Giardia intestinalis glucosamine 6-phosphate isomerase: the key enzyme to encystment appears to be controlled by ubiquitin attachment.

The cyst wall of the parasitic protozoan, Giardia intestinalis, is composed of a polymer of N-acetylgalactosamine, the precursor of which is synthesized by an inducible enzyme pathway. The first enzyme in this pathway, glucosamine 6-phosphate isomerase, is transcriptionally regulated. During encystment and in mature cysts this isomerase appears to be modified by ubiquitin attachment. Thus, it might be targeted for destruction by an ubiquitin-mediated pathway, suggesting that glucosamine 6-phosphate isomerase expression is tightly regulated.