Preferential, cooperative binding of DNA topoisomerase II to scaffold‐associated regions.

DNA elements termed scaffold‐associated regions (SARs) are AT‐rich stretches of several hundred base pairs which are known to bind specifically to nuclear or metaphase scaffolds and are proposed to specify the base of chromatin loops. SARs contain sequences homologous to the DNA topoisomerase II cleavage consensus and this enzyme is known to be the major structural component of the mitotic chromosome scaffold. We find that purified topoisomerase II preferentially binds and aggregates SAR‐containing DNA. This interaction is highly cooperative and, with increasing concentrations of topoisomerase II, the protein titrates quantitatively first SAR‐containing DNA and then non‐SAR DNA. About one topoisomerase II dimer is bound per 200 bp of DNA. SARs exhibit a Circe effect; they promote in cis topoisomerase II‐mediated double‐strand cleavage in SAR‐containing DNA fragments. The AT‐rich SARs contain several oligo(dA).oligo(dT) tracts which determine their protein‐binding specificity. Distamycin, which is known to interact highly selectively with runs of A.T base pairs, abolishes the specific interaction of SARs with topoisomerase II, and the homopolymer oligo(dA).oligo(dT) is, above a critical length of 240 bp, a highly specific artificial SAR. These results support the notion of an involvement of SARs and topoisomerase II in chromosome structure.