Unfolding of 175-base-pair nucleosomes.

Calf thymus nucleosomes containing 175 base pairs of DNA unfold in two steps as the salt concentration is lowered, as detected by electric dichroism measurements. The transition midpoints are at 2.9 and 1.1 mM ionic strength at 7 degrees C with at most a small dependence on temperature. We identify the product of the 2.9 mM transition as an expanded disklike structure similar to the product of the 1.0--1.3 mM unfolding transition of 146-base-pair nucleosomes. The product of the 1.1 mM transition in 175-base-pair nucleosomes is elongated into a more asymmetric particle.