Positive supercoiling is generated in the presence of Escherichia coli SeqA protein.

In Escherichia coli, the SeqA protein is known as a negative regulator of chromosome replication. This protein is also suggested to have a role in chromosome organization. SeqA preferentially binds to hemi-methylated DNA and is by immunofluorescence microscopy seen as foci situated at the replication factories. Loss of SeqA leads to increased negative supercoiling of the DNA. We show that purified SeqA protein bound to fully methylated, covalently closed or nicked circular DNA generates positive supercoils in vitro in the presence of topoisomerase I or ligase respectively. This means that binding of SeqA changes either the twist or the writhe of the DNA. The ability to affect the topology of DNA suggests that SeqA may take part in the organization of the chromosome in vivo. The topology change performed by SeqA occurred also on unmethylated plasmids. It is, however, reasonable to suppose that in vivo the major part of such activity is performed on hemi-methylated DNA at the replication factories and presumably forms the basis for the characteristic SeqA foci observed by fluorescence microscopy.