Toxoplasma gondii, the causative agent of toxoplasmosis, results in severe disease in neonates and the immunocompromised, including AIDS patients. Currently there are few effective chemotherapeutic options, and those that are available are frequently associated with adverse side effects. Ribonucleotide reductase is an enzyme which catalyzes the conversion of ribonucleotides to deoxyribonucleotides and therefore is essential for DNA synthesis. Ribonucleotide reductase (RNR) has proven to be a promising target for chemotherapy of herpes virus infections. The primary goal of this project is to characterize ribonucleotide reductase of Toxoplasma gondii and evaluate the enzyme as a potential target for chemotherapy of toxoplasmosis. As part of this effort we plan to clone the RNR genes and express biologically active enzyme. This recombinant enzyme will be fully biochemically characterized. In vitro RNR assays, in conjunction with parasite tissue culture will be used to screen candidate inhibitors. Effective agents and currently available anti-Toxoplasma agents will be tested for synergistic action against parasites. Any agents which appear to have promise as specific anti- Toxoplasma agents will also be tested in in vivo models of toxoplasmosis. In addition, peptides corresponding to the C-terminal sequences of the small subunit of RNR will be tested for their ability to inhibit RNR. Mutant parasites resistant to candidate drugs will be generated and analyzed in order to more fully understand the basis of drug-enzyme interaction. Our hope is that these studies will form the basis for design of new novel agents for treatment of toxoplasmosis.