We recently found that a selective photoreaction of 5-iodouracil (U-I) occurs in 5'-(G/C)AA(I)U(I)U-3' and 5'-(G/C)A(I)U(I)U-3' sequences in U-I-substituted duplex DNA. In this study, the photoreactivity of the 5'-G(A)(n)(UT)-U-I-3' sequence was examined using various U-I-containing oligonucleotides. HPLC analysis revealed that their photoreactivity largely depends on the number of As between G and U-I. The most efficient reactivity was observed when the number of As was two and this decreased with increasing numbers from three to five, as observed for the 5'-G(A)(n)(UT)-U-I-3' sequence. These results indicate that the G located 5' from U-I acts as an electron donor for U-I, as in the photoreaction of U-Br. In sharp contrast to the U-Br photoreaction, U-I was photoreactive when the number of As was zero or more than five. These results indicate that both homolytic and heterolytic pathways operate in the formation of the uracil-5-yl radical in the photoreaction of U-I in duplex DNA. In addition, the ratio of these pathways is highly dependent on DNA sequence. (C) 2007 Elsevier Ltd. All rights reserved.