The fundamental objective of the proposed research is to establish a basic level of knowledge concerning the detailed molecular structure of the enzyme dihydrofolate reductase. It is expected that this enzyme's structural variability from species to species will provide a basis for rational development of species specific chemotherapeutic drugs. We expect to solve the X-ray crystal structure of chicken liver dihydrofolate reductase containing bound NADPH. Phases from this structure will then be used to obtain electron density maps for the closely isomorphous structures of the apoenzyme and the enzyme:NADP ion binary complex. The binding of substrates and inhibitors to the chicken liver enzyme will be studied using difference Fourier techniques. Additional structural studies are in progress on a dihydrofolate reductase specified in E. coli by R-plasmid R67. This enzyme is insensitive to all diaminoheterocyclic inhibitors, is composed of four homogeneous subunits and shows no sequence homology to any known dihydrofolate reductase.