Giardia intestinalis: conservation of the variant-specific surface protein VSP417-1 (TSA417) and identification of a divergent homologue encoded at a duplicated locus in genetic group II isolates.

The stability of the gene encoding TSA417, a 72-kDa variant-specific surface protein (VSP) produced by trophozoites of Giardia intestinalis isolate WB-C6, was investigated in isolates of similar (Assemblage A / Group I) or distinct (Assemblage A / Group II) genotype. Using primers specific for the WB-C6 tsa417 gene, DNA amplified in polymerase chain reactions from genomic DNA indicated the presence, in every isolate, of an intact coding sequence possessing conserved restriction sites diagnostic for this locus (herein designated vsp417-1). Sequence analysis of the DNA amplified from the genomes of genetic Group I ("A-I") isolates revealed complete identity with the published WB-C6 tsa417 (vsp417-1(A-I)) sequence. Equivalent products, amplified from the genomes of genetic Group II ("A-II") isolates, similarly yielded an invariant and apparently allelic 2142-bp coding sequence (designated vsp417-1(A-II)) possessing 79% nucleotide identity with vsp417-1(A-I) and polymorphisms unique to Group II organisms. The encoded polypeptides (VSP417-1(A-I) and VSP417-1(A-II)) are identical at 75% of amino acid positions. Substitutions are concentrated within the N-terminal portions of the proteins, but the overall structure of VSP417-1 has changed little during the evolution of the Group I and Group II genotypes from their common clonal ancestor. An additional 0.7-kb DNA, representing a separate locus (vsp417-5) encoding a 22.3-kDa VSP, was amplified from genetic Group II genomes exclusively but only using particular primer combinations. The vsp417-5(A-II) gene exhibits >85% sequence identity with the 5' and 3' segments of vsp417-1(A-I) and vsp417-1(A-II) but it lacks a 1482-bp segment that comprises the central portion of the vsp417-1 locus. Excision of this segment seems to have occurred by intragenic recombination, possibly initiated by a stem loop formed between palindromic sequences which border the 1482-bp segment within vsp417-1 but which are contiguous in vsp417-5(A-II). The detection by Southern hybridization of additional genomic sequences that share homology with these genes reveals the existence in these two genotypes of a distinctive "vsp417" gene subset. Copyright 1998 Academic Press.