DNA methylation effects on tetra-nucleosome compaction and aggregation.

DNA CpG methylation has been associated with chromatin compaction and gene silencing. Whether DNA methylation directly contributes to chromatin compaction remains an open question. In this study, we used fluorescence fluctuation spectroscopy (FFS) to evaluate the compaction and aggregation of tetra-nucleosomes containing specific CpG patterns and methylation levels. The compactness of both unmethylated and methylated tetra-nucleosomes is dependent on DNA sequences. Specifically, methylation of the CpG sites located in the central dyad and the major grooves of DNA seem to have opposite effects on modulating the compactness of tetra-nucleosomes. The interactions among tetra-nucleosomes, however, seem to be enhanced because of DNA methylation independent of sequence contexts. Our finding can shed light on understanding the role of DNA methylation in determining nucleosome positioning pattern and chromatin compactness.