Cloning of two putative Giardia lamblia glucosamine 6-phosphate isomerase genes only one of which is transcriptionally activated during encystment

The biosynthesis of the carbohydrate component of the cyst wall of the prot ozoan parasite Giardia lamblia, a polymer of N-acetylgalactosamine (GalNac) , is by a pathway that is initiated with the conversion of fructose 6-phosp hate to glucosamine 6-phosphate by an aminating isomerase, glucose 6-phosph ate isomerase. This enzyme appears only after Giardia trophozoites are indu ced to start the production of cyst wall components after bile is added. To investigate whether induction of glucosamine 6-phosphate isomerase is by p rotein modification or by transcription activation, its gene was cloned and sequenced. Two genes, gpi1 and gpi2, encoding putative glucosamine 6-phosp hate isomerases were identified but only one, gpi1 was expressed. The trans cript for gpi1 appeared not earlier than 6 h after cells were induced with bile salts. These results show that the first enzyme in the pathway leading to GalNac synthesis in encysting Giardia cyst wall biosynthesis is under t ranscriptional control.