ACTIVE DISSOCIATION OF HOECHST-33342 FROM DNA IN LIVING MAMMALIAN-CELLS

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