Active dissociation of Hoechst 33342 from DNA in living mammalian cells.

The fluorescent dye Hoechst 33342, which binds tightly to DNA in the minor groove, can be effectively extracted from the DNA in vivo due to an energy-dependent process. It is the dissociation of the dye from DNA rather than its removal from a cell that has a critical role in this process. The dissociation can be suppressed by the topoisomerase-2 inhibitors novobiocin, ellipticine and etoposide. Breaks in the DNA also inhibit the process. The dissociation of the dye requires that DNA remain intact along a region of at least several thousand base pairs. It is proposed that DNA in mammalian cells is a dynamic, conformationally nonstable system and that topologically closed loops consisting of several thousand base pairs constantly appear and disappear in chromatin.