TELOMERIC ORGANIZATION OF A VARIABLE AND INDUCIBLE TOXIN GENE FAMILY IN THE ANCIENT EUKARYOTE GIARDIA-DUODENALIS

Giardia duodenalis is the best-characterized example of the most ancie nt eukaryotes, which are primitively amitochondrial and anaerobic. The surface of Giardia is coated with cysteine-rich proteins. One family of these proteins, CRP136, varies among isolates and upon environmenta l stress. A repeat region within the CRP136 family is interchangeable by a cassette-like mechanism, generating further diversity in repeat s ize, copy number, and sequence. Flanking the 5' region of the CRP136 f amily is a novel protein kinase gene and an ankyrin homolog, creating a conserved unit. A short spacer separates the ankyrin gene from the v ariable, tandem array of rDNA gene units at a common breakpoint within the large subunit gene, which is followed by the (TAGGC)(n) telomeric sequence. Transcriptional up-regulation of the CRP136 family is accom panied by a switch in mRNA length and promoter, or de novo expression, and suggests that CRP136 mRNA induction is under the control of a tel omerically regulated position effect, which evolved very early in the eukaryotic lineage.