. Vsp417-3(A-II) i s detected exclusively in isolates belonging to the genetic Assemblage A/Group II sublineage of G. intestinalis, and it is closely related t o a previously characterized VSP gene (tsp11, herein renamed vsp417-2( A-I)) found in Group I isolates of G. intestinalis. Analysis of DNA am plified from the genomes of Group II isolates using consensus primers also revealed products corresponding to the vsp417-2 locus (vsp417-2(A -II) allele), indicating that vsp417-2(A-II) and vsp417-3(A-II) repres ent separate loci in these organisms. Southern hybridizations revealed a single genomic copy of vsp417-3(A-II) in Group II isolates, but eff orts to detect this locus in Group I organisms were unsuccessful. Toge ther with the vsp417-1 (tsa417) locus that was first identified in the genetic Group I isolate WB and which has been detected since in all t ested genetic Group I isolates (as vsp417-1(A-I)) and in genetic Group II isolates (as asp417-1(A-II)), vsp417-3(A-II) represents a third me mber of a stable subset of tsa417-like VSP genes in genetic Group II G . intestinalis. The encoded polypeptide, VSP417-3(A-II) (667 residues, predicted M-r 69120) possesses a high cysteine content (12.1 mol%), 2 8 copies of the metal-binding -CXXC- motif, and a highly conserved C-t erminal hydrophobic domain - similar to all other characterized Giardi a VSP. A revised nomenclature for Giardia VSP genes is proposed, promp ted by the need to describe the homologues found in the various major genotypes of the species complex, G. intestinalis.