THE SIZE OF THE TOPOLOGICAL DOMAIN MODULATES THE B-Z TRANSITION OF A (TG)(N) CONTAINING REPEAT

Under negative superhelical stress,long(TG), containing repeats experi ence a stepwise multiple B-Z transitions. We have investigated the eff ect of the plasmid size on this transitional behavior. A 66-bp (TG)(n) containing repeat from the 5'-untranscribed region of mouse ribosomal DNA was inserted in a 3-kb, a 6.5-kb and a 12.5-kb plasmids and its s upercoil-driven B-Z transition was followed by OsO4 probing of topoiso mer-populations. Our results show a clear correlation between the size of the topological domain and the extent of the region that converts cooperatively into Z-DNA at the initial transition.