Nucleic Acid Reactivity and Conformation

Treatment of calf thymus DNA with sodium bisulfite gave no deamination of cytosine to uracil, under conditions where 68% of the cytosines of heat-denatured DNA were converted to uracil. This implies that the mutagenic properties of bisulfite are due to reaction with single-stranded DNA. Similarly, poly(I)·poly(C) was unreactive to bisulfite under conditions where poly(C) reacted readily. It was established by ultraviolet and circular dichroism studies that the initial step of the deamination sequence, the addition of bisulfite to the 5-6 double bond of cytosine, was inhibited in double-helical polynucleotides. In accord with this, it was found that, at 25°, bisulfite did not react with the uracils of poly(A)·poly(U) and poly(A)·2 poly(U), under conditions where it added to the 5-6 double bond of the uracils of free poly(U). At higher temperatures below the melting temperature, a slow reaction of poly(A)·2 poly(U) was observed. A number of factors may contribute to the resistance of multiple-helical polynucleotides to reaction with bisulfite. These include the inability of the modified base to take part in Watson-Crick hydrogen bonding and steric hindrance to the approach of the reagent in a multiple helix.