The nucleosomal array: structure/function relationships.

A nucleosomal array consists of core histone octamer-DNA complexes spaced at approximately 200 bp intervals along a DNA molecule. Nucleosomal arrays are the fundamental building block of chromosomal superstructures, the substrate for transcription, and the first nucleoprotein assembly laid down after DNA replication. The development of homogeneous length-defined nucleosomal arrays has led to a greatly improved understanding of nucleosomal array structural dynamics in the solution state. Under physiological salt conditions, a nucleosomal array is in dynamic equilibrium between folded, self-associated and dissociated conformational states. Folding and self-association are both critically dependent on the core histone tail domains, consistent with an essential functional role for the tail domains in the mediation of chromosomal level DNA compaction in the nucleus. Nucleosomal array folding is repressive in transcription in vitro, but can be overcome by compositional (e.g., tail domain acetylation) and configurational (e.g., histone octamer depletion) changes that are correlated with transcriptional activation in vivo. The mechanism of replication-coupled chromatin assembly also appears to be functionally linked to the dynamic properties of nucleosomal arrays. Although once thought to be both structurally and functionally inert, it is now apparent that the nucleosomal array is a key participant in the biological processes that take place within the chromosomal fibers of eukaryotes.