Nucleosomes: a solution to a crowded intracellular environment?

The emergence of eukaryotes was accompanied by two major events that concern their genome and are of crucial significance when considered in terms of macromolecular crowding: (i) a substantial increase in the amount of DNA, and (ii) its confinement within a defined space. The resulting highly crowded environment would have strongly promoted DNA self-assembly processes, leading to extremely condensed and thermodynamically stable DNA aggregates. Such structural transitions have indeed been observed in vitro, as well as in virtually all cellular systems in which a nucleosomal assembly is absent. In this appear we raise the hypothesis that upon transition from prokaryotic systems to eukaryotes, the nucleosomes were rendered essential in order to negate extensive DNA condensation processes that would have resulted from excluded volume effects. By suppressing such processes, the nucleosomes act to maintain and regulate the conformational space of the DNA, thus enabling conformational flexibility and reversible structural modulations. Copyright 1997 Academic Press Limited.