Electric birefringence of DNA and chromatin: Influence of divalent cations

The effects of divalent cations on the DNA and chromatin conformation have been investigated by electric birefringence and birefringence relaxation measurements at low and constant ionic strength (0.001). An important decrease of the intrinsic optical anisotropy of DNA has been found in the presence of Mn 2+and Cu 2+ but not with Mg 2+. A complex variation of the mean relaxation time with the ratio I/P of ion to DNA-phosphate molar concentration has been evidenced in the presence of Mn 2+ and Cu 2+, while the mean relaxation time monotonously decreased in the presence of Mg 2+ These observations are interpreted in terms of a specific organization of DNA in a compact, rigid structure, in the presence of Mn 2+ and Cu 2+, and a non-specific coiling in the presence of Mg 2+. Drastic conformational changes encountered by chromatin in the presence of Mg 2+ and Mn 2+ cations have also been evidenced through electric birefringence measurements. They are interpreted by the formation of a superhelical compact arrangement of nucleosome strings which yielded a reversal of the birefringence sign with respect to the negative anisotropy observed in the presence of Na + ions. The removal of the histone HI prevented the appearance of this quaternary structure. More extended fragments of the chromatin chain obtained by ECTHAM chromatography of sonicated chromatin could not afford such compact arrangements.