A GIARDIA-DUODENALIS GENE ENCODING A PROTEIN WITH MULTIPLE REPEATS OFA TOXIN HOMOLOG

A gene, CRP136, from the intestinal protozoan parasite Giardia duodena lis, expressed at a high level in a metronidazole-resistant line, enco des a 136 kDa protein with 23 copies of a 40 amino acid repeat. The pr otein is cysteine-rich and has the typical membrane-spanning region an d CXXC amino acid motifs of a family of Gini din cysteine-rich surface proteins (CRSPs). The repeat unit in CRP136, shares 57 %, homology wi th the gene encoding the precursor of the sarafotoxins, a group of sna ke toxins from the burrowing adder known to cause symptoms similar to those of humans acutely infected with Giardia. The sarafotoxins are lo w molecular weight sulphydryl cross-linked peptides which are proteoly tically cleaved from a precursor polyprotein. CRP136 has homology over the entire length of the sarafotoxin precursor, and the repeats are o f the same length. Thus CRP136 represents the first evidence for a pot ential Giardia toxin. The genomic copy number of CRP136 appears to be the same in both the parent and drug-resistant lines and expression of this gene, and at least one other, is associated with a conserved par tial duplication, but not amplification, of one chromosome.