A contribution of nonhistone proteins to the conformation of chromatin.

1. Changes in circular dichroism (CD) spectra and thermal melting profiles of guinea pigliver DNA reassociated with histones and/or nonhistone proteins from the cerebral of liver chromatin are described. 2. In the DNA-histone complex, positive ellipiticity in the CD spectrum at 260-300 nm is progressively lod by a red-shift of the crossover point at around 260 nm. DNA in this complex is thermally stabilised to a considerable extent, but not to such a full extent as is shown with DNA in native chromatin. 3. DNA-nonhistone complex in 0.14 M NaCl is, in contrast to DNA-histone complex, not precipitable by centrifugation at 20 000 X g. DNA in this complex shows only a slight reduction in ellipticity at 260-300 nm, and a very weak thermal stabilisation. 4. Characteristics in the CD spectrum of the native chromatin are most satisfactorily reproduced in the DNA-histone-nonhistone complex. These include a large decrease in ellipticity at 260-300 nm, a red-shift of the crossover point at around 260 nm, and a slight negative band at around 305 nm. Also, DNA in this complex is thermally stabilised to the extent comparable with DNA in the native chromatin. 5. Addition of nonhistone proteins to the preformed DNA-histone complex in 3 M urea renders a half of the complex, named DNA-histone(-nonhistone), unprecipitable upon centrifugation at 20 000 X g in 0.14 M NaCl. CD spectrum and thermal melting profile of the precipitable DNA-histone(-nonhistone) complex are similar to those of the DNA-histone-nonhistone complex, while in the unprecipitable DNA-histone(-nonhistone) comples, the ellipticity at 260-300 nm is significantly elevated and the highest melting transition (at 80 degrees C) is lacking. 6. The CD spectrum of native cerebral chromatin closely resembles that of unprecipitable DNA-histone(-nonhistone) complex, while in liver chromatin, the spec.trum is an intermediate between those of the unprecipitable and pn of chromatin by nonhistone proteins. Cerebral nonhistone proteins bind to DNA and to the DNA-histone complex more extensively than liver nonhistone proteins. 7. It is concluded that, although the basic conformation of DNA in native chromatin is determined largely by histones, nonhistone proteins also play an individual role. There is also an indication that nonhistone proteins exert an organ-specific modification of chromatin superstructure.