Effects of H1 Histone Variant Overexpression on Chromatin Structure

The importance of histone H1 heterogeneity and total H1 stoichiometry in chromatin has been enigmatic. Here we report a detailed characterization of the chromatin structure of cells overexpressing either H1 0 or H1c. Nucleosome spacing was found to change during cell cycle progression, and overexpression of either variant in exponentially growing cells results in a 15-base pair increase in nucleosome repeat length. H1 histones can also assemble on chromatin and influence nucleosome spacing in the absence of DNA replication. Overexpression of H1 0 and, to a lesser extent, H1c results in a decreased rate of digestion of chromatin by micrococcal nuclease. Using green fluorescent protein-tagged H1 variants, we show that micrococcal nuclease-resistant chromatin is specifically enriched in the H1 0 variant. Overexpression of H1 0 results in the appearance of a unique mononucleosome species of higher mobility on nucleoprotein gels. Domain switch mutagenesis revealed that either the N-terminal tail or the central globular domain of the H1 0 protein could independently give rise to this unique mononucleosome species. These results in part explain the differential effects of H1 0 and H1c in regulating chromatin structure and function.