Structure and stability of Z* DNA.

The structure and stability of the left handed Z* DNA aggregate was examined by spectroscopic methods and by electron microscopy. Poly(dGdC), upon heating in the presence of Mn++, forms a large aggregate which may be sedimented at 12,000 X g, with a circular dichroism spectrum characteristic of left handed DNA. Aggregation gives rise to turbidity changes at visible wavelengths, providing a convenient means of monitoring the transition in solution. The wavelength dependence of turbidity is consistent with the scattering behavior of a long thin rod. Electron microscopy shows that Z* DNA is a large fibrous structure of indeterminant length, with a uniform diameter of approximately 20 nm. The results obtained in solution and under the requisite conditions for electron microscopy are mutually consistent. Poly(dGdC) preparations with average lengths of 60, 240, 500, and 2000 base pairs all form Z* DNA. Poly(dGm5dC) forms Z* DNA in the presence of Mn++ without heating, but poly(dAdC)-poly(dGdT) and calf thymus DNA cannot be induced to the Z* form under any conditions tried. Kinetic studies, monitored by turbidity changes, provide evidence that the formation of Z* DNA proceeds by a nucleated condensation mechanism. Dissolution of the Z* aggregate results from the chelation of Mn++ or by the addition of the intercalator ethidium bromide. The allosteric conversion of Z* DNA to an intercalated, right handed form by ethidium is demonstrated by kinetic studies, equilibrium binding studies and circular dichroism spectroscopy. Electron microscopy provides a striking visualization of the dissolution of the Z* aggregate by ethidium.