Conformation of the HMG 14 nucleosome core complex from flow birefringence.

Flow birefringence and extinction angles have been measured for HMG 14 complexes with nucleosome core particles from chicken erythrocytes under cooperative "tight" binding conditions, and for the uncomplexed core particles used in the preparations. Results are interpreted using optical models for the observed DNA anisotropy, and are compared to recent small angle neutron scattering results. (19) The studies effectively rule out highly distorted DNA conformations and configurations in which DNA ends are unwound and extended. It is concluded that the most likely conformation of the complex is one in which the DNA superhelix is radially increased, either uniformly or bilaterally, with the DNA ends remaining tightly bound to the particle. This conformation does not require large changes in spatial relationships between the DNA ends compared to the uncomplexed core as would accompany, for example, significant unwinding of the ends. However, it may lead to more subtle but possibly highly significant differences in the angles at which the DNA exits the core particle.