Nucleosome fractionation by mercury affinity chromatography. Contrasting distribution of transcriptionally active DNA sequences and acetylated histones in nucleosome fractions of wild-type yeast cells and cells expressing a histone H3 gene altered to encode a cysteine 110 residue.

A technique for the separation of transcriptionally active and inactive nucleosomes by mercury affinity chromatography has been applied to study the nucleosomal distribution of DNA sequences from the GAL1, ACT1, HIS4, MAT alpha, and HMRa genes of yeast. In mammalian cells, the method has been shown to separate active from inactive nucleosomes and to fractionate the active nucleosomes into two classes, one retained on the mercury column because of salt-labile associations with certain thiol-reactive non-histone proteins, and the other bound by covalent linkage of the cysteine 110 thiol groups of histone H3 molecules to the mercurated support. The first class of nucleosomes is elutable in 0.5 M NaCl; the second is displaced by 10 mM dithiothreitol (DTT) (Walker, J., Chen, T. A., Sterner, R., Berger, M., Winston, F., and Allfrey, V.G. (1990) J. Biol. Chem. 265, 5736-5746). We show that, in wild-type yeast cells, in which histone H3 lacks cysteinyl residues, very little DNA and a negligible complement of nucleosomes appear in the DTT-eluate, confirming the requirement for the H3-thiols in the mercury-binding reaction. Moreover, the DTT-eluted fraction is seriously deficient in the actively transcribed GAL1, ACT1, HIS4, and MAT alpha DNA sequences. Site-directed mutagenesis was employed to create an H3 gene containing a cysteine codon in place of the alanine codon at position 110 of the yeast H3 amino acid sequence. A strain was constructed containing the mutant histone H3 gene instead of the normal H3 gene. Subsequent fractionations of the mutant nucleosomes by mercury-affinity chromatography revealed a characteristic nucleosome peak in the DTT-eluted fraction. Its content of transcribed GAL1, ACT1, and HIS4 DNA sequences was 20- to 500-fold higher than that of the corresponding DTT-eluted fraction of wild-type yeast. Although this result is in accord with the finding that, in mammalian cells, the thiol groups of histone H3 become accessible when nucleosomes "unfold" during transcription, we find that nucleosomes containing the GAL1 DNA sequences of the yeast H3-mutant also bind to the mercury column when that gene is not being expressed. We conclude that many yeast nucleosomes are maintained in a "primed," potentially active state, possibly due to the very high constitutive levels of acetylation of the core histones. However, the nucleosomes of the HMRa gene, which is not expressed in a MAT alpha yeast strain, are virtually absent from the DTT-eluted nucleosome fractions of the H3-mutant cells, indicating that prolonged silencing of the gene is accompanied by compaction and loss of H3-thiol reactivity of its nucleosomes.