Model nucleohistones: The interaction of F1 and F2al histones with native T7 DNA

Abstract

Synthetic complexes of the lysine-rich histone (F1) and of the glycine- and arginine-rich histone (F2al) with native T7 DNA have been prepared by the step-gradient dialysis procedure from 5 m-urea-3 m-NaCl and from 5 m-guanidine-HCl. Data on the physical properties of these complexes were obtained relevant to their solubility, thermal stability, sedimentation coefficients, circular dichroic spectra, and ultrastructure. Complexes of F1 with T7 DNA, prepared from urea-NaCl or from guanidine, exhibited a salt-dependent structural change between 10 −3 and 10 −1 m-sodium ions. At low ionic strengths, complexes sedimented like naked DNA and appeared to be a loose network of DNA-like fibers when examined in the electron microscope. At higher ionic strengths the complexes exhibited rapid and heterogeneous sedimentation and an altered circular dichroic spectrum, and they appeared in the electron microscope to be a mixture of large (0.1 to 0.3 μ diameter) donut and stem-like structures. Complexes of F2al with T7 DNA prepared from urea-NaCl and from guanidine revealed differences with respect to melting, sedimentation, circular dichroic, and electron microscopic properties. Complexes prepared from urea-NaCl sedimented as a single rapid heterogeneous component and resembled rosettes in the electron microscope; complexes from guanidine exhibited greater thermal stability, sedimented as two components, and revealed the presence of donut structures. Neither type of complex revealed salt-dependent structural changes of the magnitude seen with F1-DNA complexes. Poly- l-lysine and T7 DNA were combined by gradient dialysis from urea-NaCl, and also appeared as donuts and short stem structures over a wide range of ionic strengths. The present studies suggest that the structure of the annealed protein-DNA complexes strongly depends on the conformation of the basic protein during the initial stages of its interaction with native DNA. A highly disordered protein conformation results in characteristic donut and stem-like structures. F2al histone, however, aggregates in high NaCl concentrations, probably due to its hydrophobic portion (residues 46 to 102), resulting in rosette structures. The instability of F1-DNA donuts and stems at decreased ionic strength may result from the high proline content in the basic portion of F1 histone, consequent disruptions of lysine-phosphate interactions, and increased susceptibility to polyelectrolyte expansion.