Circular dichroism of poly(dG-dC) modified by the carcinogens N-methyl-4-aminoazobenzene or 4-aminobiphenyl

The conformational effect of para-substituted C8-arylguanine adducts on the B/Z-DNA equilibrium

The B form of DNA exists in equilibrium with the Z form and is mainly affected by sequence, electrostatic interactions, and steric effects. C8-purine substitution shifts the equilibrium toward the Z form though how this interaction overcomes the unfavorable electrostatic interactions and decrease in stacking in the Z form has not been determined. Here, a series of C8-arylguanine derivatives, bearing a para-substituent were prepared and the B/Z equilibrium determined. B/Z ratios were measured by CD and conformational effects of the aryl substitution determined by NMR spectroscopy and molecular modeling. The para-substituent was found to have a significant effect on the B/Z DNA equilibrium caused by altering base-pair stacking of the B form and modifying the hydration/ion shell of the B form. A unique melting temperature versus salt concentration was observed and provides evidence relevant to the mechanism of B/Z conformational interconversion.

Spectroscopic studies of STZ-induced methylated-DNA in both in vivo and in vitro conditions

Alkylating agents after formation of DNA adduct not only possess their harmful role on living cells but also can transfer this information to the next generation. Different techniques have been introduced to study the alkylated DNA, most of which are specific and designed for investigation of specific target DNA. But the exact differences between spectroscopic and functional properties of alkylated DNA are not seen in the literature. In the present study DNA was methylated using streptozotocin (STZ) by both in vitro and in vivo protocols, then methylated-DNA was investigated by various techniques. Our results show that (1) the binding of ethidium bromide as an intercalating dye decreases to methylated-DNA in comparison with normal DNA, (2) CD spectra of methylated-DNA show changes including a decrease in the positive band at 275 nm and a shift from 258 nm crossover to a longer wavelength, which is caused by reduction of water around it, due to the presence of additional hydrophobic methyl groups, (3) the stability of methylated-DNA against DTAB as a denaturant is decreased and (4) the enzyme-like activity of methylated-DNA in an electron transfer reaction is reduced. In conclusion, additional methyl groups not only protrude water around DNA, but also cause the loss of hydrogen bonding, loosening of conformation, preventing desired interactions and thus normal function of DNA.

Mutation spectra of Glu-P-1 in Salmonella: induction of hotspot frameshifts and site-specific base substitutions

We used colony probe hybridization and PCR/DNA sequence analysis to determine the mutations in approximately 1,640 revertants of the -1 frameshift allele hisD3052 and approximately 260 revertants of the base substitution allele hisG46 of Salmonella typhimurium induced by the heterocyclic amine cooked food mutagen 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1). All of the mutations were at sites containing guanine, which is the base at which Glu-P-1 forms DNA adducts. A hotspot mutation involving the deletion of a CG or GC within the sequence CGCGCGCG accounted for 100% of the Glu-P-1-induced mutations at the frameshift allele in strains TA1978 (uvr+) and TA1538 (delta uvrB) and 99% in TA98 (delta uvrB, pKM101). To explain the induction of these hotspot mutations by Glu-P-1, we describe here a more detailed version of our recently proposed correct incorporation/slippage model [Genetics:136:731, 1994]. We propose that after cytosine is incorporated correctly opposite a Glu-P-1-adducted guanine, various slipped intermediates may form (a total of 18), depending on which guanine is adducted and whether it remains within the helix or becomes extrahelical. This variety of mutational pathways may account for the high mutability of the hotspot sequence by Glu-P-1. Although the pKM101 plasmid does not influence the mutagenic potency or mutational spectrum of Glu-P-1 at the frameshift allele, it is required by Glu-P-1 to revert the base substitution allele, where Glu-P-1 induces G-C --> T-A transversions (75%) and G-C --> tA-T transitions (25%) exclusively at a single site (the second position of the CCC codon of the hisG46 allele). The limited (20-30 times less) base substitution mutagenic potency of Glu-P-1 relative to its frameshift mutagenic potency as well as the extreme site specificity exhibited by Glu-P-1 for base substitutions may have bearing on the lack of base substitutions identified in ras genes in Glu-P-1-induced rat colon tumors.