The organisation of chromatin loops: characterization of a scaffold attachment site.

Previous experiments have identified a 657-bp restriction fragment in the non-transcribed region of the Drosophila histone gene cluster that is specifically associated with the histone-depleted nuclear scaffold. The remaining fragments of the 5-kb histone repeat were shown to be readily released from the scaffold; hence it was proposed that the tandemly repeated cluster of histone genes forms a series of 5-kb loops restrained by a nuclear substructure at the sites of attachment. Here we show that the attachment fragment is tightly associated with protease-sensitive material, whereas the solubilized fragments are relatively protein-free. Exonuclease III digestion has been used to map the location of protein complexes on the attachment fragment. We have defined two regions of approximately 200 bp whose borders provide kinetic barriers to exonuclease III degradation. They are separated by a nucleaseaccessible region of approximately 100 bp. The protected regions are sufficient to mediate association of the fragment with the histonedepleted nuclei. Sequence analysis reveals an enrichment for sequences closely related to the topoisomerase II cleavage consensus in these two domains.