The antitumor antibiotic auromomycin (AUR) inhibits cell growth by causing strand breaks in DNA. The drug is composed of a non-protein chromophore, which is the cytotoxic component, and an apoprotein called macromomycin (MCR, Mr=10,700). The apoprotein, which is not involved in the antitumor activity, serves as a carrier, protecting the chromophore from degradation and regulating its release. This project is designed to study on a molecular basis the cytotoxic activity of AUR by means of crystallographic studies of the components of the antibiotic. The objectives are the determination at high resolution of the crystal structures of the holoantibiotic AUR, the apoprotein MCR and the chromophore and their comparisons with those of other proteins with antitumor activity. These structures will determine unambiguously: the chemical composition of the chromophore; the nature of the binding of the chromophore to the protein; the amino acids involved in this binding; the functional groups the chromophore employs for binding to the protein and to the DNA and for the strand scission activity; the conformational differences involved in the binding of the chromophore by the protein. Interpretation of the structures may suggest possible mechanisms for the chromophore release and identify residues involved in the interaction with the cell membrane. Once the structure of the chromophore is known its binding to DNA can be studied either crystallographically by the study of complexes with DNA model compounds or by modelling studies using graphic representations, moelcular mechanics and energy refinement. Combination of the results of these studies with biochemical and chemotherapeutic studies will result better understanding of the actions of the antitumor agent and aid in the development of potential anticancer drugs and proteins as specific drug delivery systems. The most important progress has been in the study of the apoprotein. MCR has been crystallized in six different crystal forms. Molecular replacement studies have been used to determine the crystal structures of two of these forms. Refinement of these structures has been started. AUR crystals have been obtained which are isomorphous with those of MCR.