CPG METHYLATION REMODELS CHROMATIN STRUCTURE IN-VITRO

One of the mechanisms proposed to explain how CpG methylation effects gene repression invokes a DNA methylation-determined chromatin structu re. Previous work implied that this DNA modification does not influenc e nucleosome formation in vitro, thus current models propose that cert ain non-histone proteins or a preferential affinity by linker histones for methylated DNA may mediate changes in chromatin structure. We hav e reinvestigated whether CpG methylation alters the chromatin structur e of reconstitutes comprising only core histones and DNA. We find that DNA methylation prevents the histone octamer from interacting with an otherwise high affinity positioning sequence in the promoter region o f the chicken adult beta-globin gene. This exclusion is attributed to methylation-determined changes in DNA structure within a triplet of Cp G dinucleotides. In the affected nucleosome, this sequence motif is lo cated 1.5 helical turns from the dyad axis and is oriented towards the histone core. These findings establish that DNA methylation does have the capacity to modulate chromatin structure directly, at its most fu ndamental level. Furthermore, our observations strongly suggest that a very limited number of nucleotides can make a decisive contribution t o the translational positioning of nucleosomes. (C) 1997 Academic Pres s Limited.