Transient electric dichroism studies of nucleosomal particles

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Abstract
Transient electric dichroism experiments on nucleosomal core particles of calf thymus containing 140 and 175 base pairs of DNA, and on spacerless dinucleosomes, were reported. All particles possess a permanent dipole moment. The orientation time of 140 base pair nucleosomes implies an estimated maximum dimension of a = 130 .ANG. (a must be at least 111 .ANG.), consistent with the disk model. The maximum dimension of the spacerless dinucleosome is estimated to be about 290 .ANG. (at least 180 .ANG.), ruling out a structure in which 2 disks are stacked directly on top of each other. The reduced dichroism amplitude indicates that the DNA superhelix axis in nucleosomes aligns perpendicular to the electric field, as expected for a dipole moment directed along a C2 symmetry axis across the disk diameter. Nucleosomes containing 175 base pairs of DNA show a substantially larger dichroism amplitude than do 140 base pair nucleosomes. In the context of the disk model, this result is shown to be consistent with 100 base pairs of DNA per superhelical turn, but not with 80 base pairs per turn.