Purification of yeast histones competent for nucleosome assembly in vitro

We have developed a procedure to purify nucleosomal assembly‐competent histones as a mixture of H2A, H2B, H3 and H4 from isolated nuclei of the yeast Saccharomyces cerevisiae with a purity of 70–80%. The mixture contained each of the histone subunits approximately at the equi‐molar ratio. Plasmid pBR322 DNA was assembled into nucleosomes with the purified yeast histones in the presence of nucleoplasmin from unfertilized eggs of the frog Xenopus laevis. The efficiency of assembly of yeast histones was comparable to that of core histones purified from HeLa cells. The length of DNA fragment wrapping around a core histone particle and the molar ratio of histone components in an assembled nucleosome particle were estimated to be 150 ± 10 bp long and H2A:H2B:H3:H4 = 1·0:0·9:0·9:1·0, respectively.