H1–H1 Cross-Linking Efficiency Depends on Genomic DNA Methylation

Oligonucleosomal DNA preparations from condensed-inactive chromatin were examined, before and after artificial methylation by bacterial SssI methylase, for their ability to allow cooperative H1–H1 interactions under conditions of different ionic strength. Our results support the conclusion that, within the highly methylated genomic DNA, there are some CpG's whose unmethylated state is critical for chromatin folding. Circular dichroism spectra indicate that artificial overmethylation of native oligonucleosomal DNA reduces its efficiency in inducing an ordered conformation of H1 histone. Temperature melting profiles confirm on the other hand that the native and the artificially overmethylated forms of oligonucleosomal DNA are both able to bind H1 histone.