Positive torsional strain causes the formation of a four-way junction at replication forks.

The advance of a DNA replication fork requires an unwinding of the parental double helix. This in turn creates a positive superhelical stress, a (+)-DeltaLk, that must be relaxed by topoisomerases for replication to proceed. Surprisingly, partially replicated plasmids with a (+)-DeltaLk were not supercoiled nor were the replicated arms interwound in precatenanes. The electrophoretic mobility of these molecules indicated that they have no net writhe. Instead, the (+)-DeltaLk is absorbed by a regression of the replication fork. As the parental DNA strands re-anneal, the resultant displaced daughter strands base pair to each other to form a four-way junction at the replication fork, which is locally identical to a Holliday junction in recombination. We showed by restriction endonuclease digestion that the junction can form at either the terminus or the origin of replication and we visualized the structure with scanning force microscopy. We discuss possible physiological implications of the junction for stalled replication in vivo.