(+)-CC- 1065 is an extremely potent antitumor agent produced by Streptomyces zefensis. The potent effects of (+)-CC-1065 and its alkylating analogues are thought to be due to the formation of a covalent adduct through N3 of adenine in DNA. It has been previously postulated, on the basis of modeling studies, that a phosphate may be involved in stabilization of the adduct and in acid catalysis of this reaction. In this study, using 'H NMR in combination with I70-labeled water and phosphate, we demonstrate the involvement of a bridging water molecule between a phenolic proton on the alkylating subunit of (+)-CC-1065 and an anionic oxygen in the phosphate on the noncovalently modified strand of DNA. In addition, a second ordered water molecule associated with one of the protons on N6 of the covalently modified adenine is also identified. This structure has important implications for catalytic activation of the covalent reaction between (+)-CC- 1065 and DNA and, consequently, the molecular basis for sequence-selective recognition of DNA by the alkylating subunit of (+)-CC-1065. On the basis of the example described here, the use of 'H NMR in "0-labeled water may be a powerful probe to examine other structures and catalytic processes for water-mediated hydrogen-bonded bridges that occur between small molecules and DNA or enzymes.