The size of the topological domain modulates the B-Z transition of a (TG)n containing repeat

Under negative superhelical stress, long (TG)n containing repeats experience a stepwise multiple B-Z transitions. We have investigated the effect 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 supercoil-driven B-Z transition was followed by OsO4 probing of topoisomer-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.