Changes in the helical repeat of poly(dG-m5dC) . poly(dG-m5dC) and poly(dG-dC) . poly(dG-dC) associated with the B-Z transition.

Abstract

Random sequence DNA adsorbed to surfaces such as calcium phosphate has recently been shown to be cut preferentially by nucleases1 at sites separated by 10.6±0.1 nucleotides2, corresponding to the helical periodicity of the DNA. We have now applied a variation of this technique to determine the periodicity of the alternating synthetic polydeoxynucleotides poly(dG-dC) . poly(dG-dC) and poly(dG-m5dC) · poly(dG-m5dC) · We have also obtained X-ray diffraction patterns from fibres of both polymers to compare then helical parameters. Evidence from a variety of physical measurements3–5 suggests that in appropriate solvent conditions these polymers can undergo a transition from the usual B form to a new structure tentatively identified with the left-handed Z form. We find by X-ray fibre diffraction that the methylated polymer can adopt a Z form indistinguishable from that of the unmethylated polymer. We further find that when adsorbed to calcium phosphate or calcium oxalate, the B form of each polymer has a repeat of ∼10.5 base pairs (bp), while the repeat of the Z form is ∼13.6 bp.