African trypanosomes (Trypanosoma brucei) are protozoan parasites that cause a fatal disease known as African sleeping sickness in humans and related diseases in animals. These diseases have devastating health and economic consequences. Pathogenesis of T. brucei is critically dependent on the phenomena of antigenic variation. During infection of the mammalian host the majority of the trypanosome population is destroyed by the immune response to the Variant Surface Glycoprotein (VSG) coat. In the process of antigenic variation, individual trypanosomes are able to switch the VSG gene that is expressed, from a repertoire of several hundred different VSG genes, and evade immune destruction. To be expressed, a VSG gene has to be located within a telomeric expression site (ES). However, there are approximately 20 ESs and only one is active at a time. The mechanism of selective expression and switching of silent copies of VSG is not entirely understood. Previous analysis indicates that regulation of a VSG ESs correlate with the presence of a uniquely modified DNA base, represented by the glycosylation of thymine residues and called base J. The association of J with transcriptional repression of telomeric VSG genes suggests its role in regulation of antigenic variation. We have recently shown that base J is also localized throughout the trypanosome genome enriched at regions flanking Pol II polycistronic transcription units (PTU). This suggest base J also plays a biological role in transcription initiation and termination in organisms where very little is understood concerning the regulation of Pol II transcription. The broad, long-term objectives of this proposal are to use the biochemical and genetic analyses of enzymes involved in J biosynthesis to advance our understanding of the mechanism and biology of J-function in T. brucei. These studies will contribute to a fundamental understanding of the molecular mechanisms of trypanosome gene expression, antigenic variation and pathogenesis. The potentially significant role of J to the survival of the parasite, and the absence of base J in their mammalian hosts, indicates attractive targets for the design of inhibitors with broad specificity and low toxicity. Enzymes and co-factors involved in J-biosynthesis would represent such targets. These studies may prove useful in identifying novel approaches to prevention, treatment and diagnosis of the debilitating and deadly diseases caused by these parasites.