Two genes encoding homologous 70-kDa surface proteins are present within individual trophozoites of the binucleate protozoan parasite Giardia intestinalis.

A 0.52-kb DNA sequence encoding part of a major surface antigen of Giardia intestinalis trophozoites has been amplified by the polymerase chain reaction (PCR) using primers specific for nucleotide sequences that are conserved between two apparently related genes, tsp11 (cloned previously from the Australian G. intestinalis isolate, Ad-1) and tsa417 (cloned from the Afghanistani isolate, WB). Restriction analysis revealed that the DNA amplified from each of seven axenic isolates of G. intestinalis was not homogeneous, even though the template DNA had been purified from cultures that had been established from single trophozoites. Every isolate yielded two PCR products, whose respective cleavage fragments corresponded to tsp11 (HindIII+, PstI-, KpnI-) and to tsa417 (HindIII-, PstI+, KpnI+). This was confirmed by cloning individual amplification products into the plasmid vector, pGEM-7Zf(+). The sequence of one cloned fragment (1-P4), derived from the Ad-1 isolate, but possessing restriction sites characteristic of tsa417, exhibited 98.6% identity over 425 bp with tsa417 and only 63.8% identity with the corresponding region of tsp11. The data indicate that individual trophozoites of all the isolates examined contain a copy of each of these homologous genes.