Aspects of Chromosomal Structure I. Circular Dichroism Studies

Circular dichroism spectroscopy was used to examine the chromosomal components of intact nuclei, broken nuclei, isolated chromatin, and residual chromatin complexes from which specific chromosomal proteins had been removed. The DNA component of intact nuclei displayed a positive ellipticity band at 264 nm characterizing a nonconservative DNA spectrum. Upon lysis of the nuclei the conservative DNA spectrum characteristic of DNA in aqueous solution was seen to reappear. The DNA region of the chromatin circular dichroism spectrum was little different from that of broken nuclei. These observations are taken as evidence of a unique geometry for nuclear DNA due to specific DNA packaging in the nucleus. A specific change in DNA geometry within isolated chromatin indicated by circular dichroism spectroscopy is observed concomitant with the removal of a major portion of histone IV (f2al) from chromatin, suggesting a DNA conformation determining role for this histone. The observed conformational change in the DNA component of chromatin shows a direct correlation with the RNA template activity of the chromatin. On the basis of this evidence histone IV is suggested to act as a general repressor of RNA synthesis through its conformational effect on the DNA template. The results of this study indicate that the conformation of DNA in the chromosomes of eucaryote cells is determined by both the nuclear environment of the chromosomal material and the specific interactions between the DNA and chromosomal protein components of the chromosome.