DNA phase transition promoted by replication initiator

DNA is flexible and easily subjected to bending and wrapping via DNA/protei n interaction. DNA supercoiling is known to play an important role in a var iety of cellular events, such as transcription, replication, and recombinat ion. It is, however, not well understood how the superhelical strain is eff iciently redistributed during these reactions. Here we demonstrate a novel property of an initiator protein in DNA relaxation by utilizing a one-molec ule-imaging technique, atomic force microscopy, combined with biochemical p rocedures. A replication initiator protein, RepE54 of bacterial mini-F plas mid (2.5 kb), binds to the specific sequences (iterons) within the replicat ion region (ori2). When RepE54 binds to the iterons of the negatively super coiled mini-F plasmid, it induces a dynamic structural transition of the pl asmid to a relaxed state. This initiator-induced relaxation is mediated nei ther by the introduction of a DNA strand break nor by a local melting of th e DNA double strand. Furthermore, RepE54 is not wrapped by DNA repeatedly. These data indicate that a local strain imposed by initiator binding can in duce a drastic shift of the DNA conformation from a supercoiled to a relaxe d state.