Role of nonhistone chromosomal proteins in determining circular dichroism spectra of chromatin.

Complexing of histone proteins, from WI-38 cells with pure DNA from WI-38 cells, causes a marked decrease in the amplitude of the positive ellipticity band and a red shift in circular dichroism spectra in the 250–300 nm region. Total nonhistone chromosomal proteins from WI-38 cells (without histones) cause an analogous effect, but of significantly reduced magnitude. However, the two effects are not additive, because, when DNA is complexed with both histones and nonhistones, the amplitude of the positive ellipticity band has an intermediate value, between the histone-DNA complex and the nonhistone-DNA complex. Removal of certain nonhistone proteins from chromatin of WI-38 cells, by extraction with 0.25–0.35 m NaCl, causes a decrease in the positive circular dichroism band in the 250–300 nm region. Removal of histones and other nonhistone proteins from chromatin by extraction with 0.75 and 1.5 m NaCl causes a strong increase in positive ellipticity. This suggests the existence of modest but definite effects of nonhistone proteins in determining DNA conformation in native chromatin. Taken as a whole, nonhistone chromosomal proteins have a weaker but analogous effect to that of histones, while the nonhistone proteins extractable with 0.25–0.35 m NaCl have an opposite effect.