Toroidal condensation of Z DNA and identification of an intermediate in the B to Z transition of poly(dG-m5dC).cntdot.poly(dG-m5dC)

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Abstract
A combination of spectroscopic techniques, quasi-elastic laser light scattering (QLS), and EM, shows that the B to Z transition of poly(dG-m5dC).cntdot.poly-(dG-m5dC) is accompanied by extensive condensation of the DNA in both low and high ionic strength buffers. At low concentrations of NaCl (2 mM Na+), an intermediate rodlike form, which exhibits a circular dichroism (CD) spectrum characteristic of an equimolar mixture of B and Z forms, is observed. This is produced by the orderly self-association of about 4 molecules of the polymer after prolonged incubation of a concentrated solution at 4.degree. C. On addition of 5 .mu.M Co(NH3)63+, the CD spectrum of the intermediate changes to that of the Z form, which is visualized as a dense population of discrete toroids on an EM grid stained with uranyl acetate. Addition of NaCl to a solution of poly(dG-m5dC).cntdot.poly-(dG-m5dC) in the absence of any multivalent ion condenses the polymer to toroidal structures at the midpoint (0.75 M NaCl) of the B to Z transition. Further addition of NaCl unfolds these toroids to rodlike structures, which show characteristic Z-form CD spectra. Z DNA can take up a variety of tertiary structural forms and indicate that its inverted CD spectrum is due to its left-handed helical sense rather than to differential scattering artifacts.