Materials and Methods. Organic chemicals used for synthesis of oligodeoxynucleotides were supplied by Aldrich Chemical Co. Acetonitrile, triethylamine, and distilled water, all HPLC-grade, were purchased from Fisher Chemical Co.[7-32P] ATP (specific activity 5000 Ci/mmol) was obtained from AmershamCorp. Cloned Klenow fragment of E. coli DNA polymerase I and T4 polynucleotide kinase were purchased from International Biotechnologies, Inc.; calf thymus DNA polymerase a was from Molecular Biology Resources Inc.; DNA polymerase I and deoxynucleotide triphosphates were from Pharmacia; and nuclease PI and alkaline phosphatase, type III-S, were from Boehringer Mannheim Biochemicals and Sigma Chemical Co., respec-tively. DNA polymerase/3, provided by Samuel H. Wilson, was purified as described previously (Abbots et al., 1988). A Waters 990 HPLC instrument, equipped with a photodiode array detector, was used for theseparation and analysis of modified and unmodified oligodeoxynucleotides.

Synthesis and Purification of Oligodeoxynucleotides. The DMT-phosphoramidite of 8-oxo-dA, required forDNA oligomer synthesis, was synthesized by a five-step procedure, similar to that used to prepare 8-oxo-dG. Treatment of 8-bromo-2'-deoxyadenosine with sodium benzylate in benzyl alcohol afforded 8-benzyloxy-2-deoxyadenosine in 82% yield. Transient hydroxyl protection of the latter material with