Nucleosomal arrays self-assemble into supramolecular globular structures lacking 30-nm fibers

The existence of a 30-nm fiber as a basic folding unit for DNA packaging has remained a topic of active discussion. Here, we characterize the supramolecular structures formed by reversible Mg2+-dependent self-association of linear 12-mer nucleosomal arrays using microscopy and physicochemical approaches. These reconstituted chromatin structures, which we call oligomers, are globular throughout all stages of cooperative assembly and range in size from similar to 50 nm to a maximum diameter of similar to 1,000 nm. The nucleosomal arrays were packaged within the oligomers as interdigitated 10-nm fibers, rather than folded 30-nm structures. Linker DNA was freely accessible to micrococcal nuclease, although the oligomers remained partially intact after linker DNA digestion. The organization of chromosomal fibers in human nuclei in situ was stabilized by 1 mM MgCl2, but became disrupted in the absence of MgCl2, conditions that also dissociated the oligomers in vitro. These results indicate that a 10-nm array of nucleosomes has the intrinsic ability to self-assemble into large chromatin globules stabilized by nucleosome-nucleosome interactions, and suggest that the oligomers are a good in vitro model for investigating the structure and organization of interphase chromosomes.