Distinctive higher-order chromatin structure at mammalian centromeres.

The structure of the higher-order chromatin fiber has not been defined in detail. We have used a novel approach based on sucrose gradient centrifugation to compare the conformation of centromeric satellite DNA-containing higher-order chromatin fibers with bulk chromatin fibers obtained from the same mouse fibroblast cells. Our data show that chromatin fibers derived from the centromeric domain of a chromosome exist in a more condensed structure than bulk chromatin whereas pericentromeric chromatin fibers have an intermediate conformation. From the standpoint of current models, our data are interpreted to suggest that satellite chromatin adopts a regular helical conformation compatible with the canonical 30-nm chromatin fiber whereas bulk chromatin fibers appear less regularly folded and are perhaps intermittently interrupted by deformations. This distinctive conformation of the higher-order chromatin fiber in the centromeric domain of the mammalian chromosome could play a role in the formation of heterochromatin and in the determination of centromere identity.