NUCLEOSOMES - A SOLUTION TO A CROWDED INTRACELLULAR ENVIRONMENT

The emergence of eukaryotes was accompanied by two major events that c oncern their genome and are of crucial significance when considered in terms of macromolecular crowding: (i) a substantial increase in the a mount of DNA, and (ii) its confinement within a defined space. The res ulting highly crowded environment would have strongly promoted DNA sel f-assembly processes, leading to extremely condensed and thermodynamic ally stable DNA aggregates. Such structural transitions have indeed be en observed in vitro, as well as in virtually all cellular systems in which a nucleosomal assembly is absent. In this paper we raise the hyp othesis that upon transition from prokaryotic systems to eukaryotes, t he nucleosomes were rendered essential in order to negate extensive DN A condensation processes that would have resulted from excluded volume effects. By suppressing such processes, the nucleosomes act to mainta in and regulate the conformational space of the DNA, thus enabling con formational flexibility and reversible structural modulations. (C) 199 7 Academic Press Limited.