We have identified a clear preference of
histone H1 for CpG-methylated
DNA, irrespective of DNA sequence. The conditions under which this preference is observed allowed cooperative binding of H1; the H1-DNA complexes formed were shown earlier to be 'tramlines' of two
DNA duplexes bridged by an array of H1 molecules, and multiples of these. The preference for methylated
DNA is clear in sedimentation assays, which also show that the preference is greater with increased methylation level, and in gel retardation assays with an
oligonucleotide containing a single methyl-CpG pair; it is shared by the globular domain which also binds cooperatively to
DNA. A small intrinsic preference of H1 for methylated
DNA is also apparent in Southwestern assays where the immobilized H1 presumably cannot bind cooperatively. Methylated
DNA in H1-DNA complexes was partially protected (relative to unmethylated
DNA) against digestion by MspI but not by
enzymes whose cutting sites were not methylated, consistent with a direct interaction of H1 with methylated
nucleotides; this was also true of GH1-DNA complexes. H1 variants (
spH1 and H5) from transcriptionally repressed nuclei have a stronger preference than H1 for methylated
DNA, suggesting that this may be relevant to the stabilization of
chromatin higher order structure and transcriptional repression.