GRADUAL AND ORIENTED B-Z TRANSITION IN 5'-UNTRANSCRIBED REGION OF MOUSE RIBOSOMAL DNA

The structural transition of an alternating purine-pyrimidine sequence ((CG)(5)(TG)(28)) from the 5'-untranscribed region of the mouse ribos omal DNA was analyzed by two-dimensional gel electrophoresis and chemi cal probes. The repeat undergoes a supercoil dependent gradual and ori ented B-Z transition. At a threshold level of negative supercoiling, a limited region of the repeat encompassing the (CG)(5) motif converts cooperatively into Z-DNA. As the superhelical stress increases, the Z- structure propagates along the remaining part of the repeat by success ive transitions until the full-length sequence is converted. By in sit u OsO4 probing experiments, we show also that this (TG)(n)-containing repeat adopts the Z-structure in Escherichia coli.