THE ACTIVE-ROLE OF DNA AS A CHROMATIN ORGANIZER

Histone octamers (hos) and DNA topoisomerase I contribute, along with other proteins, to the higher order structure of chromatin. Here we re port on the similar topological requirements of these two protein mode l systems for their interaction with DNA. Both histone octamers and to poisomerase I positively and consistently respond to DNA supercoiling and curvature, and to the spatial accessibility of the preferential in teraction sites. These findings (1) point to the relevance of the topo logy-related DNA conformation in protein interactions and define the p articular role of the helically phased rotational information; and (2) help to solve the apparent paradoxical behaviour of ubiquitous and ab undant proteins that interact with defined DNA sites in spite of the l ack of clear sequence consensuses. Considering firstly, that the inter actions with DNA of both DNA topoisomerase I and histone octamers are topology-sensitive and that upon their interaction the DNA conformatio n is modified; and secondly, that similar behaviours have also been re ported for DNA topoisomerase II and histone H1, a topology-based funct ional correlation among all these determinants of the higher order str ucture of chromatin is here suggested.