Circular Dichroism of Avian‐Erythrocyte Chromatin and Ethidium Bromide Bound to Chromatin

Abstract

The circular dichroism of mature gander erythrocyte chromatin was compared with that of partially dehistonized chromatin and deproteinized DNA. The ellipticity at 275 nm for native chromatin was found to be three‐times less than that for DNA. The 275‐nm transition was gradually increased in magnitude upon removal of histones. Selective removal of the very lysine‐rich histone I and the erythrocyte‐specific histone V, but not histone I alone, resulted in a significant increase in transcription in vitro and ethidium bromide primary binding properties of chromatin. Concomitant with these changes a “blue shift” of the positive DNA band from 282 nm for native chromatin to 275 nm for purified DNA was observed. Optimal conditions for circular dichroism studies in ethidium bromide bound to DNA and native chromatin were determined. The circular dichroic spectra of ethidium bromide bound to native chromatin and partially deproteinized chromatin were found to be essentially the same as that exhibited by ethidium‐bromide · DNA complexes. However, the ellipticity of the dye bound to native chromatin was smaller than that when ethidium bromide was bound to DNA. The 310‐nm dye transition was increased after the removal of chromosomal proteins. Selective removal of histone I and V was found to generate the most notable increment in ellipticity. The represent results are consistent with recent interpretations on how ethidium bromide interacts with chromatin; native chromatin has fewer ethidium bromide primary binding sites than deproteinized DNA; the mode of this dye interaction in these two systems is nearly the same, if not identical.