EFFECTS OF METHYLATION ON A SYNTHETIC POLYNUCLEOTIDE - THE B-Z TRANSITION IN POLY(DG-M5DC).POLY(DG-M5DC)

The behavior in solution of the synthetic polynucleotide poly(dG-m5dC).cntdot.poly(dG-m5dC) was compared with that of the unmethylated polynucleotide poly(dG-dC).cntdot.poly(dG-dC). In solutions containing high concentrations of salt, poly(dG-dC).cntdot.poly(dG-dC) exhibits altered circular dichroic and absorption spectra correlated with formation of a left-handed Z DNA structure. Poly(dG-m5dC).cntdot.poly(dG-m5dC) behaves similarly, but the spectral transition from the B to the Z form occurs at much lower salt concentrations, close to usual physiological conditions. Divalent and polyvalent ions are particularly effective: the B-Z transition of poly(dG-m5dC).cntdot.(dG-m5-dC) can be induced at a Mg2+ concentration 3 orders of magnitude lower than that required for the unmethylated polymer. Mixed copolymers containing both dC and m5dC were also studied. Results suggest that the sequence m5dC-dG, which occurs in eukaryotic DNA, can have a disproportionately large effect on the B-Z transition.