Site-specifically located 8-amino-2 '-deoxyguanosine: thermodynamic stability and mutagenic properties in Escherichia coli

2-Nitropropane (2-NP), an important industrial solvent and a component of c igarette smoke, is mutagenic in bacteria and carcinogenic in rats, 8-Amino- 2'-deoxyguanosine (8-amino-dG) is one of the types of DNA damage found in l iver, the target organ in 2-NP-treated rats. To investigate the thermodynam ic properties of 8-amino-dG opposite each of the four DNA bases, we have sy nthesized an 11mer, d(CCATCG*CTACC), in which G* represents the modified ba se, By annealing a complementary DNA strand to this modified 11mer, four se ts of duplexes were generated each containing one of the four DNA bases opp osite the lesion, Circular dichroism studies indicated that 8-amino-dG did not alter the global helical properties of natural right-handed B-DNA, The thermal stability of each duplex was examined by UV melting measurements an d compared with its unmodified counterpart. For the unmodified 11mer, the r elative stability of the complementary DNA bases opposite G was in the orde r C > T > G > A, as determined from their -DeltaG degrees values. The free energy change of each modified duplex was lower than its unmodified counter part, except for the G*:G pair that exhibited a higher melting transition a nd a larger -DeltaG degrees than the G:G duplex. Nevertheless, the stabilit y of the modified 11mer duplex also followed the order C > T > G > A when p laced opposite 8-amino-dG. To explore if 8-amino-dG opposite another 8-amin o-dG has any advantage in base pairing, a G*:G* duplex was evaluated, which showed that the stability of this duplex was similar to the G*:G duplex. M utagenesis of 8-amino-dG in this sequence context was studied in Escherichi a coli, which showed that the lesion is weakly mutagenic (mutation frequenc y similar to 10(-3)) but still can induce a variety of targeted and semi-ta rgeted mutations.