The tsetse fly-transmitted protozoan parasite Trypanosoma brucei is the causative agent of human African sleeping sickness and the cattle disease Nagana. The bloodstream form of the parasite uses a dense cell-surface coat of variant surface glycoprotein to escape the innate and adaptive immune responses of the mammalian host and a highly glycosylated transferrin receptor to take up host transferrin, an essential growth factor. These glycoproteins, as well as other flagellar pocket, endosomal, and lysosomal glycoproteins, are known to contain galactose. The parasite is unable to take up galactose, suggesting that it may depend on the action of UDPglucose 4 -epimerase for the conversion of UDP-Glc to UDP-Gal and subsequent incorporation of galactose into glycoconjugates via UDP-Gal-dependent galactosyltransferases. In this paper, we describe the cloning of T. brucei galE, encoding T. brucei UDP-Glc-4 -epimerase, and functional characterization by complementation of a galE-deficient Escherichia coli mutant and enzymatic assay of recombinant protein. A tetracycline-inducible conditional galE null mutant of T. brucei was created using a transgenic parasite expressing the TETR tetracycline repressor protein gene. Withdrawal of tetracycline led to a cessation of cell division and substantial cell death, demonstrating that galactose metabolism in T. brucei proceeds via UDP-Glc-4 -epimerase and is essential for parasite growth. After several days without tetracycline, cultures spontaneously recovered. These cells were shown to have undergone a genetic rearrangement that deleted the TETR gene. The results show that enzymes and transporters involved in galactose metabolism may be considered as potential therapeutic targets against African trypanosomiasis.UDP-Gal ͉ galE ͉ epimerase T he tsetse fly-transmitted protozoan parasite Trypanosoma brucei causes human African sleeping sickness and the related cattle disease Nagana. There are 300,000-500,000 cases of the human disease per year in sub-Saharan Africa (1). The bloodstream form of the parasite divides by binary fission in the blood, lymph, and interstitial fluids of the mammalian host, and causes cachexia and anaemia in cattle and neurological disturbances in man. These conditions are fatal if not treated, and existing chemotherapies are toxic and difficult to administer. The need for new, less-toxic therapeutics is widely acknowledged (1).The bloodstream form of T. brucei is rich in galactosecontaining glycoproteins, most notably the variant surface glycoprotein (VSG) that, at 5 ϫ 10 6 homodimers per cell, forms a dense cell-surface coat. The VSG coat is a macromolecular diffusion barrier that protects the parasite from the innate immune system and also enables the parasite to undergo antigenic variation. Thus, although an individual parasite only expresses one VSG gene at a time, the parasite population can stay ahead of the host's specific immune response to expressed VSGs when a few parasites switch expression to one of several hundred genes encoding immunologically ...