Nucleosome repeat length and linker histone stoichiometry determine chromatin fiber structure.

To understand how nuclear processes involving DNA are regulated, knowledge of the determinants of chromatin condensation is required. From recent structural studies it has been concluded that the formation of the 30-nm chromatin fiber does not require the linker histone. Here, by comparing the linker histone-dependent compaction of long, reconstituted nucleosome arrays with different nucleosome repeat lengths (NRLs), 167 and 197 bp, we establish that the compaction behavior is both NRL- and linker histone-dependent. Only the 197-bp NRL array can form 30-nm higher-order chromatin structure. Importantly for understanding the regulation of compaction, this array shows a cooperative linker histone-dependent compaction. The 167-bp NRL array displays a limited linker histone-dependent compaction, resulting in a thinner and topologically different fiber. These observations provide an explanation for the distribution of NRLs found in nature.