Effects of H1 histone variant overexpression on chromatin structure

The importance of histone H1 heterogeneity and total H1 stoichiometry in ch romatin has been enigmatic Here we report a detailed characterization of th e chromatin structure of cells overexpressing either H1(0) or H1c. Nucleoso me spacing was found to change during cell cycle progression, and overexpre ssion of either variant in exponentially growing cells results in a Iii-bas e pair increase in nucleosome repeat length. H1 histones can also assemble on chromatin and influence nucleosome spacing in the absence of DNA replica tion. Overexpression of H1(0) and, to a lesser extent, H1c results in a dec reased rate of digestion of chromatin by micrococcal nuclease. Using green fluorescent protein-tagged H1 variants, we show that micrococcal nuclease-r esistant chromatin is specifically enriched in the H1(0) variant. Overexpre ssion of H1(0) results in the appearance of a unique mononucleosome species of higher mobility on nucleoprotein gels. Domain switch mutagenesis reveal ed that either the N-terminal tail or the central globular domain of the H1 (0) protein could independently give rise to this unique mononucleosome spe cies. These results in part explain the differential effects of H1(0) and H 1c in regulating chromatin structure and function.